Chapter 5 - Bacterial NanoCellulose Aerogelsdownload.xuebalib.com/xuebalib.com.39157.pdf ·...

73Bacterial NanoCellulosehttp://dx.doi.org/10.1016/B978-0-444-63458-0.00005-6 Copyright © 2016 Elsevier B.V. All rights reserved.

CHAPTER 5

Bacterial NanoCellulose AerogelsFalk Liebner, Nicole Pircher, Thomas RosenauDivision of Chemistry of Renewable Resources, University of Natural Resources and Life Sciences Vienna, Tulln, Austria

A Brief Overview on 85 Years of Research on Man-Made Aerogels

The long and windy road of the development of silica aerogels—the most famous and best explored subclass of aerogels—from flattening additives for paints, cigarette filters, or thickening agents for Napalm bombs to super-insulating materials for aerospace expeditions or scaffolds for nondestructive collection of stardust started in fact with the conversion of a biopolymer-based hydrogel to the respective aerogel. It was a bet between Steven Samuel Kistler—the pioneer of aerogel research—and his colleague Charles H. Learned at the College of the Pacific in Stockton, California in the early 1930s about the question whether it would be possible to replace the water of a strawberry jelly—a gel composed of pectin networks and interstitial water containing natural flavor, fruit acids, and sugars—by air without causing any shrinkage [1].

Kistler who won the bet not only discovered that the shape of respective aquogels can be largely maintained when water is first replaced by methanol and the alcohol is then extracted at supercritical conditions, but also realized the great potential of aerogels for various applications including thermal and acoustic insulation, gas sorption and separation, or filtration [2]. However, the commercial success of first large-scale silica aerogel producers, such as Monsanto Corporation, was rather limited with the new materials due to serious technological drawbacks, such as the essential but tedious removal of salts formed during poly-condensation of alkali silicates or the costly high-pressure equipment needed for “drying” the aerogels. Owing to the comparatively harsh conditions of supercritical methanol extraction (Tcrit = 239°C, pcrit = 8.1 MPa) and the state of high-pressure process technology, the “drying” step was fraught of safety risks at that time [1].

The introduction and application of modern sol-gel chemistry based on metal alkoxides in the late 60s [3] and the establishment of supercritical carbon dioxide (scCO2) technologies in the 80s [4] can be regarded as milestones in aerogel research in general, including advancement of silica aerogels for high-performance applications, and eventually their commercial success.

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Today, silica aerogels can be still considered to be the most comprehensively studied subclass of aerogels. Their utilization by the National American Space Agency (NASA) during several space missions and 15 entries in the Guinness Book of Records for their outstanding properties further contributed to their publicity. Their high specific surface area ( 600–1600 m2 g−1), permittivity (1.007-2), and shock absorption potential on the one hand, and the low bulk density of less than 1 mg cm−3, low thermal expansion (alin = 2 × 10−6 K−1), thermal conductivity (l = 0.016–0.030 W m−1 K−1), sound propagation (v = 100–300 m s−1), and acoustic impedance (Z = 103–105 kg m−2 s−1) on the other hand are used in many applications [5,6]. Most prominent examples in this respect are the use of silica aerogels for high-performance insulation in cases where the space for placing of the insulating material is very limited or the material is exposed to extreme conditions. Silica aerogels have been therefore used by NASA during the Mars Pathfinder expedition in 1996/1997 for protecting electronic devices of the Sojourner Mars rover from the extremely low Mars surface temperature that can be as low as −140°C in polar nights during winter time. The excellent shock absorbing properties have been employed during the NASA stardust project (1999–2006) for trapping high velocity particles without vaporization of the samples for analytical purposes [7,8]. Furthermore, the different stopping distance inside the aerogel caused by different speed of the particles allows for distinguishing copious anthropogenic debris from relatively rare extraterrestrial particles. By advancing this concept alumina aerogels co-doped with Gd and Tb have been developed which are able to report the kinetic energy of hypervelocity particles as the latter generate local fluorescence signals due to certain phase transitions, such as the formation of the GdAlO3:Tb perovskite phase [9].

During the past 25 years aerogels have been prepared from a wide variety of inorganic (e.g., clay, metals, metal oxides, or chalcogens) or organic precursor compounds (e.g., phenol-formaldehyde resins), and in particular the progress in synthetic polymer chemistry in the early 90s has largely contributed to this development. While the open-porous, lightweight structure is a common feature of all aerogels, the huge variety of source materials leads to a broad diversity of other properties including mechanical strength, network architecture and morphology, pore size distribution, pore geometry, surface chemistry, or roughness of the inner and outer surface, which can be employed for a large variety of applications [10–13]. Aerogels based on silica, carbon, organic polymers, or metals are nowadays already commercialized (Aerogel Technologies LLC, Aspen aerogels Inc., Cabot Aerogel Ltd., etc.) at comparatively high price for chemisorption, chromatography, low-profile thermal and acoustic insulation, particle detection, optics, chemical spill clean-up, microwave and RF induction heating processes, oil recovery or as catalyst support [14], diffusion control and battery media, materials with engineered electromagnetic properties, desalination electrodes, and electrodes for battery and supercapacitor applications [15]. Other types of aerogels are supposed to follow soon, such as cobalt–molybdenum–sulfur aerogels which have a high ability for removing mercury from polluted water, for separating hydrogen from other gases and, catalyzing hydro-desulfurization of crude oil to date in an unsurpassed efficiency [16].

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Due to both the comparatively high price of silica aerogels (200 $ for a monolithic tile as large as a playing card with a thickness of 7 mm; [17]) reflecting also the high energy requirements of the rather nongreen silica precursor chemistry and the increasing public awareness of the finiteness of fossil resources, aerogels from renewable resources have been increasingly moved into the limelight of academic and commercial interests during the last decade. However, research on biopolymer-based aerogels is still in its infancy even though a multitude of fascinating materials with promising properties has been developed in the last few years from cellulose, starch, alginates, chitin, chitosan, pectin, agar, carrageenan [18–23], arabinoxylan [24], whey protein [25], or lignin [26]. Research on cellulosic aerogels is supposedly the most vivid field in this respect due to the abundance of this biopolymer, its structural features, its partly occurrence in high purity reducing pretreatment efforts to a minimum, and the availability of well-established environmental-friendly large-scale isolation and purification technologies.

Bacterial NanoCellulose: A Green, Cheap, and Mass Producible Porous Material

Natural porous materials are rampant in both the animal and plant kingdom. The architecture and chemical composition of load-bearing natural “construction” materials, such as human bone, the female spike of Typha (reed mace, cattletail), or the stem of the West-African legume Aeschynomene pfundii Taub are the result of evolution, adaptation, and optimization processes following the guiding principles of maximizing stiffness-to-weight ratio and minimizing local stress by efficiently dissipating forces. Most of them consist of hierarchical, open-porous networks made of sophisticated natural (composite) materials. Next to dissipation of forces and minimizing weight, these materials are also used by many living organisms in other life-sustaining functions, such as high-performance adsorption and accumulation of nutrients from highly nutrient-deficient seawater (sponges on reefs), retention and distribution of fluids reverse to gravity (wood, bamboo), thermal insulation (bark of cork oak, not open-porous), or protection of gametes and developing birds in closed compartments (eggs) with a porous shell that allows for simultaneous infiltration of air and evaporation of water. Some of the most lightweight yet mechanically resistant porous natural materials are used for high-performance acoustic and thermal insulation in house walls, floors, ceilings, and facades, such as cork or reed mace.

Bacterial nanocellulose (BNC) is another example of highly porous natural materials. It is an extracellular product of the metabolism of various bacteria [27] with Gluconacetobacter xylinus being probably the best investigated species due to its commercial use. The obligate aerobic, gram-negative, and rod-like strain metabolizes primarily glucose via the steps glucose-6-phosphate, glucose-1-phosphate, and UDP-glucose to cellulose. Extrusion of glucan chains through pores (“biological spinnerets”) arranged longitudinally along the cells and subsequent assembly of glucan chains to subfibrils, microfibrils, and microfibril bundles eventually affords comparatively stiff ribbons which, in turn, consist of loosely assembled

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microfibril bundles. As the cellulose synthesizing sites are duplicated during cell division [28], mother and daughter cells are connected to one and the same cellulose ribbon which eventually forms an interconnected three-dimensional network of highly entangled cellulose. Due to its inherently high porosity, the cellulosic matrix floats on the surface of the aqueous nutrient medium and keeps the bacterium in direct contact with air, which has been frequently considered to be an integral aspect of the survival strategy of this type of microorganisms similar to other cellulose-producing bacteria, such as the plant pathogen Agrobacterium tumefaciens, which uses the formed polysaccharide matrix for better attachment to plants.

Intriguing properties in terms of hierarchical, lightweight (ρB ≤ 5–10 mg cm−3) network morphology, open, multiscale porosity comprising micro-, meso- and macropores, as well as high purity, molecular weight (100–200 kDa), crystallinity (60–80%), and fiber strength (Et 15–30 GPa) render bacterial cellulose very interesting for many applications. In particular applications in the fields of biomedicine (e.g., wound dressing) and cosmetics (e.g., skin treatment) greatly benefit from BNC’s high purity, biocompatibility, good resistance toward microbial degradation, and great water retention capacity. The latter is the result of its high internal surface which is covered by abundant hydrophilic hydroxyl groups. Compared to plant cellulose, such as cotton linters which has a water retention of around 60%, bacterial cellulose can reach water retention values of more than 1000% [14] whereof only one-tenth of the stored water behaves like free bulk water draining under the impact of gravitation [29].

However, also apart from biomedical applications, bacterial cellulose has recently moved into the focus of material research due to its natural occurrence, facile and green cultivation which solely requires aqueous conditions, an appropriate nutrient medium, simple equipment—at least for static, batch-wise cultivation—and time.

Bacterial nanocellulose—most likely not on purpose—has been produced since thousands of years as it is inseparably linked with vinegar production whose history started around 5000 BC, when the Babylonians were using the fruit of the date palm to make wine and vinegar. Similar as at that time, conversion of alcoholic beverages to vinegar in open vessels (Orléans process) is still accomplished by adding “Mother of vinegar”—a slimy BNC-rich matrix containing various Acetobacter species-which then grows on the surface of the fermenting wine or cider. Bacterial nanocellulose is also produced during fermentation of sweetened green or black tea (Kombucha tea) by the symbiotic activity of Ascomycetes and Acetobacter species.

Cultivation of Ga. xylinus in large open tanks using coconut water as a nutrient medium and taking advantage of the local climate is a vivid business in Southeast Asian countries (Philippines, Indonesia, Vietnam) as the product Nata de Coco (Spanish: cream of coconut)—a chewy, translucent, color- and tasteless, fibrous hydrogel consisting of pure BNC and water—can be sold as dietary auxiliary. In aromatized form it can be found in many deserts and drinks, also offered by many Asia shops across Europe.


The high requirements of biomedical applications in terms of purity, the inefficiency of long-distance transportation of a material that contains more than 99% of water, the desire to develop and implement more efficient technologies of BNC production capable of controlling key properties have led to a series of innovations in this field as reviewed elsewhere [30].

Batch-wise static cultivation of Ga. xylinus species using large tanks and suitable aqueous growth media is still by far the most frequently applied technology to produce bacterial cellulose. After inoculation of the culture medium with the bacteria strain and a short lag phase, secretion of the exo-polysaccharide on the surface of the growth medium sets in. The thickness of the cellulose layer increases with the cultivation time, which can last up to several weeks. The harvested cellulose is typically purified from the bacteria culture and the remnants of the culture medium by repeated alkaline treatment (e.g., 0.1 M aqueous NaOH at 90°C, 20 min) and thorough washing with (deionized) water [31]. The purified material can be disintegrated to afford nanofibril suspensions, which can be pressed during drying to obtain membranes of uniaxially or uniplanary oriented nanofibrils. BNC can be also easily cut into sheets or geometric objects of desired size and shape which, in turn, can be converted to monolithic aerogels.

Besides (agitated) static cultivation, airlift reactors, aerosol reactors, or rotary bioreactors are increasingly used [30]. While in aerosol reactors the nutrients are sprayed onto the surface of the growing BNC pellicles (thicknesses of up to 7 cm can be reached), rotary bioreactors consist of vertically aligned rotating discs which are alternately exposed segment-wise to the culture medium and air. In membrane bioreactors, the nutrients are supplied through a membrane which separates BNC from the circulating culture medium. However, as with shaken or agitated cultures which greatly increase the growth rate of bacteria, the increased risk of mutation can lead to significant reduction in BNC yield [32]. In horizontal lift reactors BNC is produced in a continuous way by slowly pulling the BNC sheets from respective tanks containing the culture medium [30,33].

With respect to future applications there is broad consent that successful commercialization of BNC will largely depend on the crucial questions whether the efficiency of large-scale BNC production lines can be greatly improved or whether particular properties of this material can be tuned in a way that special applications could justify the still comparatively high price of bacterial cellulose. Based on these considerations, many attempts have been made to tailor the properties of bacterial cellulose (morphology, pore features, strength, surface chemistry, etc.) for particular applications already during biosynthesis. While the effects of the used bacteria strain, composition of the culture medium, oxygen supply, pH and temperature, and type of bioreactor on BNC yield, degree of polymerization, ribbon diameter, crystallinity, porosity, and 3D morphology have been comprehensively reviewed elsewhere [30,34], the impact of additives shall be summarized in more detail.

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The ultrastructure of cellulose ribbons, their size and aspect ratio, intensity of entanglement, and hence, packing density have a large impact on the mechanical properties of bacterial cellulose [35]. Therefore, all measures capable of controlling these parameters, in particular self-alignment of cellulose during biosynthesis, can potentially contribute to tailor the macroscopic properties for certain target applications.

Electric fields, for example, have been employed to control the motion of Gluconacetobacter xylinus in a way that denser fiber alignment is achieved [36]. Similarly, orientation within BNC networks was achieved by cultivation of BNC on concave honeycomb-patterned agarose scaffolds as the respective microfibrils were found to align along the substrate pore walls [37]. Uniaxial fibril orientation can be also obtained if BNC is cultured on oxygen-permeable poly(dimethylsiloxane) molds of ridged (roughness 0.3–16.8 mm) morphology [38]. Depending on the roughness of the used mold, strong birefringence with colorful images indicating liquid crystallinity of BNC was observed. At a ridge size of about 4.5 mm—coinciding with the contour length of bacteria cells—the optimum of birefringence, the highest fracture stress (σ), the highest swelling degree (q), the lowest elastic modulus (E), and the thickest BNC fibril diameter are reached. Similar results have been obtained when BNC was cultivated around oxygen-permeable silicone tubes [39].

Ribbon formation on the other hand can be suppressed by addition of xylan to the culture medium as the latter covers the surface of individual microfibrils and decreases both crystal size and cellulose Ia content of BNC formed [40]. The same effect but at higher intensity is observed when glucomannan is added to Hestrin–Schramm (HS) medium while the addition of pectin has virtually no effect [41].

Besides the well-known effect of different monosaccharides on the crystallinity of BNC— it decreases in the order fructose > maltose ≥ glucose > sucrose [42,43]—replacement of sugars by less good metabolizable organic compounds or extracts, such as rice bark extract or carboxymethyl cellulose commonly results in a strong decrease of crystallinity [44]. The addition of carboxymethyl cellulose (CMC) or hydroxypropyl methyl cellulose (HPMC) to the culture medium, however, has a similar effect, but significantly improves the rehydration ability of BNC. Winding of CMC or HPMC around microfibrils creates a larger number of amorphous regions that promote water permeation into the cellulose network and swelling [45]. The addition of micro-structuring organic sources, such as multiwalled carbon nanotubes (MWCNTs) to HS medium inoculated with Ga. xylinus was shown to afford more rigid cellulosic pore walls compared to the reference sample grown on MWCNT-free HS medium [46]. Furthermore, the presence of MWCNT weakened the intermolecular hydrogen bonds of cellulose leading to reduced crystallinity index (CrI), crystal size, and cellulose Ia content. A similar result was observed when wax spheres were added to the growth medium to control the pore size of the formed BNC sheets.


Interconnected porosity and large internal surface are key features of aerogels and the prerequisite for many applications. However, the demands with regard to size and distribution of pores can largely vary depending on the target application. While thermal super-insulating materials, for example, require small pores not substantially exceeding the mesopore range, cell scaffolding materials for tissue engineering applications can require multiscale porosity as it is the case for in vitro generation of bone and cartilage tissue. This includes the presence of micron-size pores of suitable geometry and mechanical pore wall characteristics essential to support cell attachment, ingrowth, proliferation and differentiation, as well as diffusion of physiological nutrients and gases to cells, removal of metabolic by-products from cells, cell shaping, reorganization, and gene expression [47].

Polyethylene glycol (PEG), for example, has been demonstrated to be a suitable pore-size modulating culture medium additive. Depending on the molecular weight, smaller (DP 4000) or larger (DP 400) pores can be obtained compared to bacterial cellulose cultivated on PEG-free nutrient medium [48]. Different from other auxiliaries, such as hemicellulose, MC or CMC, PEG is not incorporated into the BNC network structure. Pore widening can also be achieved using b-cyclodextrin as a growth medium additive [48].

Temporary porogens creating a well-defined porous polymer morphology before being leached by an appropriate solvent are also frequently used and include the use of water-soluble salt spheres (e.g., NaCl) [49], ice crystals [50], carbon nanotubes [46], or hydrogel beads either from natural polymers like gelatin [51] or synthetic polymers like poly(ethylene glycol) [52]. Paraffin spheres which had been originally suggested as porogens for PLLA and PLGA gels [53] turned out to be also well suited for tuning the porosity of bacterial cellulose toward macroporosity [54,55].

Prefabricated BNC hydrogels can be retroactively equipped with larger pores or channels too, such as by surfactant-assisted foaming in aqueous azodicarbonamide/sodium hydroxide solution at 60°C [56], treatment with nitrogen-containing plasma [57] as accomplished to enhance cell affinity or by pulsed CO2 laser sequences [58]. Alkaline posttreatment of freshly harvested bacterial cellulose can be used on the other hand to reduce the pore size, an effect that decreases in the following order: K2CO3 > Na2CO3 > KOH > NaOH [59].

Conversion of Bacterial NanoCellulose Aquogels to Aerogels

Porous natural materials grow and develop in biological environment and their properties are therefore optimized for aqueous and humid conditions, respectively. If the particular structure of these materials is desired for technical applications in nonaqueous environments (insulation, gas sorption, etc.) water has to be removed beforehand. This has to be accomplished in a way that largely preserves the original network structure. While the composition of beams and walls of natural cellular load-bearing materials has been adapted

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to permanent or temporary capillary forces inevitably occurring in aqueous media, most of them can be air-dried without significant shrinkage, compaction, or loss of porosity. This, however, is quite different for many man-made hydrogels if the bulk density of the network forming constituent(s) is very low no matter whether the gel is composed of inorganic constituents, synthetic organic polymers, or biopolymers. Man-made biopolymer-based gels typically suffer from extensive shrinkage similar to silica gels if thermal drying is applied. This is due to capillary forces that occur alongside the capillary walls adjacent to the solvent menisci. These inward forces at the phase boundaries are most pronounced for thermal drying due to the large differences that exist in the specific energies of the three media, that is, the void forming walls, interstitial liquid, and gas phase. According to the Young–Laplace equation the absolute values of the negative hydrostatic pressure (Ψp) are inversely proportional to the capillary radius (r) and increase with the surface tension (σ) of the liquid that fills the pore voids (Eq. 5.1). Due to the particularly high surface tension of water (72.75 mN m−1 at 20°C), hydrogels are particularly sensitive. Bacterial cellulose hydrogels, for example, obtained by static cultivation of Gluconacetobacter xylinus AX5 wild-type strain, have a density of about 8 mg cm−3 only. Assuming an average void radius of about 50 nm and neglecting the particular impact of the cellulose surface, the hydrostatic pressure (tension) that develops inside the pores of such hydrogels would be in the range of 2.3–2.9 MPa (cf. Eq. 5.1). The occurrence of such strong forces inevitably has the potential to cause pore collapsing and hence far-reaching destruction of porous materials as demonstrated in Fig. 5.1.

Freeze-drying of cellulose aquogels, that is sublimation of water from solid state is a much better alternative for converting aquogels of low cellulose content into the corresponding aerogels as shrinkage can be largely suppressed. This is due to the nonexistence of phase boundaries between liquid and gas phase (gLV) and hence, any differences between their

Figure 5.1: Impact of the “drying” method on the morphology of cellulose aerogels (left). Deformation during drying is caused by capillary forces whose magnitude depends on pore size,

surface tension of interstitial liquid, and the differences between the specific energies at the phase boundary between liquid and gas phase, respectively. Young–Laplace equation in its simple (Eq. 5.1) and spherical form (Eq. 5.2) can be used to estimate the capillary pressure. Reproduced from [60] with

slight modification and permission of Taylor & Francis LLC.


specific energies, which turns the numerator of the spherical form of the Young–Laplace equation (Eq. 5.2), and hence the capillary pressure, to become zero [61].

However, as water expands by about 9 vol.% during freezing, pore collapsing and crack formation can occur in aquogels during lyophilization. The reduction in pore volume for bacterial cellulose, for example, can account for up to 10% for freezing of respective samples at −10°C. Fast freezing using liquid nitrogen or special deep freezers (e.g., −80°C) can better prevent pore collapsing as amorphous, very small, and less sharp-edged ice particles are formed [62]. Replacement of water by solvents of a lower thermal expansion coefficient, such as tert-butanol [63] and/or higher sublimation pressure, such as ethanol, utilization of low-melting liquids, such as butane (−134°C) or the use of cryo-protectants, such as glucose [64] are further measures to reduce the extent of shrinkage.

Supercritical drying is considered to be the method of choice for drying highly porous, fragile materials, such as low-density cellulose gels. Similar to freeze-drying no liquid-gas phase boundaries exist in supercritical fluids. Hence phenomena, such as surface tension or formation of solvent menisci cannot provoke shrinkage of these materials. Furthermore, there is also no liquid-to-solid transition which could alter the open-porous cellular network structure by volume expansion and formation of sharp-edged crystals. However, certain aspects related to pressurization, drying time, temperature, and depressurization rate have to be considered in order to get maximum preservation of the original morphology.

Carbon dioxide is probably the most frequently used supercritical fluid, as it is abundant, cheap, chemically largely inert, incombustible, easily recyclable, environmentally benign, and has a low critical point (30.98°C, 7.375 MPa). While supercritical carbon dioxide (scCO2) has a dissolving power similar to that of fluids, it effuses through solids like a gas at only somewhat higher density, exhibiting a low dynamic viscosity η, and hence a much higher diffusion coefficient. This allows for a rapid mass transport which is one of the main reasons why scCO2 has found wide use in extraction protocols [65]. As a nonpolar, lipophilic solvent, scCO2 is not miscible with water. Therefore, hydrogels, such as natural bacterial cellulose have to be subjected to solvent exchange steps before, aiming at a quantitative replacement of water by a nonpolar or weakly polar organic cellulose antisolvent that is miscible with scCO2.

Replacement of Water by Solvents Miscible with Carbon Dioxide

Bacterial cellulose hydrogels consist of a large volume fraction of interstitial water filling the voids of the fibrous networks they are composed of. Replacement of water by a suitable cellulose antisolvent, nonswelling toward cellulose but miscible with both water and scCO2, requires much concern with respect to the manner in which the water is incrementally replaced as strong gradients in polarity during the solvent exchange can lead to significant shrinkage by pore collapsing and hence, decreased porosity of the final aerogel. This is due

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to the different strength of solvent–polymer interactions governed by the surface chemistry of the respective network forming polymer(s). The abundance of hydroxyl groups renders cellulose a highly hydrophilic polymer; however, crystallinity and accessibility of hydroxyl groups have a considerable impact in this respect.

According to the Hansen model of solvent–polymer interactions, the cohesive energy density (expressed as Hildebrand solubility parameter) can be calculated as the sum of a dispersion force component, a polar component, and a hydrogen bonding component [66]. Replacement of water by ethanol, for example, reduces the cohesive energy density from dSI = 48 to 26.5 MPa1/2, while the total Hildebrand parameter of acetone or THF is dSI = 20.0 MPa1/2 and dSI = 19.4 MPa1/2, respectively. The hydrogen bonding component, which is, due to the high abundance of OH groups, supposed to be of particular importance for solvent–polymer interactions, is even more affected and decreases from dH = 42.3 MPa1/2 (water) to 19.4 MPa1/2 (ethanol), 8.0 MPa1/2 (THF), and 7.0 MPa1/2 (acetone), respectively.

The effect of cellulose antisolvent interactions on the preservation of the fragile cellulose network structure has been demonstrated for the conversion of different bacterial cellulose organogels to the respective aerogels [67]. Compared to BNC aerogels prepared directly from the respective alcogels (σbulk = 7.8 ± 0.5 mg cm−3), replacement of the interstitial ethanol by acetone (9.4 ± 0.6 mg cm−3) or THF (9.6 ± 0.8 mg cm−3) prior to the scCO2 drying step (40°C, 10 MPa) afforded somewhat higher densities reflecting thus the differences in the above solvent–polymer interactions. The full miscibility with carbon dioxide at comparatively mild conditions (≥8 MPa at 40°C), low viscosity, facileness of recovery, environmental aspects, and the comparatively low price render ethanol one of the most suitable cellulose antisolvents for the conversion of cellulose aquogels to aerogels.

Supercritical Carbon Dioxide Drying

Besides the type of cellulose antisolvent used to fully replace the interstitial water and to make the former hydrogel ready for scCO2 extraction, the scCO2 extraction protocol in terms of pressurization, extraction time, and depressurization rate can decisively affect the quality of the resulting aerogel in terms of preservation of original morphology, shape, porosity, etc.

For scCO2 processes p–x,y diagrams are well suited to describe binary mixtures and their behavior upon pressure variation. Fig. 5.2 shows the phase envelope of the binary mixture CO2/ethanol enclosed by the boiling point curve ascending with pressure and CO2 weight fraction and the dew point curve. While the intersect of both curves at xCO2 = 0 displays the vapor pressure of pure ethanol, that at high CO2 concentrations represents the critical point of the binary mixture for a certain temperature. Beyond the critical temperature of CO2 both the dew point and boiling curve do no longer intersect the y axis as there is no vapor pressure defined for that state.


Below the critical pressure of the binary system the solubility of CO2 in ethanol increases with pressure according to the phase envelope (cf. Fig. 5.2, right). During the supercritical drying process the state of the interstitial liquid filling the voids of the gel must be kept in a way that it stays outside this phase envelope at all times. At the beginning of the scCO2 drying process all pores of the lyogel are filled with ethanol (Point A). Upon pressurization of the system with carbon dioxide the amount of dissolved CO2 within the liquid phase increases forming a CO2-expanded-liquid phase [68]. By further increasing the concentration of CO2 in the liquid phase, its surface tension decreases significantly [69]. After reaching pressures above the mixture’s critical pressure for the given temperature (Point B), full miscibility of the binary mixture components is assured. Ethanol can then be removed from the porous matrix by flushing the system with pure CO2. This flushing step has to be performed until the ethanol content within the pore is low enough to avoid condensation of a liquid phase upon depressurization (Point C). Finally depressurization to atmospheric pressure gives the final product (Point D).

Below the critical temperature carbon dioxide behaves like a liquid. Starting from low gas-like densities, increasing pressure leads to condensation of CO2 and separation of two distinctly different dense phases. Further compression leads to an increasing volume of the liquid phase until finally all the CO2 has liquefied. At temperatures above the critical temperature (30.98°C) increasing pressures lead to increased and finally liquid-like densities without crossing the vapor/liquid coexistence region. As no clustering of the molecules and subsequent formulation of droplets is possible at these temperatures, the density of the supercritical liquid can be freely chosen. At typical drying temperatures of

Figure 5.2: Schematic representation of the mass transfer pathways associated with pressurization of the alcogel with carbon dioxide (left). Phase envelope of the binary system ethanol/CO2 at 40°C (capital letters represent the main process steps of scCO2 drying—A: alcogel at ambient conditions, B: critical point of the binary mixture, drying starts; C: extraction of ethanol is concluded; D: return

to ambient conditions after depressurization, right). Reproduced from [60] with permission of Taylor & Francis LLC.

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40°C [31], the density of CO2 is gas-like up to pressures of about 7.5 MPa and liquid-like at pressures beyond 12 MPa. In between these pressure limits, small changes in pressure effect considerable changes in density. The density of pure CO2 at drying conditions (40°C, 10 MPa) is 628 kg m−3 which is between the liquid and the gaseous state. During depressurization of the aerogel, the relatively large density differences occurring in a small pressure range close to the supercritical pressure are considered to be one major trigger of gel shrinking. Thus depressurization should be performed as slowly as necessary to avoid mechanical overstraining of the material.

The way of how the scCO2 extraction unit, and hence the gels to be dried are pressurized, affects the extent of shrinkage, too. The drying process can be divided into three steps, namely (1) diffusion of CO2 into the liquid phase within the pores, (2) spillage of the ethanol/CO2 mixture due to the increased volume of the liquid phase, and (3) convective transport of the spilled mixture within the CO2-stream out of the matrix (cf. Fig. 5.2, left).

Starting from a wet gel, CO2 diffuses into the liquid phase within the pores upon pressurization, causing an increase of the volume of the liquid phase and a slight increase of the liquid phase density. As the volume of the liquid phase increases, diffusion and spillage occur in parallel. Streaming CO2 outside of the gel and coherent convective transport should be avoided completely until the process of CO2 diffusion is completed and the CO2 concentration within the gel matrix is constant.

As denoted in Fig. 5.3, surface tension is a function of the density difference of the two involved phases. In order to avoid gel shrinkage, the interface of the involved phases must always be outside of the gel. Although there are two phases within the porous network upon pressurization (namely liquid ethanol and the mixture of CO2 and ethanol), the

Figure 5.3: Effect of residual amounts of water during pressurization of cotton linters alcogels with CO2.

Samples soaked with anhydrous ethanol (left columns) and ethanol that contained about 1 vol.% of water (right columns). Reproduced from [60] with permission of Taylor & Francis LLC.


interfacial tension can be neglected, as the density difference is marginal as long as there is no convective transport at the open end of the pore. The process of CO2 diffusion into the liquid phase is rather fast and can be ignored for small gel dimensions. When drying larger specimen, this effect has imperatively to be taken into account. After reaching uniform CO2 concentration within the matrix, convective transport can be started.

Residual amounts of water due to incomplete solvent exchange and the resulting high differences in density within the pores—scCO2 is only poorly soluble in water—can lead to strong deformation and shrinkage of the fragile porous BNC gels (cf. Fig. 5.3).

The removal of ethanol from the matrix by purging the system with pure carbon dioxide follows pressurization. This step is limited by several mass transfer resistances, which can be summarized to an effective diffusion coefficient (Deff) [70,71]. A full model for supercritical drying of aerogels following the above approach was proposed by Mukhopadhyay and Rao [72]. Exemplarily, a drying time of 120 min is recommended for cylindrical mesoporous cellulose aerogels of up to 2 cm3 volume and densities of up to about 100 mg cm−3.

Depressurization as the final step in the preparation of aerogels should be started only when the residual content of the scCO2 miscible solvent originally filling the pores of the gel (ethanol in this case) is surely low enough to be outside the phase envelope as shown in Fig. 5.2 (right). If this is not the case, capillary condensation might cause some pore collapsing, loss of specific surface area, and macroscopic shrinkage of the sample. Furthermore, depressurization should be accomplished at a fairly slow rate, not significantly exceeding 0.2 MPa min−1. This is particularly important at pressures close to the critical pressure of CO2 as small changes in pressure translate into comparatively significant changes in volume, which can be another reason for pore collapsing especially at very low density of the samples. Cooling below the critical temperature as it can occur by fast depressurization and the provoked Joule–Thomson effect can be another reason for volume reduction of the samples during scCO2 extraction.

Properties of Bacterial NanoCellulose Aerogels

The conversion of bacterial cellulose hydrogels to aerogels was first mentioned in the Japanese Journal of Polymer Science and Technology in 2006. It was the group of Shoichiro Yano at the Nihon University (Tokyo, Japan) who reported that replacement of water by ethanol and subsequent extraction of the latter under supercritical conditions (243°C, 6.38 MPa) largely preserve the cellulose network morphology of the former hydrogel and afford ultralightweight (ρB ≥ 6 mg cm−3), highly open-porous (PV ≤ 99%) materials composed of 20–60 nm thick microfibrils [73]. The harsh drying conditions, however, were suspected to effect considerable alterations of the native cellulose structure caused by intra- and intermolecular elimination of water [74–79] as the compressive stress measured at 60%

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strain (σ60%) strongly increased after converting the hydrogel (20 kPa) to the respective aerogel (54 kPa; [73]). This is assumed to be caused by hornification which is a well-known phenomenon occurring in (ligno)cellulosic materials upon thermal treatment beyond 180°C. It is known to result in considerable stiffening of the polymer structure [80] and decrease of both hydrophilicity and rewettability of BNC aerogels [76]. In general, thermal modification and degradation of cellulose takes place in three steps: (1) removal of physically bound water (up to 150°C), (2) intra- and interring dehydratization and loss of hydroxyl groups (180–240°C; [79]), and (3) pyrolytic cleavage of the polymer chain (240–400°C; [74]). While intensive dehydratization and polymer degradation during graphitization of Ryon under nitrogen atmosphere starts already at 270°C [75], cotton linters and bacterial cellulose require considerably higher temperatures of 325°C [78] and 360°C [77], respectively.

Drying with supercritical carbon dioxide at 40°C and 10 MPa is a good alternative in this respect as it is capable of fully preserving the chemical integrity and hence, nanomorphology of bacterial cellulose [31]. Depending on the way of BNC cultivation, the used BNC-producing bacteria strain, work-up procedure, and scCO2 drying conditions, bulk densities of down to 5.4 mg cm−3 [81] can be obtained.

Interestingly, bacterial cellulose resists much better volume reduction during solvent exchange and subsequent scCO2 extraction than it is the case for aquogels of comparable density obtained by coagulation of cellulose from solution state. While aquogels from coagulated commercial pulps three times denser than BNC lose at least 25% of their original volume [82,83], shrinkage is very little for bacterial cellulose (Fig. 5.4; [31]). The full preservation of porosity for BNC gels throughout the aerogel preparation is evident from the fact that after soaking the BNC aerogels in water the weight of the original aquogel is

Figure 5.4: scCO2 drying at 40°C and 10 MPa virtually fully preserves the cellulosic network structure of BNC alcogels and affords ultralightweight materials that can be kept in suspense just

by surface roughness (left encircled and middle). Shrinkage of aerogels obtained from 3 wt.% cotton linters containing Lyocell dopes during storage at different levels of relative humidity (right).

Reproduced from [60] with permission of Taylor & Francis LLC.


virtually fully recovered [84]. Bacterial nanocellulose aerogels were furthermore shown to resist shrinkage during long-term storage even under humid conditions. Whereas cotton linters (CL) aerogels (ρB ≈ 50 mg cm−3) obtained by coagulation (ethanol) of cellulose from respective 3 wt.% CL containing Lyocell dopes and scCO2 drying suffered from a significant reduction by volume during 14 days of storage (15.5% at 30%, 50.9% at 65%, and 84.2% at 98% relative humidity), BNC aerogels of much lower density (ρB ≈ 10 mg cm−3) were confirmed to resist shrinking virtually completely even at 98% relative humidity (r.h.) for at least several days. The almost zero shrinkage during scCO2 drying and open-air storage is supposed to be mainly due to the high portion of crystalline domains, the high number of entanglements, and the obvious higher stiffness of individual BNC ribbons. Its open-porous structure and the full wettability of BNC aerogels can be employed for controlled release applications. The release profiles of d-panthenol and l-ascorbic acid from BNC gels of different thicknesses, for example, have been shown to be largely independent of the amount of loaded compound due to negligible surface–solute interactions but highly dependent on the thickness of the aerogel layers [84]. It has been furthermore demonstrated that the mainly diffusion-controlled release of these two compounds can be reliably predicted using the Korsmeyer model which considers both the diffusion of water into an open-porous matrix and simultaneously that of a given organic compound out of it, using an experimentally determined effective diffusion coefficient [84,85].

Small-angle X-ray scattering (SAXS), nitrogen sorption at 77 K (calculation of the specific pore surface area by applying the models developed by Brunauer, Emmett, and Teller; BET, and Benjamin, Johnson and Hui; BJH), and thermoporosimetry with o-xylene as “confined” solvent confirmed that the dimension of the voids between the nanofibers corresponds to interconnected micro-, meso-, and macropores. In particular smaller macropores of around 100 nm in diameter contribute mostly to the BNC aerogel’s overall porosity (Fig. 5.5, left) which is in good agreement with different series of SEM and ESEM pictures [67]. Recent investigations (not yet published) using micro-computer tomography (m-CT, independently performed at Vienna University of Technology and University of Applied Science Upper Austria) and Magnetic Resonance Imaging (MRI, performed at CERMANU, Naples, Italy) confirmed these results even though the low density of BNC aerogels considerably impeded the measurements. However, careful examination of the MRI results (prevalently spin-density or spin-echo or self-diffusion or Fisp pulse sequences) permitted to identify the spin echo MSME (MultiSliceMultiEcho) technique as the most suitable pulse sequence to analyze the porosity of native bacterial cellulose aquogels and alcogels. Based on the evaluation of 20 representative pores with the MRI software (Bruker© Topspin Paravision v.2.1), two macrogroups of average size were identified in both materials: a smaller-sized group (20–70 nm) and a larger-sized group (100–160 nm). The respective average sizes for the BNC aquogel and BNC alcogel samples were 46 and 38.33 nm for the smaller-size group, and 105 and 138 nm for the larger-sized group. Direct investigation of macroporosimetry

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by mercury intrusion is not feasible due to extensive pore collapsing which is in agreement with Sescousse et al. [86], similar to cellulose acetate aerogels [87]. However, indirect reconstruction of the macropore size distribution based on the buckling theory as proposed by Pirard and Pirard [88] is an approach which can also contribute to a better picture of the true pore size distribution in ultralightweight cellulose aerogels. In brief, this approach determines the largest pores remaining after compaction caused by application of the mercury intrusion technique (which leads to collapsing of most of the pores) by nitrogen sorption experiments at 77 K using an experimentally determined buckling strength constant at defined pressure [89].

Compression tests along the three orthogonal spatial directions revealed that batch-wise static cultivation of BNC on HS growth medium, subsequent replacement of water by ethanol, and scCO2 drying (40°C, 10 MPa, 2 h) affords transversely isotropic aerogels (Fig. 5.5, right). The latter are characterized by two directions of similar, higher stiffness and strength, and a third direction of lower values. The weaker network direction was found to correspond to the growth direction of bacterial cellulose which is perpendicular to the interface between culture medium and air. While a Young’s modulus of E = 0.057 ± 0.007 MPa and yield strength of RP,0.2 = 4.65 ± 0.48 kPa were observed along the growth direction, the respective values for the other two spatial directions were significantly higher (A: E = 0.149 ± 0.023 MPa, RP,0.2 = 7.05 ± 0.55 kPa; B: E = 0.140 ± 0.036 MPa, RP,0.2 = 7.84 ± 1.06 kPa). The overall smoothness of the response curve toward compressive stress and the absence of a peak after the linear elastic region indicate that the material deforms in a ductile way, in contrast to brittle foams and silica aerogels.

Figure 5.5: Pore size distribution of BNC aerogels as determined by thermoporosimetry.The respective materials were obtained from BNC pellicles formed by a variety of pure or mixed

G. xylinus strains (left). Anisotropic response of BNC aerogels toward compressive forces (direction 3: direction of growth; lines are mean values of stress, gray background represents standard deviation,

sample size n = 5; right). Reprinted from (F. Liebner, N. Aigner, C. Schimper, A. Potthast, T. Rosenau, in: F. Liebner, T. Rosenau (Eds.), Functional Materials from Renewable Sources, ACS Symposium Series 1107,

American Chemical Society, 2012, pp. 57–74) with permission of the American Chemical Society.


Interestingly, no sample buckling was observed during compression. The Poisson ratio, which describes the change of the cross-section area upon application of mechanical stress, being in the range of 0.1–0.3 for silica aerogels, was approximately zero for BNC aerogels independent of the loading direction. This is in good agreement with Sescousse et al. [86] who also reported a zero Poisson ratio for cellulosic aerogels from [EMIM][OAc] solution, similar to cork.

Reinforcement of Bacterial NanoCellulose Aerogels

Cellulosic aerogels and in particular BNC aerogels are comparatively prone to mechanical stress which is a drawback with regard to many potential applications. Depending on the envisaged type of aerogel utilization and the required mechanical properties, different reinforcing approaches are generally applicable, such as insertion of an interpenetrating network consisting of a second polymer, cross-linking, preparation of all-cellulose composite materials, or the incorporation of network stiffening inorganic or organic particles (MWCNT, e.g., discussed earlier). However, all of the aforementioned techniques are still in their infancies regarding their adaptation to cellulose aerogels, as controlling the mechanical properties under preservation of the interconnected, high porosity—certainly the most valuable feature of aerogels—is a challenging task.

Hydrophobization and Oleophilization

Hydrophobization of cellulose aerogels is considered an effective means to increase their resistance toward microbial degradation and to prevent them from extensive shrinkage in humid environment. Surface trimethylsilylation is a common technique in this respect and has been tested for hydrophobization of BNC aerogels as well [90]. Immersion of BNC cryogels in a solution of trimethylchlorosilane and a catalytic amount of triethylamine in boiling dichloromethane, subsequent washing with ethanol, solvent exchange to aqueous tert-butanol, and freeze-drying afforded aerogels whose morphology largely resembled that of the original BNC aerogels in terms of high surface area (≥69.1 m2 g−1), high porosity (≈99.6%), and low density (≤6.77 mg cm−3). The greatly reduced surface energy imparts strong hydrophobicity and oleophilicity as evident from both the high water contact angle of 146.5 degree and high oil sorption capacities of up to 185 g g−1. Materials of similarly high water contact angles of 145 degree were obtained for BNC-silica composite aerogels which were prepared by acid-catalyzed polycondensation of sodium silicate after soaking the BNC in a solution containing the silica precursor compound. Hydrophobization of these BNC-silica composite aerogels was accomplished by immersing the BNC-silica aerogel in a methanolic silica sol prepared from methyltrimethoxysilane, a catalytic amount of oxalic acid and ammonia, using, followed by aging of the formed gel at room temperature for 24 h [91]. Freeze drying of BNC aquogels in the presence of methyltrimethoxysilane (MTMS) as recently proposed for

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aqueous suspensions of nanofibrillated cellulose [92] and chemical vapor deposition (CVD), such as octyltrichlorosilane [93] or palmitoyl chloride [94] are further alternatives that could be adapted for hydrophobization of BNC aerogels. However, inhomogeneous grafting of aerogels is an issue with CVD despite strict control of reaction conditions [90]. Chemical surface modification with the paper sizing agent alkyl keten dimer (AKD; [95]) or surface impregnation with cellulose acetate (CA; [67]) using scCO2 as both solvent (AKD) and antisolvent (AKD, CA) are further approaches to strongly hydrophobic BNC aerogels.

Interpenetrating Networks

Immersion of BNC aquogels in silica sols containing different amounts of silica nanoparticles has been shown to be a suitable approach to increase the strength of BNC aerogels [96]. However, it turned out to be difficult to load the BNC with more than 10 wt.% of silica in this way. Loadings of up to 50% silica were achieved when the cellulose-producing bacteria strain (Ga. xylinus) was incubated in a medium that contained the respective silica sol Snowtex 0 (ST 0, pH 2–4) or Snowtex 20 (ST 20, pH 9.5–10.0). Enhanced elastic moduli were observed for silica contents below 4% (ST 20) and 8.7% (ST 0), respectively, whereas higher silica contents led to reduced strength and modulus of the aerogels. Interpenetrating networks have been also obtained with (derivatized) natural and synthetic polymers. Cationic starch, such as 2-hydroxy-3-trimethyl-ammoniumpropyl starch chloride (TMAP starch) added to the growth medium of Ga. xylinus forms stabilized double-network composites and is incorporated into the wide-mashed BNC prepolymer already during the first 2 days of incubation [48]. Similarly, the addition of 0.5, 1.0, and 2.0% (m/v) CMC or methyl cellulose (MC) to the culture medium has been reported to increase the yield, the fraction of amorphous domains, water retention ability, and ion absorption capacity [97].

Reinforcement with biocompatible polymers, such as polylactic acid (PLA), polycaprolactone (PCL), cellulose acetate (CA), and poly(methyl methacrylate) (PMMA) using scCO2 antisolvent precipitation as a core technique for forming an interpenetrating secondary polymer network has been recently demonstrated to be a suitable approach to improve the mechanical properties of BNC aerogels [67]. BNC/CA and BNC/PMMA composite aerogels featured the highest gain in specific modulus (density-normalized modulus, Eρ) compared to pure BNC aerogels. For a BNC/polymer ratio of 1:8 the respective Eρ was found to be as high as 50 and 122 MPa cm−3 g−1, respectively. The specific modulus of cotton linters aerogels (obtained by coagulation of cellulose from a 1 wt.% solution in calcium thiocyanate) reinforced with cellulose acetate at the same BNC/polymer ratio exceeded that of the respective BNC/CA composite samples by a factor of 3 [67]. The formation of open-porous, interpenetrating networks of the second polymer was confirmed by treating selected BNC/PMMA hybrid aerogels with the cellulose solvent 1-ethyl-3-methyl-imidazolium acetate (EMIM acetate). Even at a high PMMA/BNC ratio of about 8, representing one of the least


favorable cases with regard to easiness of cellulose dissolution, cellulose was extracted by the ionic liquid at 50°C, leaving behind organogels which were largely transparent prior to drying and whose morphologies corresponded to those of the respective composites (Fig. 5.6). ATR-IR analysis of the extracted material confirmed the extraction of pure cellulose (more than 90% of the amount of cellulose originally present in the composite aerogel) during this process.

Sleeving of BNC fibers with acrylate polymers by in situ atom transfer radical polymerization of methyl methacrylate and n-butyl acrylate (BNC-g-PMMA, BNC-g-PBA, BNC-g-PMMA-co-PBA) [98] or UV-induced cross-linking radical polymerization of different methacrylate monomer mixtures swollen in BNC (glycerol monomethacrylate, 2-hydroxyethyl methacrylate, 2-ethoxyethyl methacrylate) are further examples for the creation of interpenetrating networks in BNC-based aerogels [99].

Cross-Linking

Cross-linking of never-dried, microfibrillated, TEMPO-oxidized bacterial cellulose (to-BNC) with chitosan (CTS) using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) as cross-linking mediator affords dimensionally stable, highly macroporous cellulosic scaffolds promising for tissue engineering applications [100]. Following cross-linking at room temperature in slightly acidic aqueous medium (pH 5.5–6)

Figure 5.6: BNC aerogel reinforced by interpenetrating networks of PMMA during extraction of BNC with the ionic liquid EMIM acetate.

Opaque regions represent residual amounts of BNC. SEM pictures: morphology of a BNC/PMMA composite aerogel (A) and of an aerogel as obtained from (A) after extraction of BNC by EMIM

acetate. Reproduced from [67] with permission of Elsevier.

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[100,101], subsequent dialysis (removal of excess of EDC and NHS) and concentrating the suspension by dialysis against aqueous polyethylene, the to-BNC/CTS slurry was degassed and cast into molds. The samples were deep frozen (e.g., −30°C) to solidify the solvent and to induce liquid–solid phase separation. to-BNC/CTS aerogels containing 60% of to-BNC, for example, had an average pore diameter of 284 ± 32 µm, which is more than three orders of magnitude larger than that of native bacterial nanocellulose and meets the requirements of cell scaffolding materials [47]. Microfibrillation of bacterial cellulose prior to TEMPO oxidation and subsequent cross-linking with chitosan using the previous EDC/NHS mediator system can be used to further tune the microstructure (porosity of 120–280 nm) and mechanical properties of respective scaffolds for tissue engineering [102,103].

Potential Applications of Bacterial NanoCellulose Aerogels

The huge application potential of aerogels in general and of silica aerogels in particular has been comprehensively described by several authors [10,104,105]. However, different from silica or metal oxide aerogels which are increasingly commercialized for many applications-headed by high-performance thermal insulation-cellulosic aerogels have hitherto not entered any technical application. This is not surprising considering the comparatively short time period that has elapsed only since the beginning of systematic research in this field.

With regard to future market perspectives the authors believe that aerogels from the most abundant biopolymer on earth will enter numerous applications within a short period of time. However, it is important to keep in mind that inorganic and organic aerogels do not necessarily share the same fields of applications. While applications, such as insulation toward heat or catalysis of high-temperature processes are usually limited to silica or metal oxide aerogels which can resist temperatures of more than 1000°C, organic aerogels from synthetic polymers or biopolymers are advantageously used for thermal insulation toward low temperature, in medical applications (e.g., tissue engineering, slow release of bioactive compounds) due to their biocompatibility and biodegradability, or in electrochemical applications employing carbonized aerogels (carbon aerogels) as electrode material, such as in electrical double-layer capacitors or proton exchange fuel cells.

However, the boundaries between inorganic and organic aerogels with regard to applications are blurred. Thus, biopolymer-based aerogels have been tailored and successfully tested in selected catalysis applications as well. Topological loading of monovalent copper ions onto the surfaces of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCN) in aqueous suspension and subsequent freeze-drying, for example, affords TOCN-Cu+ aerogels which were shown to exhibit excellent catalytic efficiency for azide-alkyne [3 + 2] cycloaddition reactions [106]. Nano-fibrillary chitin aerogels featuring internal surface areas of about 280 m2 g−1 on the other hand have been recently demonstrated to exhibit high catalytic performance via their surface amino functionalities in flow-mode


Knoevenagel condensation reactions between aldehydes or ketones and active methylene compounds [107].

Surface Loading of Inorganic or Organic Compounds for Catalysis, Bio-Sensing, Visualization, or Controlled Release Applications

Their particular morphology in terms of interconnected nano-porosity and high pore surface area render cellulosic aerogels excellent matrices for controlled deposition of finely spread nanoparticles of noble or ignoble metals or metal oxides aiming at their use in electronic, optical, sensor, medical, or catalysis applications.

The preparation of cellulose II aerogels furnished with silver, gold, and platinum nanoparticles shall exemplarily illustrate the general approach to this type of materials. The route proposed by Cai et al. [108] comprises (1) dissolution of cellulose 4 wt.%, filter paper pulp in precooled (−10°C) aqueous solutions of LiOH (4.6 wt.%) and urea (15 wt.%), (2) casting, (3) coagulation of cellulose using ethanol as an antisolvent, (4) hydrothermal reduction using either cellulose or NaBH4 as a reductant, and scCO2 drying. The obtained cellulose/metal hybrid aerogels were reported to feature high transmittance, porosity (95%), surface area (360–400 m2 g−1), moderate thermal stability, and good mechanical strength [108].

Following a similar approach, flexible magnetic BNC aerogels equipped with nonagglomerated ferromagnetic cobalt ferrite nanoparticles (diameter 40–120 nm) were prepared that could be used in micro-fluidics devices or electronic actuators. BNC aquogels obtained by cultivation of Ga. xylinus FF-88 were freeze-dried and subsequently immersed in aqueous solutions of varying FeSO4 and CoCl2 content. Heating to 90°C converted the initially formed water-soluble iron/cobalt hydroxides to nonmagnetic metal oxyhydroxide complexes that precipitated on the surface of the 20–70 nm BNC nanofibrils. The particular morphology of the latter was found to have a templating effect as they prevented the growing particles from agglomeration and afforded particles whose size did not much exceed the thickness of the BNC fibrils. Subsequent immersion in alkaline (NaOH) solutions of potassium nitrate and heating to 90°C converted the oxyhydroxides into ferrite crystal nanoparticles. The obtained highly flexible aerogels can sustain large deformation, can be actuated by a small magnet, adsorb water, and release it upon compression [109]. A comprehensive review on magnetic responsive cellulose nanocomposites and their applications can be found elsewhere [110].

Besides their use as carrier matrix for catalytically active compounds or magnetic nanoparticles, aerogels literally invite to modify their large internal surface with quantum dots (QDs). The latter are semiconductor nanoparticles whose size is small enough to confine excitons (electron–hole pairs attracted to each other by electrostatic Coulomb forces) generated by incoming photons which, in turn, impart QDs quantum mechanical properties. Depending on the energy of the incoming photons, QDs can respond in various

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ways including photoluminescence, Foerster energy transfer, photosensitization, photoelectric ionization, or generation of electron–positron pairs. Photoluminescence is of particular interest if respective cellulose-quantum dot hybrid aerogels are considered for bio-sensing or true volumetric display applications.

The preparation of both BNC and cellulose II aerogels carrying covalently immobilized (ZnS)x(CuInS2)1−x/ZnS (core/shell) quantum dots whose photoluminescence properties can be tuned within a wide range of the visible light has been described by Wang et al. [111,112].

Grafting of the alloy-based core/shell QDs onto cellulose has been accomplished via a-mercapto-g-(trialkoxylsilyl)propyl ligands added to the QDs ZnS shell by partial replacement of a-mercaptododecyl ligands introduced for surface deactivation during QD synthesis. While cellulose II /QD hybrid aerogels were prepared by (1) dissolution of a commercial kraft pulp in 1-hexyl-3-methyl-1H-imidazolium chloride, (2) addition of a homogenous dispersion of quantum dots in the same solvent, (3) molding, (4) coagulation of cellulose using ethanol as a cellulose antisolvent, and (5) scCO2 drying of the resulting composite alcogels (Fig. 5.7; [111]), BNC alcogels have been used to demonstrate that largely homogeneous loading and subsequent grafting can also be achieved by simply immersing the BNC alcogels in a suspension of the QDs in iso-propanol followed by heating to 45°C [112]. Conversion of the hybrid alcogels into strongly photoluminiscent aerogels has been accomplished by scCO2 drying at 40°C and 10 MPa without loss of any QDs (Fig. 5.7).

Thermal insulation

High-performance thermal insulation is the target application number one of all manufacturers of silica aerogels. However, the high price of current products and the rather nongreen and costly chemistry and technological steps behind the manufacture of alkoxysilanes are serious

Figure 5.7: Schematic presentation of the process steps to photoluminiscent cellulosic aerogels containing covalently grafted (ZnS)x(CuInS2)1−x/ZnS (core/shell) quantum dots.

Reproduced with slight modification from [111].


drawbacks of silica aerogels and motivation for the development of alternative products. Therefore, the potential of cellulosic aerogels, in particular of BNC aerogels for high-performance thermal insulation shall be briefly examined.

Cellulose is obviously one of the oldest insulation materials and is still used in various application forms such as loose-fill, wet-spray, stabilized cellulose, or low-dust cellulose. Even though loose-fill cellulose has a considerably low effective thermal conductivity of 35–50 mW m−1 K−1 [113–115], current research efforts aim to develop facile routes to monolithic cellulose aerogels for high-performance thermal insulation.

Thermal super-insulating aerogels featuring effective thermal conductivities lower than that of air (25 mW m−1 K−1) can be obtained by classic sol-gel techniques using appropriate low-molecular source compounds. Baekeland polymerization of resorcinol and formaldehyde or acid-catalyzed hydrolysis of alkoxysilanes and subsequent polycondensation are just examples for synthetic routes to two types of aerogels that can reach leff values as low as 14 mW m−1 K−1. Super-insulating monolithic aerogels can also be prepared from polymeric precursor compounds. Different from classic sol-gel processes which are typically based on nonlinear step-reaction polymerizations, the respective polymeric precursor compounds are molecular-dispersing dissolved in an appropriate solvent and forced to coagulate by physical or chemical cross-linking. Cellulose acetate (CA) aerogels, for example, reaching leff values of around 11 mW m−1 K−1, have been prepared by chemical crosslinking of CA with diisocyanates [87]. Physical cross-linking of low-esterified pectins rich in carboxylate groups using Ca2+ ions has been recently reported to afford aerogels of similarly low effective thermal conductivity (20 mW m−1 K−1) which is why pectin aerogels can be regarded to be the first thermally super-insulating aerogels derived from a nonderivatized biopolymer [89].

Cellulose aerogels have not yet succeeded to conquer the super-insulator boundary. This includes ultralightweight aerogels from bacterial cellulose without and with reinforcement by interpenetrating networks of cellulose acetate, similar as for aerogels obtained by coagulation of cotton linters from respective 0.5, 1.0 and 1.5 wt.% solutions in a melt of Ca(SCN)2·8H2O and reinforced with different amounts of cellulose acetate (unpublished results of the authors). While unmodified bacterial nanocellulose featured leff values of about 29 mW m−1 K−1 all other monolithic aerogels up to a bulk density of 100 mg cm−3 had thermal conductivities of 30–40 mW m−1 K−1 only. This is in good agreement with a recent study that investigated the interrelationship between thermal properties of cellulosic aerogels obtained by coagulation of cellulose from aqueous solutions in sodium hydroxide and their bulk densities/extent of cross-linking accomplished with epichlorohydrin. The result of this study confirmed that the dependency between bulk density and effective thermal conductivity runs through a minimum which was found to be at around ρB = 0.18 g cm−3 [116] similar as for silica aerogels [117], resorcinol-formaldehyde aerogels [118], or cellulose acetate aerogels [87,119].

However, the results of recent studies on pectin aerogels [89] and even more impressive on MOx/(MOx–SiO2)/SiO2 core–shell metal oxide aerogels [120] suggest that controlled

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nano-structuring can be a key toward super-insulating cellulosic aerogels, also at very low bulk densities. Cross-linking of pectin by calcium ions in aqueous solution that can be described by the egg box model [121] is one example in this respect and has been shown to afford super-insulating aerogels even at very low bulk densities of 50 mg cm−3, strongly deviating from the above leff/ρB dependency of many aerogels. Novel MOx/(MOx–SiO2)/SiO2 core–shell metal oxide aerogels obtained by alkoxide chemical liquid deposition techniques are another example demonstrating that nanostructuring of highly diluted sols can give access to aerogels of extremely low thermal conductivity. Core-shell nanostructured Al2O3 aerogel/mullite fiber/TiO2 composite aerogels, for example, feature ultralow thermal conductivity of down to 5.8 mW m−1 K−1, which are the lowest values for inorganic aerogels ever reported [120].

Due to the competition with much cheaper established products of similarly low thermal conductivity, such as loose-fill cellulose, commercialization of bacterial cellulose aerogels will probably only succeed, if their properties can be tuned in a way that renders them super-insulating materials. This can be possibly achieved by finding suitable cultivation conditions in terms of bacteria strain, composition of growth medium, type of additives, and cultivation technique capable of affording largely isotropic cellulose networks hosting nanopores of narrow size distribution with maximum pore diameters not exceeding 70 nm. The latter limit results from the considerations that (1) thermal conductivity in cellulose aerogels is mainly driven by solid and gas conduction (ca. 70%) and (2) that gas conduction decreases significantly when the average pore size is significantly smaller than the mean free path of the gas molecules (Knudsen effect; [116]). Low bulk density is beneficial in this respect as high porosity and small beam diameters restrict the propagation of phonons in the fragile scaffold and hence the contribution of solid phase radiation.

Biomedical Applications

Even though most commercial BNC products for medical and cosmetic applications are still shipped and used in hydrated form, dry materials, that is, aerogels or dry microspheres instead of hydrogels would be superior in some respect, in particular with regard to long-term storage in sterile environment. This, however, would demand full preservation of all morphological and chemical features of native BNC throughout both drying and rehydration required prior most applications, such as drug delivery systems or tissue engineering. Combined use of BNC for wound healing (including skin substitute) and transdermal drug delivery is of particular interest as BNC does not only prevent moisture from evaporation, avoid external contamination and maintain intimate contact with the exposed, inflamed, or diseased area [122], but features large internal surfaces as well that could be used for reversible adsorption of drugs.

Monolithic aerogel sheets prepared from respective hydrogels are superior to BNC nanoparticles as drug release can be controlled in a more predictable way due to their nonexisting tendency to aggregation [123].


The preparation of monolithic BNC aerogel sheets via freeze-drying has been shown to preserve more than 90% of the accessible pore volume. However, replacement of water by ethanol and subsequent scCO2 drying of respective BNC alcogels is superior to freeze-drying as virtually the entire pore volume can be retained [84]. Simultaneous deposition of bioactive compounds onto the large internal surface of BNC is a particular advantage of supercritical fluid drying technologies. It is based on the phenomenon that the solvent power of a binary mixture, such as of ethanol/liquid CO2 sharply decreases close to the critical pressure of the binary mixture at a given temperature and turns CO2 to act as an antisolvent causing the solute to precipitate [124]. For mixtures of CO2 and ethanol, the critical pressure is reached at 8 MPa for 40°C [125]. As the interfacial tension between liquid and supercritical phase—a prerequisite for supercritical drying [126]—approximates zero already somewhat above the pressure range of precipitation [69], supercritical antisolvent precipitation and supercritical drying can be performed in one run. Following this approach, loading and release of d-panthenol and l-ascorbic acid onto/from respective BNC alcogels and rehydrated aerogels was studied [84]. Loading isotherms evidenced that deposition of d-panthenol and l-ascorbic acid, respectively, is mainly governed by diffusion and not by specific interactions between solid matrix and solute. Similarly, the exponential shape of the release curves was interpreted as purely diffusion-driven release kinetics which are independent of the amount of loaded substance and can be thus controlled by the thickness of the gel layer. This has been confirmed for both of the studied drugs and BNC gels of different thicknesses using experimentally determined effective diffusion coefficients (D-panthenol: Deff = 6.9×10−4 cm2 min−1; l-ascorbic acid: Deff = 5.94×10−4 cm2 min−1) and the Korsmeyer model which considers both diffusion of water into a porous matrix and the reverse way that is unloading of a drug by diffusion (Fig. 5.8).

Figure 5.8: Release of d-panthenol (left) and l-ascorbic acid (right) from bacterial nanocellulose aerogel sheets of different thicknesses.

Experimental values versus release predicted using the Korsmeyer model. Reprinted from [84] with permission of Wiley-VCH.

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Besides drug delivery, cellulosic aerogels have been also studied as cell scaffolding materials in regenerative medicine, such as for in vitro generation of skin, cartilage, or bone tissue. Regenerative medicine is considered superior to artificial replacement materials and autogenous or allogeneic transplantation as it may enable the engineering of replacement tissues that restores the normal anatomy and physiology of damaged tissue.

Open-porous cell scaffolding materials providing structural support for attachment, spreading, migration, proliferation, and differentiation of pluripotent stem cells play a key role in regenerative medicine. However, they have to meet multiple requirements with regard to biocompatibility, mechanical properties, pore size and geometry, surface chemistry, and micro-topology. Diffusion of physiological nutrients and gases to cells, removal of metabolic by-products from cells, in vitro cell adhesion, cell ingrowth, and in vivo neovascularization, for example, require interconnected, spread porosity and microstructured surfaces [47]. Beyond that, cell survival, signaling, growth, propagation, reorganization, shaping, and gene expression are greatly governed by the average pore size, pore size distribution, and shape of pores.

Due to the brittleness and comparatively low mechanical stability of inorganic cell scaffolding materials, such as of bioglasses and -ceramics like CaO-P2O5-SiO2 [127], Bioglass 45S5 [128], or SiO2-CaO-P2O5-MgO [129], ductile organic materials of higher strength have been recently moved into the limelight of material research. Besides synthetic polymers, biopolymers, such as collagen, fibrinogen, starch, chitosan, or cellulose are of particular interest due to their natural abundance and low immunogenic potential [130]. Amongst them, cellulose is probably the most interesting candidate last but not least due to its macromolecular homogeneity.

As the average pore size of common bacterial cellulose is too low for most applications in tissue engineering, several approaches have been studied aiming at the preparation of materials that feature hierarchical interconnected porosity and consist of a large portion of macropores, preferably in the range of 50–400 mm. This includes emulsion freeze-drying starting from aqueous BNC suspensions [131], unidirectional or 3D laser perforation and cutting of BNC hydrogels using a pulsed CO2 laser capable of creating channels of about 220 mm in diameter [58], or tuning of pore size and pore interconnectivity during biosynthesis of BNC [132]. The utilization of porous wax spheres as easily removable porogens added to the growth medium is another promising approach in this respect, as the obtained dual-porous materials combining nano- and micro-scale porosity were demonstrated to support migration, proliferation, and differentiation of human smooth muscle cells [133].

The capability of cellulose to facilitate the formation of calcium-deficient hydroxyapatite (cd-HAp) under physiological conditions as demonstrated for BNC [134,135] is highly


desired in bone tissue engineering as cd-HAp is osteoconductive, biocompatible, bioactive and increases the expression of mRNA which encodes the bone matrix proteins osteocalcin, osteopontin, and bone sialoprotein [136,137]. Deposition of cd-HAp increases also the mechanical strength of the otherwise fragile cell scaffolding materials which improves their handling throughout the whole process chain from sterilization, cell seeding, immersion in different aqueous growth, and differentiation media up to the final implantation. Furthermore, moderate rigidity at sufficient remaining elasticity is beneficial for osteogenic differentiation of stem cells [133].

The formation of cd-HAp can be promoted by furnishing the surface of cellulosic scaffolds with negative charges which can act as nucleation sites for calcium containing minerals [103,133] and simultaneously increase both biodegradation and molecular recognition [130,138]. Introduction of surface charge can be accomplished by adsorption of respective polymers, such as carboxymethyl cellulose to the large internal surface of cellulosic aerogels [133] or by chemical derivatization affording carboxylated [139,140], sulfated [141] or phosphorylated [142–144] cellulosic matrices. Limited periodate oxidation of BNC pellicles permits the formation of cd-HAp nano-crystallites under physiological conditions as well, but renders both the 2,3-dialdehyde cellulose (DAC) scaffold and its mineralized counterpart better biodegradable and thus more suitable for bone regeneration [134]. In how far cellulose lyogels or aerogels can be additionally equipped with homogeneously distributed cd-HAp particles following the approach of Ma et al. [145] who demonstrated that cellulose-hydroxyapatite nanocomposites can be obtained by microwave-assisted thermal treatment (150°C) of a solution of microcrystalline cellulose, CaCl2 and NaH2PO4 in N,N-dimethylacetamide (DMAc), has not been approved yet.

However, despite recent advances in engineering of cellulose-based cell scaffolding materials featuring improved cell attachment, growth, and osteogenic differentiation, the major problem with cellulose aerogels for bone tissue engineering is the insufficient binding and crystallization tendency of cd-HAp on the cellulose matrix [146,147]. Even if cd-HAp nanoparticles are homogeneously formed and deposited within the porous matrix, the missing chemical linkage between cellulose as the “organic” and hydroxyapatite as the “inorganic” part would impede full biomineralization and osseointegration [148].

Grafting of negatively charged phosphorous-containing groups onto cellulose improves the mineralization of respective matrices. This has been demonstrated by Wan et al. who activated BNC pellicles in a solution of urea in DMF at 110°C and phosphorylated them subsequently using a solution of 98 wt.% H3PO4 in DMF (136°C, 1 h; [147]). Phosphorylation was furthermore shown to induce the formation of cd-HAp in simulated body fluid for cotton linters [149,150], Avicell PH-101 [148], and bacterial cellulose

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[146,147] (Fig. 5.9), similar as with phosphorylated synthetic polymers, such as poly(ethylene terephthalate) [151] or biopolymers, such as chitin [152]. Phosphorylated Avicell PH-101 was furthermore confirmed to be nontoxic in cultured human osteoblasts and fibroblasts [142]. Similar results have been reported by [142,148] who studied the formation of cd-HAp on the surface of regenerated cellulose discs that had been phosphorylated using a mixture of H3PO4, P4O10, and triethyl phosphate in hexanol [153]. Interestingly, cd-HAp formation was found to be suppressed for water-soluble cellulose phosphates of high DS or for phosphorylated cellulose that had not been pretreated with CaCl2 and is highest at moderate degrees of surface phosphorylation [148]. Similarly, strongly charged, hydrophilic cellulosic matrices featuring a high degree of surface phosphorylation are inferior to samples of lower DS with regard to cell attachment and proliferation as demonstrated for cultured human bone marrow stromal cells (HBMSC; [143]).

In a recent study the authors confirmed that spin-coated layers of phosphorylated cotton linters and hardwood prehydrolysis kraft pulp (DS 0.2–0.4) support a robust growth and osteogenic differentiation of human bone-marrow derived mesenchymal stem cells (MSC) similar as with clinically used tissue culture polystyrene. Respective cellulose phosphate aerogels (CPA) showed a good hemocompatibility (human whole blood) in terms of hemostasis and inflammatory response. Surprisingly, the low degree of phosphorylation was sufficient to suppress any significant inflammatory response via the alternative pathway for the CP aerogels which is typically an issue with comparable products of nonderivatized cellulose [144].

Carbon Aerogels

Carbon particles and monoliths of interconnected porosity and controlled morphology are of increasing interest for many applications. This includes gas separation and adsorption

Figure 5.9: SEM pictures of phosphorylated BNC showing the growth of cd-HAp after soaking in simulated body fluid (left). TEM image of a single BNC microfibril embedded in hydroxyapatite

particles (right). Reprinted from [147] with permission of Elsevier.


[154,155], catalysis [155–158], hydrogen storage [159–161], batteries [162], proton exchange membrane fuel cells (PEMFCs; [163–165] or electrical double-layer capacitors (EDLCs; [162,166–168]). Their use in PEMFCs and EDLCs is a particularly hot topic of current material research due to the globally increasing energy consumption and new energy generation technologies.

Proton exchange membrane fuel cells (PEMFCs) utilizing hydrogen as a fuel are being developed to replace batteries in portable electronic devices and internal combustion engines in automobiles on account of their high energy efficiency, low pollutant emission, and low working temperature. PEMFC electrodes which catalyze both of the half-cell reactions, that is, hydrogen oxidation and oxygen reduction are typically porous materials covered by a thin film of platinum. For commercial and electrochemical reasons this platinum film should be as thin as possible. The latter is due to the fact that the catalyst must have simultaneous access to hydrogen and both of the conducting media (H+, e−). If the platinum film would be not thin enough, the rate of proton diffusion within the catalyst layer, the mass transfer rates of the chemical reactants and products to and from the active sites would result in a loss of energy. This, in turn, can contribute to a significant over-potential or -polarization of the electrodes, which can limit the cell performance, particularly at high current densities [169].

Double-layer capacitors are also referred to as “super-capacitors” that store energy via separation of charges across a polarized electrode/electrolyte interface and bridge the gap between batteries (accumulators) and conventional capacitors [170]. They are able to store more energy than conventional capacitors, release a higher voltage than batteries, store electrical energy almost lossless for a long period of time, and can be (dis)charged very quickly. Potential applications of super-capacitors are uninterruptible power supplies for bridging electrical power outage, short-term supply of high electrical power, such as for starting up industrial machinery and storage of relatively short energy impulses. In addition to voltage, the surface of the interface between electro-conductive solid and surrounding electrolyte is the main criterion determining charge storage.

However, despite the huge variety of porous carbon-based electrode materials (carbon nanotubes, CNT; activated carbon powders, ACP; activated carbon fabrics, ACF) that have been developed to date, none of them are ideal candidates for PEMFC or EDLC applications [171]. This is mainly due to morphological deficiencies which arise from the poor control of the network and pore’s characteristics during the respective preparation processes. Both EDCL and PEMFC applications require high mesoporosity as a compromise of the required high surface area and wettability of pores which is particularly difficult for polymeric electrolytes or proton conductors, such as the fluoropolymer–copolymer Nafion® used in PEMFCs. While the specific capacitance of purified CNT powders is not impressive (20–80 F g−1) [172], activated carbon fabrics (ACF) are more promising in this respect. However, the high production costs for rayon or PAN-based ACFs restrict their use in EDLCs to very specific applications.

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Carbon aerogels derived from highly porous organic precursor materials are of increasing interest as the formation of ordered networks featuring interconnected porosity can be better controlled. This improves ionic motion [172] and is the reason for high power capabilities of carbon aerogel-based electrodes [171].

Carbon aerogels are commonly prepared by pyrolysis of appropriate organic aerogels at 1000–2000°C depending on the desired degree of graphitization. Suitable aerogel forming precursor compounds are polymers that consist of a high percentage of aromatic or heteroaromatic moieties, functionalities that contribute to radical formation and subsequent cross-linking upon pyrolysis, and a molecular design that prevent the polymer from excessive thermal decomposition. Surface doping with hetero-elements like nitrogen which can be accomplished by treatment of the carbon aerogel with 4N HNO3 and ammonia at 400°C (3 h) have been demonstrated to increase both internal surface and microporosity [173].

Mesoporous cellulose-based carbon aerogels of moderate internal surface area (117–165 m2 g−1), suitable for EDLC applications have been recently obtained by pyrolysis of cross-linked cellulose acetate (CA) under nitrogen atmosphere (4°C min−1, Tmax 1000°C) [173]. Cross-linking of CA was accomplished with polymeric diphenylmethane diisocyanate in dry acetone, catalyzed by 1.4-iazabicyclo[2.2.2]octane (DABCO).

Highly crystalline native cellulose including bacterial cellulose, algal cellulose, and ramie fibers have been also demonstrated to be suitable raw material for the preparation of carbon aerogels, as the ultrastructure of the parent materials is largely retained throughout the carbonization (500°C) and graphitization (2000°C) steps [174].

Bacterial cellulose carbon aerogels featuring porosities of up to 0.83 cm3 g−1, pore surface areas of up to 670 m2 g−1 and carbon fibril diameters of 20 nm have been successfully tested as anodes in lithium ion batteries where they deliver superior capacity retention (decline from 386 mA h g−1 to 359 mA h g−1 after 100 cycles) and rate performance (reversible capacities of 288, 228, 94, and 34 mA h g−1 at current densities of 0.375, 0.75, 1.875, and 3.75 A g−1) compared to other carbon-based materials. The network of carbonized cross-linked nanofibers obtained by pyrolysis of BNC cryogels under nitrogen atmosphere was shown to boost the transport of electrons and offer a short diffusion distance for lithium ions rendering high electrochemical performance and stability to the carbon aerogel [175].

Bacterial cellulose nanofibers carbonized at 800°C exhibit also superior desalination performance with electrosorption capacities of 12.81 mg g−1 in 1000 mg L−1 NaCl solution, much higher than those of carbon nanotubes (3.78 mg g−1) and electrospun carbon fibers (6.56 mg g−1) [176]. The excellent performance has been ascribed to the high specific surface area, low charge transfer resistance, and superior hydrophilicity of the investigated material.

Cellulose-based carbon aerogels doped by platinum nanoparticles have been furthermore reported to be promising substrates for clean-energy technologies based on oxygen reduction


reactions (ORR), such as fuel cells or metal-air batteries as their properties can compete with state-of-the-art Pt-doped carbon black materials [163,164,177]. They are typically obtained by pyrolytic conversion (830–1050°C) of respective cellulose II aerogels (e.g., Avicell Ph-101) in nitrogen atmosphere [177]. Doping with platinum particles is accomplished for example by consecutive thermal activation in CO2 atmosphere, impregnation with H2PtCl6, and reduction of Pt4+ using either hydrogen (300–400°C) [178] or NaBH4 [163,177].

Nitrogen-doped carbon aerogels can greatly promote the commercialization of ORR technologies as they overcome serious obstacles of platinum-based electro-catalysts like prohibitive cost and scarcity. It has been shown that direct pyrolysis of bacterial cellulose as a cheap, green, and mass-producible biomass followed by NH3 activation of the carbon aerogel largely preserves the three-dimensional nanofibrous network of bacterial cellulose, affords high BET surface areas of up to 916 m2 g−1 and imparts the material a high density of N-containing active sites (5.8 at.%). The obtained carbon aerogels have high ORR activity (half-wave potential of 0.80 V versus reversible hydrogen electrode), selectivity (electron-transfer number of 3.97 at 0.8 V), and excellent electro-chemical stability (only 20 mV negative shift of half-wave potential after 10,000 potential cycles) in alkaline media. Furthermore, the ORR activity of NH3 activated BNC carbon aerogels is three times higher compared to NH3-treated carbon blacks, carbon nanotubes, and reduced graphene oxide aerogels [178].

Outlook

Even though research and development in the field of cellulosic aerogels is still in its infancy and respective products are not yet commercialized, it can be assumed from the multitude of promising studies that this particular type of porous solids made from the most abundant biopolymer on earth will soon enter numerous technical and biomedical applications. This includes bacterial cellulose as a unique natural resource which can be easily produced in various shapes and morphologies. Due to its intriguing properties, BNC aerogels are expected to find use in high-performance thermal insulation, as matrix material for gas separation, carrier for magnetic particles (electro actuators), catalysts, quantum dots (bio-sensing, volumetric displays) or bioactive compounds (controlled drug release). BNC aerogels are furthermore promising cell scaffolds (tissue engineering) and precursor materials for the manufacture of carbon aerogels (electrochemical applications).

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