Materials Science and Engineering C 33 (2013) 1842–1852

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Materials Science and Engineering C

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Review

High melting lipid based approach for drug delivery: Solid lipid nanoparticles

Sacheen Kumar, Jaspreet Kaur Randhawa ⁎Centre for Material Science and Engineering, National Institute of Technology, Hamirpur 177005 (H.P), India

⁎ Corresponding author. Tel.: +91 9418085224; fax:E-mail address: jaspkaur2001@yahoo.com (J.K. Rand

0928-4931/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.msec.2013.01.037

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 June 2012Received in revised form 11 January 2013Accepted 16 January 2013Available online 24 January 2013

Keywords:Solid lipid nanoparticlesDrug delivery systemSurfactants

Poor solubility of newly developed drug molecules is the main problem in recent drug discovery research, sonovel drug delivery approaches are being used to deliver these molecular entities for pharmacological action.Colloidal carriers (emulsion, suspensions, liposomes, polymer nanoparticles and solid lipid nanoparticles)have been used to administer poorly soluble drugs, but solid lipid nanoparticles are found to be the most re-liable carriers for this type of drugs due to its advantages over other carriers. Solid lipid nanoparticles havethe potential to solve the drug delivery problems with safe excipients used in its formulation. In this reviewall the aspects of solid lipid nanoparticles production, stability, characterization, differentiation based onroute, preservation and storage have been discussed.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18432. Types of SLNs based on route of administration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1844

2.1. Parenterally administered SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18442.2. Orally administered SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18442.3. Ocular SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18452.4. Topically applied SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18452.5. SLNs in molecular medicine and gene therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1846

3. Principle of solid lipid nanoparticles formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18464. Processing and Analysis of solid lipid nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1846

4.1. Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18464.1.1. High pressure homogenization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18464.1.2. O/W micro-emulsion breaking method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18464.1.3. Solvent emulsification–diffusion technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18474.1.4. Solvent injection method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18474.1.5. W/O/W double emulsion method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18474.1.6. Ultrasonication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18474.1.7. Super critical fluid technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18474.1.8. Membrane contactor method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18474.1.9. Electro-spray technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1847

4.2. Formulation ingredients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18474.3. Entrapment of water soluble drugs in SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18474.4. Characterization of SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1847

4.4.1. Photo correlation spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18474.4.2. Differential scanning calorimetric analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18484.4.3. Microscopy analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18484.4.4. X-ray photoelectron spectroscopy (XPS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18494.4.5. BET surface area analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1849

4.5. Stability of SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18494.5.1. Polymorphic modification of lipids during storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1849

4.6. Drug release from SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1849

+91 1972 223834.hawa).

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4.7. Preservation and storage of drug loaded SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18494.7.1. Sterilization of SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18494.7.2. Freeze drying of SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18504.7.3. Spray drying of SLNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1850

5. Conclusion and future prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1850List of abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1850Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1850References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1850

1. Introduction

Nanotechnology offers another challenge to achieve a certain goal andto deliver a drug at the right place at the right time. Nanoparticles are de-fined as a particle of 1 nm to 1000 nm and can bemade of biodegradablematerials like natural or synthetic polymers, lipids or phospholipids evenmetals. The drug can either be integrated in thematrix or attached to thesurface of the submicron particle which possesses very high surface tovolume ratios. As a consequence the dissolution rate is increasedaccording to Noyes Whitney and Kelvin equation. For example, a poorlysoluble compound like paclitaxel, cyclosporine or amphotericin B showsan increase in dissolution rate and absorption in the gastrointestinaltract when formulated as a nano-suspension. Depending on the particlecharge, surface properties, and relative hydrophobicity, nanoparticlescan be designed to adsorb preferentially on organs or tissues. The maindisadvantages of nano-scaled particle are difficult production, storageand administration because of physical instability phenomena such as ag-gregation. On the other hand, the main advantage is their ability to crossmembrane barriers, particularly in central nervous systemand the gastro-intestinal tract. Overcoming these barriers gives a deeper understandingof the normal and physiological application such as enzyme immobiliza-tion and DNA transformation. In the age of genetic manipulation and so-matic gene therapy, transfection systems using nano-scale particles arecustoms tailored by the use of designed polymers for specific applications.

Nano-carrier based drug delivery system is categorized as non-biodegradable nanocarrier based drug delivery system and biode-gradable nanocarrier based drug delivery system (Fig. 1).

Carbon nanotubes, silica nanoparticles, and magnetic nanoparticlesare non-biodegradable nanocarrier based drug delivery systems and lipo-somes, biodegradable polymers nanoparticles, solid lipid nanoparticles(SLNs), nanoemulsion and nanocrystals are biodegradable nanocarrierbased drug delivery systems. An ideal drug delivery system must have

Fig. 1. Biodegradable and non-biodegradable nanoparticulate drug delivery system.

the properties of sufficient drug loading, stable in environmental condi-tions, selective for the site, biodegradable, non-toxic, non-immunogenic,controlled and targeted release, easy and inexpensive scale up procedure[1]. Biodegradable polymer, liposomes, nanoemulsion, nanocrystals andsolid lipid nanoparticles are nanocarrier based drug delivery systemswhichhave tendency tomeet the requirements of ideal drug delivery sys-tem. Drug molecules loaded in different nanoparticles which act as vehi-cles for drug delivery are shown in Table 1.

Solid lipid nanoparticles have an edge above the others, as comparedto polymer toxicity which is low, inexpensive excipients and organicsolvents are not used. SLNs give prolonged drug release andmore stableformulations as compared to liposomes with no sterilization problems.Nanoemulsion has erratic drug release and nanocrystals were not ableto solve the issue of solubility of drug in biological fluids. So that SLNscan provide better profile as a drug delivery system over the othernanocarriers based drug delivery system. SLNs are the colloidal systemof nanoparticles (diameter 50–1000 nm) made up of solid lipid as ma-trix medium which is stabilized in aqueous media by surfactants. Thesurfactant system got immobilized on SLNs surface during the solidifi-cation of lipid matrix. SLNs have the advantage of controlled release ki-netics, target specific delivery, stability, and tolerability, protection oflight and acid sensitive drugs. In SLNs solid lipids used are complex glyc-eride mixtures, highly purified triglycerides, or waxes which do notmelt at body temperature. SLNs are prepared by GRAS graded solidlipids and surfactant. The drug is embedded in voids of solid lipidmatrixcrystals. Drug loading in SLNs depends upon type of solid lipids, solubil-ity of drug in lipid, method of processing, and polymorphic change inlipid crystals.

Some of the disadvantages of conventional SLNs are low drug pay-load, drug leaking on cooling or crystallization and polymorphicchange of the lipid matrix leads to changes in drug release kineticsfrom lipid matrix. So nowadays SLN based on liquid lipid–solid lipidmixture, p-acyl calixarenes, high content of lecithin and cyclodextrinsare prepared to solve the limitations of conventional SLN. Nanostruc-tured lipid carriers (NLCs) were invented to overcome the disadvan-tages of SLNs. SLN differs from nanostructured lipid carriers (NLCs)by the physical state of core lipids. NLC contains the complex lipidswhich have high drug loading capacity as compared to SLNs. Basedon structure NLCs is three types imperfect type NLC, amorphoustype NLC and multiple type NLC. In SLN lipophilic and highly hydro-philic potent drugs can be incorporated.

SLN are most commonly used to incorporate lipophilic drugswhich are not easy to administer by commonly used dosage forms.SLNs has used in the delivery of peptides like calcitonin, cyclosporineA [20], insulin [74], luteinizing hormone-releasing hormone [75], so-matostatin [76] and proteins like BSA, lysozyme [77]. SLNs are alsoused as vector for transfection, cationically modified SLN producedby hot homogenization using Compritol ATO888 or paraffin as lipidmatrix, tween 80 and span 85 as tenside and cetylpyridinium ascharged carrier [78]. The cationic lipids used in liposomal transfectionagents were also used to formulate cationic solid lipid nanoparticles(SLN) for gene transfer. It's better to use cationic SLN as transfectingagent as compared to liposomal counterpart [79]. Cationic SLN bindelectrostatically to DNA, sreptavidin and biotinylated ligands whichcan interact with the surface receptors. So SLN acts as targeted

Table 1Nanoparticulate drug delivery vehicle for delivery of some of the drug molecules.

Vehicle Degradable Drug References

1. Liposomes Biodegradable Doxorubicin, Daunorubicin, Amphotericin B, Econazole, Progesterone, Betamethasone dipropionate, Dexametha-sone, 5-fluorocytosine, Tetracaine, Triancinolone acetonide, Cyclosporin-A

[2–12]

2. Solid lipidnanoparticles

Biodegradable Azidothymidine palmitate, Camptothecin, Acyclovir, Piribedil, Vitamin A, Prednisolone, Tetracaine, Etomidate,Cyclosporine, Diazepam, Doxorubicin, Paclitaxel, Vitamin E, Resveratrol, Idebenone, Ergocalciferol, Ketoconazole,Ubiquinone

[13–29]

3. Polymericnanoparticles

Biodegradable ornon-biodegradable

Doxorubicin, Risperidone, Paclitaxel, Olanzapine, Estradiol, Clarithromycin, Insulin, streptomycin, Clofazimine,Celecoxib, Docetaxel, Budesonide, Tamoxifen, Ketoprofen,

[30–41]

4. Carbonnanotubes

Non-biodegradable Paclitaxel, Doxorubicin, Cisplatin, Methotrexate, Dexamethasone, Dopsone, Theophylline, Amphotericin B,Vitamin E, 10-hydroxycamptothecin (HCPT), Hexamethylmelamine, β carotene,

[42–52]

5. Polymerosome Biodegradable ornon-biodegradable

Pravastatin, Calcein, Bleomycin, Paclitaxel, Doxorubicin [53–57],

6. Niosomes Biodegradable Insulin, Paclitaxel, Caffeine, Acetazolamide, Nystatin, Ganciclovir, Capsaicin, Minoxidil, Hydroxycamptothecin,Naltrexone, Flurbiprofen, Tenofovir, Tretinoin, Diclofenac

[58–70]

7. Silicananoparticles

Non-biodegradable Insulin, Camptothecin, Paclitaxel, [71–73]

1844 S. Kumar, J.K. Randhawa / Materials Science and Engineering C 33 (2013) 1842–1852

non-viral gene delivery vector [80]. Protein-free low-density lipopro-tein based cationic solid lipid nanoparticles (SLN), were used to deliv-er small interfering RNA (siRNA). The siRNA and polyethylene glycolform a reducible conjugate and (siRNA–PEG) was anchored onto thesurface of SLN via electrostatic interactions, resulting in stable com-plexes in buffer solution and in even 10% serum. Flow cytometry re-sults also showed that siRNA–PEG/SLN complexes were efficientlytaken up by cells. Surface-modified and reconstituted protein-freeLDL mimicking SLN could be utilized as noncytotoxic, serum-stable,and highly effective carriers for delivery of siRNA [81]. So that SLNshave a wide range of applications in delivery of lipophilic drugs, hy-drophilic molecules, DNA and siRNA.

2. Types of SLNs based on route of administration

2.1. Parenterally administered SLNs

Parenterally administered drug has a rapid onset of action andhigh bioavailability with rapid rate of clearance from body, so dosingfrequency is high. So potent drugs with low solubility have to be ad-ministered by specific vehicle which overcome the challenges associ-ated (i.e. rapid clearance, high dosing frequency and poor solubility)with parenteral delivery of drugs. SLNs have the ability of controlledrelease kinetics with enhanced solubility of drugs in plasma to main-tain the drug concentration in therapeutic window. Clearance of drugloaded SLNs from circulation mainly depends upon size and surfacecharacteristics of SLNs. Parenterally delivered SLNs are metabolizedlike chylomicrons, after injection triglycerides rich SLNs adsorb apoli-poproteins from high density lipoproteins then, these SLNs are rapid-ly metabolized by lipoprotein lipase. If the target site of drug release isliver, no special surfactants are required in SLNs suspension. Reticulo-endothelial system (RES) cells may recognize SLNs as foreign parti-cles and remove them from systemic circulation due to lipophilicsurface of SLNs. Adsorption of opsonins increases the clearance ofSLNs from circulation. So to achieve sustained and targeted releaseof drug from the SLNs, both clearance routes should be controlled.Metabolic clearance can be decreased by increasing the particle sizeand choosing the higher molecular weight triglycerides, and RES clear-ance of SLNs can be inhibited by using stearically stabilized surfactantswhich inhibit the opsonins binding on particle surface. Poloxamers andpoloxamines are polyethylene oxide chains containing non-ionic sur-factants which sterically stabilized the nanoparticles and reduce the ad-sorption plasma proteins or opsonins on the surface of nanoparticles byproviding hydrophilic property to the surface of nanoparticles. The ten-dency of adsorption of proteins on the surface of SLNs was found to beinversely proportional to the chain length of PEO of poloxamers andpoloxamines [82].

Parenterally administered lipid nanoparticles encapsulated withanticancer agents, imaging agents, anti-parkinsonism, antiHIV, anti-psychotics, anti-rheumatoid arthritis agents, antiparasitics, antihy-pertensive agents and antibiotics have been studied. SLNs preventthe hydrolysis of camptothecin at physiological pH at body tempera-ture and provide the controlled drug release kinetics profile [83].Comparative study of stealth and non-stealth doxorubicin loadedSLNs with doxorubicin solution injected to albino rats shows doxoru-bicin concentration in plasma after 24 h whereas doxorubicin was ab-sent after 180 min in animals injected with doxorubicin solution.Doxorubicin concentration was more in case of stealth nanoparticlesas compared to non-stealth nanoparticles. Stealth SLN showed moreconcentration of doxorubicin than non-stealth one [22]. SLNs loadedwith curcuminoids were prepared by nanoemulsion technique andtested for antimalarial activity on malaria induced albino mice showeda 2-fold increase in the survival period than the free curcuminoids at thesame dose. Curcuminoids loaded SLNs exhibit controlled release kinet-ics for longer period which improve bioavailability of drug in activeform [84]. Controlled release lipid formulation of Itraconazole was pre-pared by tristearin [85] and triolein using high pressure homogeniza-tion technique. Triolein acts as a liquid lipid core, and released kineticsdepends on the liquid lipid content. ITZ incorporated at a concentrationof 20 mg/g, better results were obtained with 1% liquid lipid contenthaving smallest size and highest drug loading [86]. Artemether, an anti-malarial agent has very poor solubility marketed as oily suspension.Artemether loaded in lipid nanoparticles show enhanced anti-malarialactivity than the simple solution of drug administered parenterally.Artemether loaded in NLC (Nanoject) formulated by employing amicroemulsion template technique. Nanoject shows controlled releaseprofile and lower haemolytic potential with higher anti-malarial activi-ty. Nanoject showed a significantly higher survival rate (60%) even after31 days as compared to the marketed formulation which showed 0%survival (100% mortality) [87]. Artemether-loaded lipid nanoparticles(ARM-LNP) composed of 5% (w/v) lipidmass were produced by amod-ified thin-filmhydrationmethod using glyceryl trimyristate (solid lipid)and soybean oil (as liquid lipid in a concentration ranging from 0 to 45%(w/v) with respect to the total lipid mass). The particles were loadedwith 10% of the anti-malarial ARM and surface-tailoredwith a combina-tion of non-ionic, cationic or anionic surfactants [88]. So that SLNs haspromising applications in drugs administered by parenteral route.

2.2. Orally administered SLNs

SLNs can be administered orally in the form of dispersion or in theform of tablets, pellets, capsules. For acid sensitive drugs (Insulin),peptidase sensitive peptide drugs (Penicillin), drugs which undergoesfirst pass effect (Clozapine) [89], short acting drugs, impalpable drugs(Chloromphenicol). The drug must be solubilised to be absorbed in

1845S. Kumar, J.K. Randhawa / Materials Science and Engineering C 33 (2013) 1842–1852

GIT, so in lipid based formulation drug is absorbed from the formula-tion in the solid state. Digestion of triglyceride formulations starts instomach by gastric lipase, mechanical mixing of gastric contentswith amphiphilic products of lipid digestion leads to formation ofcrude emulsion. In small intestine pancreatic lipase and its co-factorco-lipase complete the breakdown of triglycerides to diglycerides,monoglycerides and fatty acid. For oral delivery of lipophilic drugs se-lection of lipid is a critical step, drug absorption is efficient with lowlipid content in case of long chain triglycerides but in case of mediumchain lipids high lipid concentration needed for the drug to beabsorbed in the intestine. Due to colloidal nature of SLNs, they en-hance drug absorption by affecting enterocyte based transport, affect-ing the drug transport to systemic circulation [90]. Due to the largesize of colloidal SLNs in the intestine they are transported throughthe lymphatic system to systemic circulation, so the first pass metab-olism is neglected and bioavailability of the drug will be higher. Oralcyclosporine SLNs gives better release kinetics as compared to drugnanocrystals, encapsulation rate of SLN was found to be 96.1% andnanocrystals are composed of 100% of the drug. In this study therewas a comparison of cyclosporine SLNs and nanocrystals with com-mercial Sandimmun Neoral/Optoral used as reference have beenperformed, SLN as a drug carrier for oral administration of cyclospor-ine A a low variation in bioavailability of the drug and simultaneouslyavoiding the plasma peak typical of the first Sandimmun formulationcan be achieved [91]. SLNs prepared by the “emulsion solvent diffu-sion” technique loaded with antitubercular drugs with considerableentrapment efficiency i.e. rifampicin (51±5%), isoniazid (45±4%)and pyrazinamide (41±4%). Conventional free drug require 46daily doses for required therapeutic benefit whereas drug loaded inSLNs require 5 doses each after 10 days, so SLNs reduced the dosingfrequency and improving patient compliance for better managementof tuberculosis [92]. Oral apomorphine delivery by SLNs enhanced thebioavailability up to 12–13 folds, antiparkinsonism activity studied inrats with 6-hydroxydopamine-induced lesions. Apomorphine loadedin SLNs shows reduced first pass effect and increased oral bioavail-ability [93]. Choice of emulsifiers also affects the bioavailability oforally administers lipid nanoparticles, lovastatin loaded nanostruc-tured lipid carriers (NLCs) stabilized by myverol (24%) have high bio-availability as compared to soybean phosphatidylcholine (13%). Ingastric environment myverol system has more stability [94].Octadecylamine-fluorescein isothiocyanate (ODA-FITC) loaded SLNsprepared by solvent-diffusion technique were used as marker tostudy the transport mechanism of SLNs. Majority of SLNs (77.9%)reach to the systemic circulation through lymphatic system andothers directly goes to blood [95]. Oral bioavailability of SLNs dependson size, surfactant and lipid matrix material. SLNs of size 150 nm havehigher bioavailability than 500 nm and 100 nm size range particles[96].

2.3. Ocular SLNs

Challenges for the ocular delivery of a drug alone or in a dosage formare different layers of the cornea, sclera, retina including blood aqueousand blood–retinal barriers, choroidal and conjunctival blood flow, lym-phatic clearance, tear dilution and efflux pumps in conjunction. For oc-ular delivery active pharmaceutical compound drug delivery systemshould provide extended contact timewith drug and increased penetra-tion of drug from tear phase to eye. Lipophilic drugs are permeable tocornea but formulations of these lipophilic drugs are problematic. Col-loidal dosage forms provide sustained and controlled release of thedrug at the targeted site, reduced frequency of administration, abilityto overcome blood–ocular barriers, and efflux-related issues associatedwith the parent drug [97]. So these drugs are formulated in suspensionform. SLNs have advantages of high viscosity to increase retention time,controlled release kinetics and corneal permeability. Cyclosporine Aloaded NLC prepared by melting emulsion-technology made-up of

Precifac ATO 5 and Miglyol 840 was studied on human corneal epithe-lial cell lines and rabbit cornea. Incorporation of liquid lipid innanoparticles enhances the bioavailability and prolonged retentiontime on eye surface [98]. SLNs containing tobramycin for ocular deliveryto treat eye infection gives better results than the solution of it.Tobramycin loaded as an ion pair with hexadecylphosphate in SLNs ata concentration of 0.3% was administered in rabbit eye and aqueoushumor concentration of tobramycin analyzed up to 6 h which hasgiven better results than solution of the same concentration oftobramycin [99]. Diclofenac sodium loaded in SLNs prepared by goatfat and phospholipid was analyzed on bioengineered human cornea, re-sults shown high permeability and sustained release was obtained[100]. Sustained release flurbiprofen NLC made up of stearic acid asthe solid lipid and a mixture of Miglyol® 812 and castor oil as the liquidlipids were prepared by hot homogenization method found to benon-toxic in case of in-vitro experiments [101].

2.4. Topically applied SLNs

Topical route delivers the drug in the localized area of skin, inorder to deliver the drug through the skin retention time of formula-tion and penetration of drug through the skin are important parame-ters which should be considered. Emulsion has the problem of lowviscosity which results to less retention time, however, SLNs can beused in the formulations to increase the viscosity. Occlusive com-pounds affect the skin hydration and penetration of compounds intoskin. An occlusion results in the retention of water content of theskin, due to the retention of water content stratum corneum swellsand leads to drug permeation. SLNs have small particle size and filmforming properties which make it best for topical applications. SLNcan be used in conventional topical formulation which results in syn-ergistic effect. Solid state of dispersed nanoparticles and small size ofthese film forming nanoparticles makes SLN as suitable for topical ap-plication [102]. Vitamin A loaded SLNs made up of glyceryl behenatewere tested for drug penetration in porcine skin. SLNs increase thepenetration of retinyl palmitate in skin due to occlusion effect [103].Prednicarbate was incorporated in SLNs to increase drug targeting.Prednicarbate absorption and loaded SLNs toxicity was evaluated onkeratinocyte, fibroblast monolayer cultures, reconstructed epidermisand excised human skin. Prednicarbate penetration to skin increasedup to 30% as compared to cream [18]. Advantages of dermal route areavoidance of first-pass metabolism and minimization of side effects.In dermal system the main challenge is impermeability of stratumcorneum towards many drugs in sufficient amount. The bioavailabil-ity of drugs penetrating into viable skin can be enhanced whenusing colloidal drug delivery systems because the small particle/vesiclesize (i.e., below 1 μm) ensures close contact to the stratum corneumand thus, the amount of encapsulated drug reaching the site of actionwill be increased. Aqueous SLN or NLC dispersions applied topicallywill create a mono-layered lipid film onto the skin, which avoidswater evaporation, and thus increases skin's moisture and hydration[104]. The podophyllotoxin-loaded SLN prepared by high pressurehomogenization stabilized by 0.5% poloxamer 188 and 1.5% soybean lec-ithin and 2% polysorbate 80. SLNs can be used for epidermaltargeting of podophyllotoxin, it penetrate in to skin through stratumcorneum and hair follicle route, which was confirmed by fluores-cence microscopy [105]. Cosmetic products need short time to be re-leased in the market and regulatory hurdle are also less in cosmeticproducts. SLNs based products are used as sunscreen, occlusive daycream, antiaging creams, controlled release formulation for perfumesand insect repellents and skin disease creams. Antioxidant all-trans ret-inol can be stabilized by loading it in SLNs, co-loading of anotherantioxidant also increases its stability [106]. Retinoic acid is a skin ir-ritant and its loading in SLNs is also low, so to solve these problemsCastro used lipophilic amine i.e. stearyl amine to form ion pairingwith retinoic acid. Entrapment efficiency increases from 13% to 94%

1846 S. Kumar, J.K. Randhawa / Materials Science and Engineering C 33 (2013) 1842–1852

by using stearyl amine and irritation due retinoic acid was also reducedas compared tomarketed retinoic acid cream [107]. SLNs dispersion canbe used in water based topical formulations, to replace water from theformulations. SLNs loaded with morphine accelerate wound healing,low cytotoxicity and low irritation towards reconstructed humanfull-thickness skin equivalents [108].

2.5. SLNs in molecular medicine and gene therapy

Synthetic non-viral vectors have advantages over viral vectors i.e. lowtoxicity, oncogenicity potential, easier regulatory controls and size inde-pendent delivery of nucleic acid. Lipoplexes, polyplexes and dendriplexeswhich are complex of nucleic acid with cationic lipids, cationic polymersand dendrimers respectively are non-viral vectors which are used intransfection [109]. Cationic solid lipid nanoparticles (cSLNs) have applica-tion as synthetic non-viral vector for transfection and delivery ofmolecu-lar medicines to target site. It has advantages in toxicity, stability andleakage issues over the liposomes and polymers. cSLNs for DNA andRNA transfection has to be prepared by phase inversion and solvent diffu-sion–emulsification methods because these methods don't require themechanical force which may damage DNA and RNA strands [110].Benzalkonium chloride, cetyl trimethylammonium bromide (CTAB),cetylpyridinium chloride (CPC), dimethyldioctadecylammoniumbromide (DDAB), N,N-di-(β-stearoylethyl)-N,N-dimethyl-ammoniumchloride (Esterquat 1, EQ 1), and N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium chloride (DOTAP) are some of the cationicsurfactants which can be used in the formulation of cationic solid lipidnanoparticles [111]. The cell targeting of cSLN-DNA can be enhancedby conjugation of ligands on the surface of cSLNs which supportcommunication of nanovectors with the target cells. Mannan modifiedcSLNs showed higher gene expression and lower toxicity over thenon-modified ones [112]. DNA delivery by cSLNs depends upon theDNA condensation with cationic lipid which affects transfection capac-ity. DNA to cationic lipid (e.g. N-[1-(2,3-Dioleoyloxy) propyl]-N,N,N-trimethyl-ammonium chloride) ratio affect the transfection capacityof cSLNs [113]. cSLNswhich are like lipoplexesmay internalize into cellsby clathrin mediated endocytosis. cSLNs internalized via the clathrin-mediated route will be fully transfection effective. As shown by Rejmanet.al. DNA complexed in the DOTAP lipoplexes prevented from lyso-somal degaradation due to acidic environment [114].

3. Principle of solid lipid nanoparticles formulation

Normally interfacial tensions are less than surface tensions be-cause adhesive forces between two liquids at interface are more ascompared to gas and liquid. Molecules at interface have more poten-tial energy as compared to bulk phase molecules, so these moleculesconstitute the surface free energy of interfacial system. So during ag-itation these systems attain a spherical system, to minimize the sur-face free energy. So work has to be done to increase the surface ofdispersed particles

W ¼ γ � ΔA

whereW is work done in ergs, γ is suface tension in dynes/cm and ΔAis the increase in surface area in cm2.

At constant temperature and pressure surface tension is the Gibbsfree energy per unit area

γ ¼ dGdS

� �P;T

:

Surfactant molecules are amphiphilic in nature so they areadsorbed at the interfaces, because adhesive forces between watermolecule and surfactant polar head group are lower than the cohesiveforce between individual water molecules. For amphiphilic molecules

to be adsorbed at interface surfactant molecule must be balancedwith water and oil soluble groups.

HLB scale of Griffin provides the measurement of hydrophilic andlipophilic balance of surfactants, so efficiency of HLB range of surfac-tant can be determined. High HLB leads to hydrophilic and low HLBleads to lipophilic molecules.

Nonionic surfactant whose hydrophilic portion is only polyoxy-ethylene

HLB ¼ E5

E is the percentage by weight of ethylene oxide.Polyhydric alcohol fatty acid esters

HLB ¼ 20 1− SA

� �

where ‘S’ is saponification number of ester and ‘A’ is acid number offatty acid.

SLNs are similar to emulsion which are formulated by solid lipids(instead of oils), which are solids at room temperature and body tem-perature having melting point in range of 40 °C–65 °C. When bothwater and melted lipid phases are mixed pressure agitation, resultsin fine droplets of dispersed phase in dispersion medium. But due tointerfacial tension both phases separate out and form individuallayer, so that third component surface active agent is used to stabilizethe system reduce the energy of the disperse system. Anionic andnon-ionic surfactants are generally used in the formulation of SLNs;sometimes cationic surfactants are also used. Alkyl phenols, straightchain alcohols, propylene glycols and fatty acids can also acts assurfactants.

4. Processing and Analysis of solid lipid nanoparticles

4.1. Production

High pressure homogenization (HPH), oil in water (o/w) micro-emulsion breaking method [97], solvent emulsification–diffusiontechnique [115], solvent injection method [116], water in oil in water(w/o/w) double emulsion method [117], ultra-sonication [118], super-critical fluid technique [119], membrane contactor method and [120],electrospray technique [121] for preparation of SLNs. High pressure ho-mogenization technique for the production of SLNs was used reproduc-ibly with narrow size distribution to load drug [122].

4.1.1. High pressure homogenizationHPH is a technique in which high pressure is used to apply stress

to produce the nanoparticles. Shear stress and cavitational forces in-duced by high pressure results in to reduction of particles size. HPHcan be performed at high temperature or below room temperaturecalled as hot-HPH and cold-HPH respectively. In HPH lipid is forcedat high pressure (100–1000 bar) through narrow space (few μm)for 3–5 times which depends upon formulations and required prod-uct. Before homogenization the drug is dispersed or dissolved inlipid melt. For heat sensitive drugs cold-HPH is preferred for prepara-tion of SLN.

4.1.2. O/W micro-emulsion breaking methodThis method was invented by Gasco, firstly the microemulsion was

prepared by mixing lipid melt with drug, surfactant and co-surfactantmixture preheated to temperature equal to melting point of lipid,then obtainedmicroemulsion dispersed inwater at the temperature be-tween 2–10 °C. Lipid used were stearic acid, trilaurin, cetyl alcohol,stearyl alcohol etc., sodium cholate, sodium deoxycholate, tween 80,tween 20 etc., were surfactant used and butanol, hexanol, butyric acid

1847S. Kumar, J.K. Randhawa / Materials Science and Engineering C 33 (2013) 1842–1852

etc., were co-surfactants used in the formulations of SLNs prepared bymicroemulsion method. SLNs dispersion produced by this method isvery dilute, so that final preparation has to be concentrated by ultra-filtration or lyophilisation. The main problem with this method is thehigh concentration of surfactant and co-surfactant [97]. Gasco et al.also designed an apparatus that consisted of a thermo-stated aluminumchamber, a pneumatic piston and a needle to prepare the SLNs at largescale by this method. Chamber temperature, piston pressure, needlegauge and volume of dispersing water were the main influencing fac-tors to affect the diameter and polydispersity index of final preparation.Small size SLNs were produced at high temperature and pressure withnarrow gauge needle [123]. Device temperature, water temperature,and delivery rate of concentrated microemulsion to cold water werekey factors in the production of SLNs by this method [124]. Tretinoinloaded SLNs prepared by this method with considerable drug load(37–48%) by using glyceryl mono-stearate Tween 20 (as a surfactant)and Transcutol P (a co-surfactant), these excipients were bio compati-ble to prepare SLNs by microemulsion method [125].

4.1.3. Solvent emulsification–diffusion techniqueIn this method lipid is dissolved in organic solvent saturated with

water, the solution obtained is further emulsified with water saturatedwith organic solventwith stirring. Lipid nanoparticles were obtained byadding water to the emulsion prepared, which later results in the diffu-sion of organic phase into continuous phase. SLNs dispersion can be pu-rified by ultra-filtration using a dialysis membrane of approximately1,00,000 KDa cut-off [115].

4.1.4. Solvent injection methodIn this method lipids were dissolved in water-miscible solvent and

dissolved lipid was injected through an injection needle in to stirringaqueous solution with or without surfactant. Process parameters fornanoparticles synthesis in this method were nature of injected sol-vent, lipid concentration, injected amount of lipid solution, viscosityand diffusion of lipid solvent phase to aqueous phase [116].

4.1.5. W/O/W double emulsion methodThis method is mainly used for preparation SLNs loadedwith hydro-

philic drugs. SLNs were produced from w/o/w multiple emulsions bysolvent in water emulsion diffusion technique, insulin was dissolved ininner acidic phase of w/o/w multiple emulsion an lipids was dissolvedin water-miscible organic phase, then SLNs were produced by dilutingthe w/o/w emulsion in water. It results in diffusion of organic solventto aqueous phase and precipitated the SLNs. Nature of solvent and inter-action of hydrophilic drug with solvent and excipients affect the prepa-ration by this method [117].

4.1.6. UltrasonicationSound waves can be used reduce the size of particle size, high pres-

sure homogenization and ultrasonication simultaneously used forproduction SLNs. Preparation of lipid nanopellets by ultrasonicationresulted in homogeneous particles with size range from 80 to 800 nm[118].

4.1.7. Super critical fluid techniqueSuper critical carbon dioxide has a tendency to dissolve the lipophil-

ic drugs, by combination with ultrasonication technique it can be usedto prepare SLNs. Xionggui loaded SLNs have been prepared by usingsuper critical carbon dioxide fluid extraction and ultrasonication [119].

4.1.8. Membrane contactor methodIn this methodmembrane contactor was used to prepare SLNs, lipid

was pressed at the temperature above the melting point of lipidthrough the membrane pores, water circulated beyond the pores flowwith the produced droplets of melted lipid which was further cooledat room temperature [120].

4.1.9. Electro-spray techniqueIt is the recent novel technique for the preparation of SLNs, in this

technique electrodynamic atomization was used to produce narrowlydispersed spherical SLNs less than 1 μm size. In this method SLNswere directly obtained in powder form [121].

4.2. Formulation ingredients

SLNs contain lipids which remain solid at room temperature andbody temperature, the lipids are pure triglycerides (tristearin, tri-palmitin, trimyristin, tricaprin, trilaurin), long chain alcohols (cetylalcohol), waxes (bees wax, cetyl palmitate), calixarenes and sterols(cholesterol). Surfactants generally used are poloxamer 188, poloxamer182, poloxamer 407, bile salts, cetyl trimethylammonium bromide(CTAB), soybean lecithin, phosphatidyl choline, polyethylene glycols,polyvinyl alcohols, tween 80, tween 60, brij 30, brij 98 and brij 76.Fig. 2 shows the SLN for the delivery of antiHIV agent at targeted site[126]. Sometimes combinations of surfactants are also used to stabilizethe system. Organic solvents are not essential to be used in the formu-lation SLNs, excipients used to prepare SLNs are safe and under Gener-ally Recognized as Safe (GRAS) status approved by the Food and DrugAdministration (FDA). Lipophilic drug components, labile drugs, lessanti-oxidants, peptide drugs and proteins can be entrapped in matrixof solid lipid nanoparticles.

4.3. Entrapment of water soluble drugs in SLNs

Cold high pressure homogenization technique is used for loading ofhydrophilic drugs, but loading of drug is still very low. Some approacheswere used to increase the loading hydrophilic drugs. Organic counterion approach has been used to form ion pair with charged drug mole-cule. Loading of doxorubicin HCl and idarubicin HCl in stearic acidSLNs has improved bymonoalkyl phosphate esters (i.e. decyl phosphateand hexadecyl phosphate) [127]. Formation of insoluble lipid-drugcomposite by salt formation or covalent linking like ester linkage resultsinto lipid drug conjugate nanoparticles after mixing with aqueous sur-factant by homogenization or related methods of SLNs preparation.Diminazene diaceturate lipid-drug-conjugate (LDC) with stearic acidand oleic acid has used for brain delivery of diminazene tomice infectedwith Trypanosoma brucei gambiense [128]. 2-D electrophoresis has usedto check interaction of plasma proteins with brain delivered LDC [129].Recently polymer lipid hybid nanoparticles has used to increase theloading of hydrophilic molecules, it involves the complexation of ionicpolymer and drug. The drug charge neutralized with counter ion ofpolymer and resulted complex has encapsulated in SLNs [130]. In caseof polymer lipid nanoparticles (PLNs) drug loading capacity of a lipidmatrix depend on the drug-ionic polymer binding and interaction be-tween complex and lipid [131].

4.4. Characterization of SLNs

Characterization is a critical step in the formulation of SLNs disper-sion which gives complete analytical detail of stability, loading capac-ity, and release kinetics, polymorphism and particle size. Crystalbehavior of lipid affects the drug loading, release kinetics and stabilityof system. Highly crystalline system has less drug loading and slowrelease, so more stability in the system.

4.4.1. Photo correlation spectroscopyPCS measures fluctuation of scattered light from dispersed parti-

cles in a medium are recorded and analyzed in correlation delaytime domain, these dispersed particles have different refractiveindex from medium and stable during the measurement. Particles,emulsions and molecules in suspension undergo Brownian motion.This is the motion induced by the bombardment of the solvent mole-cules that are moving due to their thermal energy. If the particles or

Fig. 2. Schematic illustration of HSA/NVP-SLN or HSA/NVP-NLCs [after permission from [126]].

1848 S. Kumar, J.K. Randhawa / Materials Science and Engineering C 33 (2013) 1842–1852

molecules are illuminated with a laser, the intensity of the scatteredlight fluctuates at a rate that is dependent upon the size of the parti-cles as smaller particles are “kicked” further by the solvent moleculesand move more rapidly. Analysis of these intensity fluctuations yieldsthe velocity of the Brownian motion and hence the particle size usingthe Stokes–Einstein relationship.

D ¼ kBT6πηr

where D is diffusion constant, kB is Boltzmann's constant, T is temper-ature, h is viscosity of solvent, r is radius of particles.

The diameter that is measured in Dynamic Light Scattering is calledthe hydrodynamic diameter and refers to how a particle diffuses withina fluid. The diameter obtained by this technique is that of a sphere thathas the same translational diffusion coefficient as the particle beingmeasured. The translational diffusion coefficient will depend not onlyon the size of the particle “core”, but also on any surface structure, aswell as the concentration and type of ions in the medium. This meansthat the size can be larger than measured by electron microscopy, forexample, where the particle is removed from its native environment.PCS can measure size range from 1 nm to 4 μm [132].

4.4.2. Differential scanning calorimetric analysisDSC can accurately determine melting point and polymorphic

state. DSC also used to determine oxidative stability of lipids, mainlyused for vegetable oils. DSC measures the heat effect associatedwith phase transitions and chemical reactions which are temperaturecontrolled. Temperature of sample and reference increased at a con-stant rate. At constant pressure heat flow is equivalent to enthalpychange.

dqdt

� �¼ dH

dt

Difference in heat flow between sample and standard

ΔdHdt

¼ dHdt

� �sample− dH

dt

� �reference:

Difference in heat flow may be positive or negative. DSC is classi-fied into two modes depending upon the methods used for measure-ment power compensation differential scanning calorimetry andheat-flux differential scanning calorimetry.

DSC is used to determine the degree of crystallinity of SLNs disper-sion. About 5 mg needed for dried SLN powder and 40 mg needed fordispersions. Drug encapsulation efficiency of nanoparticles was deter-mined by DSC in NLC which was related to crystal states of lipids in-gredients used in formulations. DSC static and dynamic modes wereused to prove that oil nanocompartment in NLC influencing thedrug distribution into nanoparticles system [133].

4.4.3. Microscopy analysisDue to small size scanning electronmicroscopy (SEM), transmission

electron microscopy (TEM), and atomic force microscopy (AFM) areused to characterize the nanoparticles. It's tough to analyze the lipid-based nanoparticles by electron microscopy due to heating of samplesby electron beam which may damage the shape and structure of lipidnanoparticles. So cryo-field emission scanning electron microscopy(cryo-FESEM) was used to characterize the SLNs and NLC [134]. SLNscontaining high amounts of ceramides were characterized by scanningelectron microscope and show no change in morphology of nano-particles [135]. Cyclodextrins formulated SLNs were analyzed by SEMand AFM shows that alteration in structure occurs during the sampleprocessing. During vacuum drying of SLNs for SEM sample preparationresults in shrinkage of nanoparticles which were confirmed by AFM.Sample preparation for AFM by deposition leads to cluster formationof SLNs [136]. Existence of soft surfactant layering around the SLNscan be observed through phase imaging view of non-contact atomicforce microscopy [137].

1849S. Kumar, J.K. Randhawa / Materials Science and Engineering C 33 (2013) 1842–1852

4.4.4. X-ray photoelectron spectroscopy (XPS)X-ray photoelectron spectroscopy involves exposure of sample by

soft X-ray beam and energy analysis of photoelectron emitted fromthe surface. The components of XPS are vacuum vessel, sample holdingchamber, X-ray source, energy analyzer, magnetic lenses and electrondetector. Interaction between X-ray beam and sample causes ejectionof electrons with discrete energies corresponding to electron lines[138]. Common applications of XPS are quantitative measurement ofcomposition of organic and inorganic samples, determination of oxida-tion states, surface analysis and depth profiling of coatings. Several re-searchers have observed surfactant adsorption at interface and on thesurface of particles by XPS [139–142]. Kuo et al. used XPS to show thecationic lipid interaction with other components of SLNs (cacao butter,cholesterol, stearylamine, and esterquat 1) and surface distribution of it.Bio-conjugation of SLNs surface by stearylamine could also be con-firmed by XPS [143]. Same group has also observed the grafting ofdocetaxel and ketoconazole loaded SLNs by folic acid [144]. In case ofSLNs surfactants which have elements other than the basic elements(C, H, O) could be detected over the surface of SLNs. XPS could beused for the functionalization of nanoparticles and physical or chemicaladsorption of the molecules on the interfaces.

4.4.5. BET surface area analysisIt is the analysis technique which was used to determine the sur-

face area of particles based on nitrogen adsorption on surface. Thismethod was provided by Brunauer, Emmett, and Teller, thus, this the-ory is known as the BET theory. BET equation is the extension of Lang-muir theory which is the mono-layer adsorption for multi-layergaseous adsorption. For the BET analysis the sample has to be homo-geneously distributed without aggregation and dried to maintain gaspassage at the surface of nanoparticles.

1

v pop

� �−1

h i ¼ c−1vm

ppo

� �þ 1vmc

where p is the equilibrium pressure, po is the saturation pressure, v isthe adsorbed gas density, vm is the monolayer adsorbed gas densityand c is the BET constant.

c ¼ eE1−ELRT

� �

where E1 is the heat of adsorption and EL is the heat of liquefaction.Surfactant stabilized nanoparticle surface area could be measured

after removing the solvent from the surface of nanoparticles which willexpose the surface of nanoparticles to gaseous system. Some literatureis available where the surface area of surfactant stabilized nanoparticleswas studied [145–149]. But in case of solid lipid nanoparticles BET anal-ysis technique has not been used till now. SLNs have to be freeze dried toobtain the uniformly distributed un-aggregated particles where the gashas to be absorbed for the measurement of surface area.

4.5. Stability of SLNs

Stability of SLNs is affected by lipid matrix materials, surfactantsand storage conditions. So that optimum concentrations of surfac-tants, optimum temperature and light should be considered for longterm stability of SLNs and should be also considered during storageand processing.

4.5.1. Polymorphic modification of lipids during storageDuring the storage the lipids undergo polymorphic transition from

less stable to higher stable crystal state [150]. α (alpha), β′ (betaprime), β (beta) crystal modifications are present in triglycerides. α pos-sess hexagonal sub-shell packing in which fatty acid chains are perpen-dicular to methyl end group plane and are assumed to be oscillating

with a high degree of molecular freedom, β′ (beta prime) possesses or-thorhombic sub-shell packing inwhich fatty acid chains are tiltedwith re-spect tomethyl group plane and fatty acid chains are in different plane asthe zigzag structure, β (beta) possesses triclinic packing with fatty acidchains in the same plane as the zigzag structure. Lipids crystallize intoless stable α modification, when rapid cooling occurs during SLNs pro-duction which changes to more stable β modification on storage. Poly-morphic modification of lipids depends on chemical structure of lipids.Addition of electrolytes to SLNs dispersion leads to gel formation and in-crease in particles size. Sodium, calcium and aluminum chloride wereadded in varying concentrations to a Compritol formulationwhich resultsto destabilization. A pronounced destabilizing effect was observed withincreasing electrolyte concentration and increasing valence of cation, soAl3+>Ca2+>Na+. The mechanisms of the destabilizing effect of theelectrolytes are reduced electrostatic repulsion and transformation ofthe lipid Compritol to the β′ modification promoting gel formation[151]. Chemical stability of excipients used in SLNs has been determinedby gas chromatography in combinationwith lipid extractionmethod, theformulationwhich have triglycerides as lipid content show negligible de-composition of the structure during incubation at 25 °C [85]. Higher con-centration of surfactant influences leads to dense coverage at theinterface which may influence crystal structure and stability of solidlipid nanoparticles via surface-mediated crystal growth [152].

4.6. Drug release from SLNs

Release of drug from SLNs depends upon the crystal structure ofsolid lipids; during the preparation crystallization of lipids occurs par-tially in higher energy modifications with imperfections in crystalstate. Crystal structure could be affected by temperature, rate of crys-tallization and additives. The structure of SLN matrix mainly dependson the difference between melting point of lipid and homogenizationtemperature. Controlled release pattern of drug release also dependsupon the solubility of drug in molten lipid and interaction betweendrug–lipid-surfactant. A burst effect of drug release was observed indrug enriched shell model of drug containing SLNs. Large surfacearea, lower matrix viscosity, high partition coefficient and small dis-tance of diffusion results into fast release of drug from reservoir. Pred-nisolone loaded in SLNs releases the drug in a controlled manner upto 5 weeks in the absence of enzymes, it may be a drug-enrichedcore model of SLNs [153]. Due to polymorphic transitions drug expul-sion from SLNs occurs.

4.7. Preservation and storage of drug loaded SLNs

4.7.1. Sterilization of SLNsNanoparticle preparations which are to be administered by paren-

teral, ocular or pulmonary routes must be sterilized. Nanoparticle for-mulations which have particle size below 200 nm can be easilysterilized by membrane filtration but for larger particles autoclavingor gamma sterilization is needed. Excipients of SLNs can handle thehigh temperature of autoclaving, so autoclaving is better to be usedas sterilization method for SLNs. During autoclaving the temperatureof system rises to form hot oil in water emulsion, which may modifythe size of final product. The stability of SLNs during autoclaving de-pends on surfactant used to stabilize the system and lipid matrixmaterial. Particle size increased slightly after autoclaving but not toa reasonable extent and polydispersity index also increased. SLNsprepared by microemulsion by using stearic acid, behenic acid orAcidan N12 as lipid matrix material, epikuron 200 as surfactant andtaurodeoxycholate as co-surfactant were sterilized by autoclaving.After autoclaving the particle size of stearic acid SLNs and behenicacid SLNs increased whereas Acidan N12 SLNs remain unaffected.Zeta potential remains unaffected after autoclaving of SLNs [154].For thermo labile materials gamma sterilization is used to sterilize

1850 S. Kumar, J.K. Randhawa / Materials Science and Engineering C 33 (2013) 1842–1852

the SLNs, but there are chances of free radical production which resultin oxidation of excipients used in SLNs production.

4.7.2. Freeze drying of SLNsShelf life of SLNs suspension is less because the drugs may hydrolyze

and/or surfactant may not formulate a stable system for longer duration.So freeze drying is needed to store the SLNs dispersion in dry form for lon-ger duration. Trehalose (15%) is the best cryoprotectant which is to beused in freeze drying of SLNs dispersion. During freeze drying resultsvery slightly change in particle size, but degree of crystallinity of particlesincrease. So drug loaded SLNs result in different size particles because freedrug concentration leads to increase in particle size. The zeta potential offinal productmay decrease due to free drug content in suspension, so freedrugmust be removed from the product before freeze drying. Crystalliza-tion of the lipid matrix during freezing also leads to drug expulsion andzeta potential of the system decreases [19]. Protective effect of cryopro-tectant, freezing velocity, and thermal treatment affects the reconstitutedproduct of SLNs powder obtained after dispersing itwithwater [155,156].Saccharide cryoprotectants affect the aggregation behavior of SLNs duringfreeze drying, crystalline nature of cryoprotectant leads to aggregates ofnanoparticles. Xylose on freeze drying leads to regular crystals andnanoparticles dispersed in suspension comes out to form aggregates[157].

4.7.3. Spray drying of SLNsSpray drying is a one step process used to dry the suspensions in dry

form which may be constituted at the time of use. In case of SLN disper-sion spray drying, melting point of lipid must bemore than 65 °C. Duringspray dryingmoisture forms a film around SLNwhich lowers the temper-ature below 15–20 °C lower than the temperature of the outer environ-ment. So thermo-labile drug also gets preserved and lipid will not meltduring the drying process. Kinetic energy and shear forces are maximumwhile the process of spray drying is used. Dilute SLNs suspension resultsinto a dispersible product because there were fewer chances of aggrega-tion and particle damage during spray drying. Temperature also resultsin aggregations of nanoparticles during spray drying either by meltingof lipid or damage of surfactant film on surface of nanoparticles, inorder to avoid this problem, carbohydrate or alcohol content was usedin spraying [158].

5. Conclusion and future prospective

Solid lipid nanoparticles are colloidal particles for delivery of lipo-philic and labile drugs with controlled and targeted release kineticswhich do not have the disadvantages of emulsions, suspensions, lipo-somes and polymer nanoparticles. SLNs based formulations are compat-ible with all the routes of administration due to non-toxic excipientsused in its formulation. The only disadvantage of SLNs is the polymor-phicmodifications of solid lipidwhich can create problems related to re-lease kinetics and loading, but these limitations can be overcome byusing nanostructure lipid carriers. Newly synthesized molecules whichhave low solubility and labile nature could be brought as marketed for-mulation by SLNs which saves cost and time for a better active pharma-ceutical ingredient to be available in the market. SLNs have been alsoused in the delivery of nucleotides to cells whichmake it a perfect vectorfor gene therapy. RNAi is a promising process in treating many diseases,but its stable delivery vehicle SLNs can solve the purpose of stable RNAat target site. Storage and shelf life of SLNs can be improved by somemodification in the formulation and downstream processing.

List of abbreviations

SLNsGRASLDC

o/ww/o/wNLCHPHDSCPCSTEMFESEMAFM

Acknowledgment

The authors are highly thankful to the Director, National instituteof Technology Hamirpur (H.P) India for providing the laboratory facil-ities and the Ministry of Human Resources and Development for fi-nancial support.

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