REVIEW

Review: Radiation-induced surface modification of polymersfor biomaterial application

Saravana Kumar Jaganathan • Arunpandian Balaji •

Muthu Vignesh Vellayappan • Aruna Priyadarshni Subramanian •

Agnes Aruna John • Manjeesh Kumar Asokan • Eko Supriyanto

Received: 10 September 2014 / Accepted: 10 November 2014

� Springer Science+Business Media New York 2014

Abstract The field of biomaterials is one of the fast

growing and continuously dominating in medical arena for

the last five decades. Biomaterials utilize various kinds of

materials ranging from metals, polymers, ceramics and

biological substances as an alternative for replacing/

assisting the functions of different parts of human system.

Major issues associated with biomaterials are their prop-

erties and the biocompatibility which have to be addressed

and resolved before promoting it to the market or clinical

setting. In this scenario, polymers have emerged as a better

candidate with versatile properties that make them ideal

choice for biomedical applications. However, still the

problem of biocompatibility limits the use of polymers in

the human body. Several surface modification strategies are

continuously evolving to improve the biocompatibility of

polymers. This review initially outlines the polymers’

biomedical applications and also elicits the immune aspects

of biocompatibility. Further, a thorough attempt is made to

summarize the radiation-induced surface modification of

the polymers. This review will help us to keep abreast of

the recent advances in the radiation-induced surface mod-

ification and also in promoting radiation as a probable

candidate to enhance the biocompatibility of polymers.

Introduction

Biomaterial can be described as a combination of sub-

stances originating from natural, inorganic or organic

materials, which is biocompatible in exactly or partially,

contact with the body during the healing time. They

involve complete or part of a living organism or bio-

medical device which performs, augments or replaces any

natural functions [1]. Recent reports of MarketsAndMar-

kets indicates that by 2017, the estimated global market

for biomaterials will be 88.4 billion US$ with a com-

pound annual growth rate (CAGR) of 15 %. Further, it is

forecasted that the Asian market will grow at the highest

CAGR of 21.5 % because the number of people with

medical problems is steeply increasing in Asia demanding

more biomaterial-based medical products [2]. Nowadays,

biomaterials are commonly used in various medical

devices and systems namely synthetic skin, drug delivery

systems, tissue cultures, hybrid organs, synthetic blood

vessels, artificial hearts, cardiac pacemakers, screws,

plates, wires and pins for bone treatments, total artificial

joint implants, skull reconstruction, dental and maxillo-

facial applications [3]. Among various applications, the

application of biomaterials in cardiovascular system is

most significant. The use of cardiovascular biomaterials is

projected to be the predominant category of the bioma-

terial market in 2014, with a worth of about $20.7 billion

[2].

Biomaterials (CB) broadly fall into the four main types

namely metals, ceramics, polymers and biological sub-

stances. Metals have unique atomic structure which gives

them characteristic strength and properties which enable

them specifically for load-bearing applications like ortho-

paedics. However, the corrosion associated with the use of

metals limits their utility. Ceramics have evolved as better

S. K. Jaganathan (&) � A. Balaji � M. V. Vellayappan �A. P. Subramanian � A. A. John � E. Supriyanto

IJN-UTM Cardiovascular Engineering Centre, Faculty of

Bioscience and Medical Engineering, Universiti Teknologi

Malaysia, 81310 Johor Bahru, Malaysia

e-mail: jaganathaniitkgp@gmail.com; saravana@utm.my

M. K. Asokan

Department of Research and Development, PSNA college of

Engineering and Technology, Dindigul 624622, Tamil Nadu,

India

123

J Mater Sci

DOI 10.1007/s10853-014-8718-x

biomaterials because of their bioinertness and compatibility.

However, due to its brittleness and low strength, ceramics is

losing its popularity [4]. Polymers have widespread appli-

cations in the field of biomaterials. Properties of polymers

are dependent on the unit macromolecule present in the long

chain of the polymer. Polymers with a wide range of

mechanical and chemical properties are available in the

market and the selection criteria for different biomedical

applications decide the polymer for its specific needs. Bio-

logical substances include the valves of bovine and porcine

directly implanted to replace heart valves. For all types of

biomaterials, the surface plays a key role in determining the

biocompatibility of a particular implant.

The biocompatibility and cellular interactions of the

biomaterial vary depending on the surface physico-

chemical properties. The properties such as surface

roughness, hardness, temperature, surface chemistry,

surface reactivity (inert or active), wettability and surface

charge (surface free energy) are some of the essential

surface properties. Each of the above-mentioned proper-

ties plays a key role in cell adhesion, cell spreading, cell

proliferation and tissue formation. The appropriate sur-

face physico-chemical properties of a biomaterial depend

on the corresponding biomedical application. For

instance, cell adhesion is expected to be good for car-

diovascular and artificial joint implants. In contrast, for

catheters, cell adhesion should be as small as possible in

order to avoid contaminations that can cause infections.

In general, if an application requires good cell interac-

tion, proliferation and spreading, then the surface of

implant should have high wettability (hydrophilic), better

surface roughness, promoting protein adsorption, highly

reactive and good surface energy. In order to have a

better understanding, readers may refer the following

cited articles [5–7].

So, based on the requirements, the surface properties of

a material can be tailored by employing appropriate surface

modification techniques. Before going into classification

and different types of surface modification modalities

available, a brief insight about the requirements and

problems associated with the biomaterial is summarised in

successive section in order to understand the need of doing

surface modification.

Requirements and challenges of biomaterials

First and foremost, a biomaterial must be biocompatible

i.e.it should not elicit an adverse response from the body,

and vice versa. Additionally, it should be nontoxic and

non-carcinogenic. Secondly, the biomaterial should pos-

sess adequate physical and mechanical properties to serve

as augmentation or replacement for body tissues. Because

of the deficit of above characters, many engineering

materials lose their reputation as good biomaterials. So, in

order to meet various requirements as biomaterials,

polymers are mostly preferred because of their diversified

properties and versatility. Polymers are long-chain mole-

cules consisting of a number of small repeating units

called ‘‘monomer’’ which are covalently bonded chains of

atoms. The macromolecules interact with one another by

weak secondary bonds such as hydrogen and Van der

Waals bonds to form entanglement structure. The com-

position, structure and organization of constituent mac-

romolecules specify the properties of polymers. The

biomedical applications of various polymers are men-

tioned in (Table 1) [8–18].

Although polymer is a versatile choice among other

biomaterials, there are some aspects like biocompatibility

and blood compatibility limiting its popularity. The field of

biocompatibility and blood compatibility is quite vast and

its thorough explanation may not be the context of this

review. However, basic insight of biocompatibility and

blood-mediated reactions is briefed in successive sub-sec-

tions in order to insist the importance of this review.

Table 1 Polymers and their biomedical applications

Body parts Polymers used predominantly

Ear and ear parts acrylic, polyethylene, silicone,

poly vinyl chloride (PVC)

Denture acrylic, ultrahigh molecular

weight polyethylene

(UHMWPE), epoxy,

Polymethyl methacrylate

Facial prostheses acrylic, silicone, nylon,

Polyurethane

Polytetrafluoroethylene

Tracheal tubes acrylic, silicone, nylon

Vascular graft Polytetrafluoroethylene,

Polyethylene terephthalate.

Breast prostheses Polydimethylsiloxane.

Heart and heart components polyester, silicone, PVC

Heart pacemaker polyethylene, acetal

Lung, kidney and liver parts polyester, polyaldehyde, PVC

Oesophagus segments polyethylene, polypropylene

(PP), PVC

Blood vessels PVC, polyester

Gastrointestinal segments silicones, PVC, nylon

Finger joints silicone, UHMWPE

Bones and joints acrylic, nylon, silicone, PUR,

PP, UHMWPE

Hip joint replacement Polyethylene

Knee joints Polyethylene,

Polydimethylsiloxanes

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Biocompatibility

Biocompatibility is a vital factor which determines the

quality of a biomaterial and its application in various are-

nas. There are a number of statements to define biocom-

patibility. It may be defined as the ability of the material to

perform at a specific region with an appropriate host

reaction. Biocompatibility may also be defined as the

ability of a biomaterial to perform its desired function with

respect to a medical therapy, without eliciting any unde-

sirable local or systemic effects in the recipient or benefi-

ciary of that therapy, but generating the most appropriate

beneficial cellular or tissue response to that specific situa-

tion and optimizing the clinically relevant performance of

that therapy [19]. Biocompatibility has been mentioned in

many works with increasing interest in evaluating the

characteristics of medical materials and devices and also

the responses caused by its components. But the ideal

pattern for determining these properties has not yet been

determined; however, various methods have been sug-

gested for this purpose. Biocompatibility encompasses

many aspects of the material, including its physical,

mechanical and chemical properties, and potential cyto-

toxic, mutagenic and allergenic effects, so that no signifi-

cant injuries or toxic effects on the biological function of

cells and individuals arise [20–22].Until the biocompati-

bility of a material is proven, it must be subjected to var-

ious studies ranging from in vitro assays to clinical trials,

involving distinct areas such as pharmaceutics, biology,

chemistry and toxicology to justify its use as biomaterial.

Blood compatibility

The term biocompatibility has been defined by consensus,

but not blood compatibility. The interactions between

blood and a surface depend on the blood composition, the

blood flow and the surface of the material defined by its

physicochemical feature [23]. The design of blood-com-

patible materials is clearly a challenge to increase success

in all medical devices that come in contact with blood and

to answer unsolved problems in vascular reconstruction.

To explain blood compatibility from a different perspec-

tive, consider a material that is not blood compatible, i.e.

a thrombogenic material. Such material would produce

specific adverse reactions when placed in contact with

blood: formation of clot or thrombus composed of various

blood elements; shedding or nucleation of emboli

(detached thrombus); the destruction of circulating blood

components and activation of the complement system and

other immunologic pathways. Thus, we can define blood

compatibility as the ability of the material to work in a

particular place without eliciting any blood-related com-

plications. Indeed, biocompatibility of blood-contacting

devices relates mainly to the thrombotic response induced

by the materials. Whenever the blood comes in contact

with the implants (biomaterial), it will lead to the fol-

lowing complications: (1) Interaction of blood compo-

nents with surfaces resulting in protein and water

adsorption (2) Blood cells interfere with the surface of

biomaterial (3) These actions lead to haemostasis and

coagulation.

Several distinct but interrelated thrombotic and anti-

thrombotic systems exist to prevent the formation of

intravascular clots expected in response to vascular trauma.

Haemostasis is the sum of these mechanisms and serves to

limit blood loss following injury. Once regulation is initi-

ated, these same mechanisms combine first to localize the

clot at the site of injury, then to terminate coagulation and

finally to remove the clot once it has served its purpose.

These haemostatic mechanisms include platelet activation,

coagulation, fibrinolysis and local vascular effects. Blood

clotting, platelet adhesion and giant cell formations are

major problems involved in blood compatibility. These

problems frequently arise in cardiovascular implants since

the material is always in contact with blood and its com-

ponents [24]. The process of clotting can occur through two

different pathways namely intrinsic and extrinsic. The

intrinsic pathway occurs whenever the blood comes in

contact with the foreign material, whereas the extrinsic

pathway initializes at the site of injury or damaged blood

vessel. The intrinsic pathway starts with the conversion of

factor XII to factor XIIa. The factor XIIa then converts

factor XI to factor XIa which in turn forms factor IXa from

factor IX. The formed factor IXa along with factor VIIa

converts factor X to factor Xa through proteolysis. The

factor Xa converts prothrombin into activated thrombin

which then reacts with fibrinogen to form fibrin finally.

Fibrin eventually gets stabilized as a red thrombus or clot.

The extrinsic pathway starts with the release of tissue

factor, collagen and von Willebrand (vWF) from the

damaged blood vessel. Tissue factor acts as cofactor to

activate factor X. There is also a possibility for factor VII

to activate factor IX which in turn activates factor X.

Except for factor VII, all factors of extrinsic pathway are

similar to intrinsic pathway leading to the formation of

thrombus (Fig. 1) [25, 26]. In order to reduce above

complications and to increase the quality of a biomaterial,

several modification techniques have been framed, which

are summarised in the next section.

Surface modification

To improve the biocompatibility of polymer, the following

surface modification strategies (Fig. 2) are available: (1)

Physico-chemical methods, (2) Mechanical methods and

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(3) Biological methods. These three modes were further

classified into a number of sub-groups. The physico-

chemical method is classified into three sub groups:

(a) treatment with active gases and vapour (or) radiation,

(b) solution treatment (or) bulk phase desorption and

(c) combination of first and second method. The treatment

with active gases and vapour (or) radiation type can be

achieved by the following methods: (i) deposition of

polymers from active gases and vapours (e.g. Gas dis-

charge, chemical vapour, flame spray), (ii) active gas (or)

accelerated ion treatments (e.g. etch, ablate or oxidize,

corona discharge or ion beam, ion implantation) and (iii)

crosslinking of surface molecules (e.g. ionizing radiation,

UV). The solution treatment (or) bulk phase desorption

method can be achieved by the following four methods:

(i) Solution deposition of polymers and amphiphiles (e.g.

polymer coatings, surfactants), (ii) Desorption of surface-

active compounds from bulk (e.g. Desorption of surfactants

from bulk to surface), (iii) chemical treatments to modify

surface groups (e.g. Oxidize, sulfonate, chlorinate, acety-

late, quaternize) and (iv) chemical conjugation of mole-

cules to surface groups (e.g. silanating agents, PEG). The

mechanical modifications were further classified into two

sub-groups namely roughening (e.g. From micro-rough to

porous surface) and micromanipulation (e.g. Using STM,

AFM probes). The biological method is classified into fourFig. 1 Intrinsic and extrinsic pathway associated with the blood

clotting process

Fig. 2 List of various polymer

surface modification techniques

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sub-groups: (i) physical adsorption of biomolecules (e.g.

proteins, peptides, ligands, receptor, drugs and lipids), (ii)

physical adsorption and self-crosslinking of biomolecules

(e.g. Proteins, peptides, ligands, receptor, drugs and lipids,

where feasible), (iii) chemical conjugation of biomolecules

to surface groups (e.g. Proteins, peptides, ligands, receptor,

drugs and lipids) and (iv) cell seeding and growth to con-

fluence. [27]

Among various methods of surface modifications dis-

cussed above, in our review, we have concentrated only on

surface modification of the polymer surface by radiation

treatment which comes under physico-chemical modifica-

tion type. There is plenty of literature dealing with bio-

compatibility improvement using radiation. This review

will highlight and collate some important works, those of

which utilized radiation as a tool to modify the biomaterial

surface.

Radiation

Radiation is a form of energy that comes from a source and

travels through some medium or through space. Radiation

is emitted from an atom when the electrons drop from

higher energy to lower energy. And the radiation is divided

into two types, based on the energy and ionizing power,

namely ionizing and non-ionizing radiation [28].

Ionizing radiation is produced by unstable atoms.

Unstable atoms differ from stable atoms because they have

an excess of energy or mass or both. Ionizing radiations are

high-energy radiations which are able to remove the elec-

trons from an atom or a molecule to form an ion [29]. Non-

ionizing radiation, in contrast to ionizing radiation, is

electromagnetic radiation that does not have sufficient

energy to remove electrons from an atom or a molecule to

form an ion (or charged particle) during collision [30].

Both ionizing and non-ionizing radiations can be harmful

to organisms and can result in changes to the natural

environment. Among the above, ionizing radiation is far

more harmful to living organisms because of its high

energy. The types of ionizing and non-ionizing radiations

are mentioned in Fig. 3.

In biomaterial surface modification, the following radi-

ation types like laser, UV, microwaves, gamma and plasma

have been frequently utilized for improving biocompati-

bility of polymers. Hence, the effect of the above-men-

tioned radiation on the various polymer surface and its

biocompatibility changes are reviewed.

Laser treatment

Laser is a type of radiation used for various medical

applications. In recent days, it plays a vital role in polymer

surface treatment to improve biocompatibility. Khorasani

et al. used CO2-pulsed laser on polydimethylsiloxane

(PDMS) surface, and the results of in vitro assays indicated

that the platelet adhesion was reduced on laser-treated

PDMS. Hence, they concluded that laser irradiation on

silicone rubber is a versatile technique to produce anti-

thrombogenic surface for biomaterial applications [31]. In

a sequel, the same group studied the rate of platelet

adhesion by grafting acrylamide (AAm) on PDMS. Their

result depicted further decrease in platelet adhesion when

compared to unmodified polymers [32]. In further experi-

mentation, authors examined the PDMS coated with NaCl

solid particles. The results showed that the impact of laser

ended in morphological changes of the surface leading to

decreased wettability. scanning electron microscope

(SEM) micrographs and water drop contact angle mea-

surements depicted uniform porosity and super-hydropho-

bic nature on the surface of PDMS. They observed that the

hydrophobicity of the samples depends on the laser

intensity. ATR–FTIR spectra revealed the modified PDMS

surface contains carbonate groups which enrich the oxygen

content of the surface. Energy dispersive X-ray analysis

(EDXA) analysis confirmed the higher percentage of

oxygen on the surface of the modified PDMS [33]. Suggs

et al. used Kr-F excimer laser (248 nm) for surface mod-

ification of polymers namely polymethylmethacrylate

(PMMA), glycol-modified polyethylene (PE) terephthalate

(PETG) and polytetrafluoroethylene (PTFE). They

observed increased surface roughness of treated PMMA

and PTFE, compared to untreated. The right balance of

surface chemistry, surface free energy and surface rough-

ness has been attained which resulted in increasing cell

adhesion on these polymer surfaces [34]. Dadsetan et al.,

Fig. 3 Classification of radiation based on their ionizing power

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studied polyethyleneterephthalate (PET) surface by

exposing it to CO2 -pulsed laser. ATR-IR (attenuated total

reflectance infrared spectroscopy) spectrum showed that

the crystallinity in the surface region decreased due to laser

irradiation. The water drop contact angle also decreased

with increasing laser pulse radiation [35]. Wang et al.

studied effects of PMMA exposed to femtosecond laser

pulses at various laser fluencies and focus distances. It

resulted in the controlled modification of surface wetta-

bility of PMMA. This change in the wettability was sus-

pected to be caused dominantly by laser-induced chemical

structure modification and not by a change in surface

roughness [36]. Mirzadeh et al. used CO2-pulsed laser on

ethylene–propylene rubber (EPR) which was surface

grafted with AAm and 2-hydroxyethylmethacrylate

(HEMA). In grafted EPR, macrophage adhesion and cel-

lular damage decreased after laser irradiation. They also

observed that there is no acute or chronic inflammatory

response at the site of implantation [37]. Tavakoli et al.

used different types of laser source on various polymers to

study the changes caused by the radiation. In general, pre-

treatment with the excimer laser (general order of effec-

tiveness ArF [ KrF [ XeCl) was effective in increasing

adhesion to various substrates under optimised conditions.

Characterisation of high-density polyethylene (HDPE)

showed that at low laser pulse frequencies, there was little

or no physical effect on the surface. Analysis of surfaces of

low-density polyethylene (LDPE) revealed that surface

contamination had been removed by laser treatment. There

was also evidence of an increase in the concentration of

oxygenated surface functional groups with the increase in

the pulse frequency of the laser treatment. Further, they

studied the effects of laser-induced surface topography on

neutrophil, chemokinesis and fibroblast adhesion using two

polymeric substrates; polycarbonate and polyetherimide.

Human neutrophils isolated from blood were exposed to

the surface for 20 min and tracked using image processing

and analysis techniques. They calculated the mean speed

for each cell on each surface and the obtained data were

statistically analysed using multivariate analysis of vari-

ance to determine any significant effect on the speed of

movement due to surface topography. They inferred

improvement in speed and the cells also spread freely on

treated surface compared with untreated which was later

confirmed by SEM and confocal micrographs of the laser-

treated surfaces. The SEM images also revealed increase in

cell adhesion. The results obtained from fibroblasts dem-

onstrated that the textured polymer surfaces showed good

cytocompatibility [38]. The above summary cover few

researches carried on polymers that come under silicone,

polyester, synthetic fluoropolymer and polyacrylate family.

From the results and observations, we can infer that laser

treatment increases surface roughness, surface chemistry

and surface energy of polyester and polyacrylate families

which will boost their cell compatibility. Moreover, for

silicone polymers, laser treatment observed to increase

antithrombogenic property and also reported to produce

super-hydrophobic surfaces. In specific, the PDMS sample

exposed to CO2 laser beam of wavelength 9.58 lm

exhibits reduced platelet adhesion and wettability which

varies depending on the number of pulses and percentage

of peroxide formed on the surface. In case of polyester

polyacrylates and synthetic fluoropolymers like PMMA,

PET and PTFE, the samples treated with CO2 and Kr-F

(248 nm) laser beams observed to have high wettability,

surface roughness, better cellular adhesion. Hence, laser

treatment can be used to tailor the material properties of

the above polymer families which can be utilized for cell

adhesion and antithrombogenic application.

UV treatment

Among the various strategies employing radiation and dis-

charges, UV-assisted surface modification seems to be an

easily applicable and economical method [39]. UV-induced

surface modification has been studied extensively for vari-

ous industrial and biological applications. In industries,

applications of UV may include but not restricted to disin-

fection of packaging materials, disinfection of surfaces,

curing and activation of surfaces [40]. Literature dealing

with UV-induced surface changes of various polymers has

been reviewed to bring out the importance of this method-

ology. To start with, a recent study depicted the modifica-

tions of the surface of flurocarbon polymer using UV. It has

been found that there is a slight decrease in the flourine

content with the formation of carbonyl groups resulting in

the increase of hydrophilicity of polymer fragments [41].

Ramanathan et al. studied UV surface modification of

Polystyrene (PS), polyurethane, polysulfone (PSU) and

polypropylene (PP) in the presence of acrylic vapour, and

further polyurethane and polysulfone samples alone were

treated in presence of trimethoxy propyl silane (TMPSi).

These polymers presented a permanent hydrophilic surface

even after 65 days of treatment process in presence of

acrylic vapour, whereas the polyurethane and polysulfone

samples became hydrophobic when treated in the presence

of (TMPSi) [42]. Olbrich et al. 2007 investigated the UV

surface modification of new nanocomposite for cytocom-

patiability. Their results showed that there was an increase in

the hydrophilicity with the formation of new hydrophilic N

and O groups without altering the surface morphology.

Moreover, human umbilical vein endothelial cell (HUVEC)

line EA.hy926 growth significantly increased in the UV-

treated nanocomposite between 3 and 8 days after seeding

[43]. In a similar study, hydrophobic recovery of biomate-

rials was investigated by Connell et al. They showed that

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hydrophobic recovery proceeds at a different rate for each

polymer, it is generally a two-phase process and that sur-

faces are still more hydrophilic after 28 days than the ori-

ginal untreated state [44]. In another study, Heitz et al.

examined the cell adhesion behaviour of HUVECs and

human aortic smooth muscle cells (HASMC) after exposing

PTFE or Polyethylene terephthalate to excited xenon dimer

(Xe2?) Excimer lamp at a wavelength of 172 nm. UV-

exposed PTFE favoured increased adhering of HUVEC or

HASMC 1 day after seeding and also in the formation of a

confluent cell layer after 3–4 days. In case of PET, HUVEC

adhesion and proliferation rates were almost similar to the

untreated samples but the cells were spread homogenously

over the samples [45]. Subedi et al. showed that UV of

shorter wavelength (254 nm) was efficient in improving the

wettability of the polycarbonate compared to longer wave-

length (375 nm). Further they found that with the increasing

time interval of UV irradiation with shorter wavelength

resulted in the increasing wettability of polycarbonate [46].

Works summarised in this section comprise the researches

carried out on polymers coming under synthetic aromatic,

thermoplastic, polyesters and fluorocarbon families. From

the outcome of the above-mentioned researches, we can

infer that UV treatment improves the wettability and cell

compatibility of the polymers especially those that come

under thermoplastic polymer family. In specific, when the

thermoplastic polymers like polysulfone and PP are treated

with UV in the presence of acrylic acid vapours, their wet-

tability is found to increase significantly which results in

better cellular adhesion and proliferation. Moreover, the

wettability is found to increase with the decrease in wave-

length of UV rays in case of polycarbonate urethane and

polycarbonate when subjected to 172 and 254 nm, respec-

tively. In addition, the fluorocarbon polymers treated with

UV rays of wavelength 254 nm were found to exhibit

increased hydrophilicity due to incorporation of oxygen

molecules on the surface. In contrast, for synthetic aromatic

polymer like PS, the UV treatment increases the hydro-

phobicity resulting in the formation of super-hydrophobic

surfaces. For fluoropolymer like PTFE, the UV treatment

increases cell spreading, adhesion of HUVEC and HASMC

cells, but for PET, it did not produce much difference when

compared to the untreated one. In case of plastic polymers

like polyurethane and thermoplastic polysulfone, it is pos-

sible to produce better hydrophobic surfaces by exposing it

to UV rays in the presence of TMPSi vapour for photolysis

time of 120 and 60 min, respectively.

Plasma treatment

Plasma-surface modification (PSM) is an effective and

economical surface treatment technique for many materials

and of growing interests in biomedical engineering

especially for polymers to increase the biocompatibility.

Plasma treatment involves the modification of a polymer

surface with non-polymerizable gases such as argon, oxy-

gen, nitrogen or fluorine, in a vacuum system. Melnig et al.

treated the polyurethane surface by helium plasma at

atmospheric pressure and implant Ar ? ion in order to

increase biocompatibility using the lactate segment (i.e.)

poly (lactaturethane). The treated surfaces have modified

morphology and promoted cell adhesion. Argon ion beam

surface treatment intensified the cell attachment and

increased the cell growth. It was observed that there was an

increased adherence of fibroblasts and a decreased intensity

of inflammatory reaction on the treated surface compared

with the untreated one. They concluded that the Ar? ion

implantation on membranes improves the biocompatibility

and it made the poly (lactaturethane) suitable for tissue

replacements [47]. Arefi et al. treated the surface of PP with

nitrogen plasma which resulted in increased wettability and

altered the surface conductivity and the adhesive properties

of the polymer [48]. Siegel et al. studied the properties of

polyethylene LDPE, polytetrafluorethylene, PS and poly-

ethyleneterephtalate (PET) modified by Ar plasma. They

found that under the plasma discharge, the polymers were

ablated and their surface morphology and roughness were

changed dramatically and eventually leading to the increase

in the wettability of the polymer surface [49]. Kolska et al.

used argon plasma discharge on PTFE samples. The contact

angle of the modified PTFE decreased with increasing time

of the plasma treatment. After the plasma treatment, dra-

matic changes in the PTFE surface morphology and

roughness were observed. The total oxygen content and

hydrophilicity of the polymer increased with increasing

time of the plasma treatment. However, the contact angle

increased with the ageing of the plasma-modified PTFE

[50]. Rezinckova et al. studied surface modification of

polyethylene (PE), PTFE, PS, PET and PP treated by Ar

plasma. They inferred that under the plasma treatment, the

polymers are ablated and their surface morphology and

roughness were changed dramatically [51]. Khorasan et al.

used radio frequency (RF) plasma treatment in which

O2 was applied to modify the surface of poly (L-lactic acid)

(PLLA) and poly (d, I-lactic acid-coglycolic acid) (PLGA).

They reported that there is an improvement in cell adhesion

which was attributed to the combination of surface chem-

istry and surface wettability during plasma treatment. Cell

culture results showed that B65 nervous cell attachment

and growth on the plasma-treated PLLA was much higher

than on unmodified surface [52]. Slepicka et al. treated PP

surface with argon plasma, the result initially showed

increase in the oxygen concentration on the PP surface;

however, with ageing, the oxygen concentration decreases.

On the other hand, initially the contact angle decreased

rapidly but it increased with the ageing of PP [53]. Su et al.

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exposed PMMA to plasma treatment before being coated

with TiO2 films. Antibacterial properties of the plasma-

treated PMMA surface with TiO2 films were estimated

using two different strains namely S. aureus (gram positive)

and E. coli (gram negative). The results showed the photo-

induced antibacterial activity (after 2 hours of illumination)

was 100 % on both bacterial strains compared with the pure

PMMA. In addition, the surface characterization studies

revealed the superior anti-adhesion capability of TiO2-

coated PMMA surface [54]. Slepicka et al. determined the

surface properties of polymers after plasma treatment. They

examined PET, HDPE, PTFE and PLLA. Their results

indicated that the plasma treatment was immediately fol-

lowed by a sharp decrease of contact angle of the surface.

In the course of ageing, the contact angle increased due to

the re-orientation of polar groups into the surface layer of

polymer. The PLLA samples exhibited saturation of wet-

tability (aged surface) after 100 h, while the PET and PTFE

achieved constant values of contact angle after 336 h. The

changes in the surface roughness and morphology were

observed. Finally, they concluded that plasma exposure had

significantly improved the PTFE biocompatibility [55].

Kasalkova et al. studied the effects of plasma treatment and

subsequent Au nanoparticle grafting of polyethylene (PE).

It leads to changes in surface morphology, roughness and

wettability, thereby significantly increasing the attractive-

ness of the material for cells [56]. Junkar et al. used

radiofrequency (RF) oxygen and nitrogen plasma on PET

surface. Their results showed that by oxygen and nitrogen

plasma treatment, the surface chemistry, wettability and

morphology are altered. Furthermore, plasma treatment

improved the proliferation of fibroblast and endothelial

cells and altered the adhesion properties of platelets.

Interestingly, adhesion of platelets was noticeably reduced

on oxygen plasma-treated surfaces, while adhesion on

nitrogen plasma-treated surfaces was similar to the

untreated ones. They concluded that oxygen plasma treat-

ment is a promising way to improve haemocompatible

properties of PET surface [57]. Lahann et al. investigated

the influence of plasma on metallic stents coated with an

ultra-thin polymer layer by chemical vapour deposition

(CVD). The layer obtained [poly (2-chloroparaxylylene)]

was modified by treatment with a sulphur dioxide plasma,

which resulted in increased hydrophilicity. They also

inferred the improvement of haemocompatibility after

treatment with sulphur dioxide plasma. Platelet adhesion

also seemed to decrease from 85 % for the metallic surface

to 20 % for the CVD-coated and sulphur dioxide plasma-

treated surface [58]. De et al. studied the application of

plasma treatment for confluent cell growth with strong

adhesion to the substrate, in order to withstand the arterial

blood flow shear stress. They revealed that the helium

plasma treatment improved the wettability, oxidized the

surface and enhanced the endothelial cell adhesion on the

polyurethane surfaces. The adhesion of the endothelial cells

on plasma-treated polyurethane varies with plasma treat-

ment time. Stronger adhesion is observed with longer

plasma treatment time [59]. Bilek et al. demonstrated the

antibacterial ability of (air) plasma-treated LDPE grafted

with polyallyamine. The treated samples were coated with

four common antibacterial agents like benzalkonium

chloride, bronopol, chlorhexidine and triclosan. The poly-

allyamine grafting observed to improve the immobilization

of antibacterial agent. The triclosan- and bronopol-coated

samples exhibit the highest and lowest antibacterial activ-

ity, respectively, against gram-positive Staphylococcus

aureus (S. aureus) and gram-negative Escherichia coli

(E. coli) strains. In a sequel, they grafted air plasma-treated

LDPE surface with monomers like allylamine (AA), N-

allylmethylamine (AMA) and N,N-dimethylallylamine

(DMAA) using triclosan for improving the antibacterial

activity. Furthermore, they analysed the ability of each

graft on triclosan anchoring and its corresponding anti-

bacterial activity. Their results depicted that triclosan

anchoring was better on AMA- and DMAA-grafted sur-

faces when compared to the AA-grafted one [60, 61].

From the above concluded researches, it can be inferred

that a wide variety of polymer families such as synthetic

aromatic, plastic, thermoplastic, fluoropolymer and poly-

ester families were subjected to plasma treatment. For

polymers like PP, both the argon and nitrogen plasma

treatment inferred to produce surfaces with better rough-

ness and wettability. Especially, the nitrogen plasma-trea-

ted PP induces more surface conductivity than the pristine

one. This results in increased cell adhesion, proliferation

and spreading. In case of fluoro and polyester polymers like

PTFE, PET, PLLA and PLGA, the plasma treatment

observed to increase surface reactivity, surface roughness

and wettability with the increase in treatment time. The

PET samples treated with N2 plasma of frequency

27.12 MHz at 75 pa were inferred to have good adhesion of

fibroblast and endothelial cells whose presence is highly

valuable for tissue regeneration. The elevated adhesion of

fibroblast and endothelial cells was also reported in plastic

polymers like Polyurethane (700 HZ for 4 min) and

polylactaturethane after Helium plasma treatment. In

addition, the synthetic aromatic polymer like PS and

thermoplastic polymers like PE (coated with Au nanopar-

ticles), PMMA, LDPE and HDPE inferred to have

improved surface morphology, roughness and wettability

after plasma treatment. Moreover, photo-induced antibac-

terial activity against Staphylococcus aureus (gram posi-

tive) and Escherichia coli (gram negative) was observed in

plasma-treated PMMA samples coated with TiO2. Simi-

larly, better antibacterial activity of plasma-treated LDPE

was demonstrated by Bilek et al.

J Mater Sci

123

Microwave treatment

Microwaves are the electromagnetic (EM) waves of wave-

lengths ranging as long as one metre to as short as one

millimetre or equivalently and have the frequencies between

300 MHz (0.3 GHz) and 300 GHz. Generally, microwaves

are used in medical applications like computed tomography

(CT), microwave ablation and surgery, but recently it has

been proved that microwaves can also be used successfully

for surface modification of polymers and fabrics. Mohandas

et al. studied the effects of microwave treatment on metal-

locene polyethylene (mPE). The results of contact angle

studies revealed a decrease in the contact angle of the treated

samples insinuating an increase in the hydrophilicity of the

polymer. The SEM analysis showed increasing surface

roughness and hole formation. The coagulation assays por-

trayed an increase in the clotting time of the microwave-

treated surfaces. They concluded that the microwave-assis-

ted surface modification of mPE resulted in enhanced blood

compatibility [62]. Rabiei et al. used microwaves on poly-

etheretherketone (PEEK) deposited with hydroxyapatite

(HA). Cell culture tests showed a significant increase in the

initial cell attachment and growth on the microwave-

annealed coatings, compared with untreated PEEK. They

concluded that microwaves can be used to increase the

biocompatibility of polyetheretherketone (PEEK) [63]. Ke-

shel et al. exposed polyurethane surface to microwave

plasma with oxygen and argon gases. AFM images showed

improved surface roughness of samples modified with

oxygen plasma in comparison with those modified with

argon plasma. Contact angle analysis showed significant

reduction in the contact angle for samples modified with

oxygen plasma. Cellular investigations showed better

adhesion, growth and proliferation among samples espe-

cially radiated by oxygen plasma [64]. Ginn et al. used an

unmodified ‘‘kitchen microwave oven’’ as the source of

microwaves to modify the surface of PDMS. The radiation

in the microwave oven produced oxygen plasma from the

residual gas. The results illustrated a profound increase in

the hydrophilicity of the PDMS surface. The cellular adhe-

sion property of the polymer was also observed to be

increased [65]. Mutel et al. treated the PP surface with low

pressure microwave cold nitrogen plasma. They used this

process to create adhesion of epoxy resins on PP surfaces to

improve the cell adhesion properties. The microwave treat-

ment increased the wettability and hydrophilicity of the

polymer surface [66]. Badey et al. modified the surface of

PTFE by microwave plasma treatment. They too observed

an improvement in the wettability and hydrophilicity of the

surface. Further, it has been revealed that oxygen content on

the treated surface was higher than that on the normal sur-

face. The XPS analysis also confirmed the oxygen enrich-

ment on the treated surface [67]. Microwave treatment

utilized for various polymers that come under different

polymer families has been summarised in this section. The

treatment on polyolefin polymers like mPE was observed to

have better contact angle, increased surface roughness and

good blood compatibility. Moreover, the above properties

were found to increase with the increase in treatment time

and better results were obtained from the sample treated for

15 min. Likewise, treated samples of thermoplastic poly-

mers, fluoropolymer, silicon polymers and plastic polymers

such as PEEK, PTFE, PP, PDMS and PU show improved

wettability, good cellular adhesion and proliferation. In

addition, the wettability is found to increase with the

increase in exposure time.

Gamma radiation

Gamma radiation is widely used for sterilization of medical

devices in order to eradicate the problem associated with

the pathogen attachment. Recently, Cristina et al. used

gamma radiation on the porous polymer membranes

obtained through alloying poly (hydroxy-urethane) (PHU)

and poly (vinyl alcohol) (PVA) in different concentrations.

These membranes were found to have varying hydrophilic

character, surface energy, resilience and initial elastic

module based on their alloying concentrations. As the dose

of gamma radiation increases, they observed improvements

in the porosity and hydrophilic properties of the sample.

Moreover, roughness of the samples also decreased sig-

nificantly [68]. Since there is a scarcity of studies involving

gamma radiation as a tool for surface treatment, it is dif-

ficult to analyse the advantages of gamma radiation over a

wide variety of polymers.

Conclusion

Biomaterials include metals and its alloys, ceramics,

polymers, composites and biological substances. Polymers

have been used widely as an implant material because of

their tailor-made properties. However, the quality of a

polymeric-implant is decided based on its performance

inside the body with the appropriate host response which

completely depends on the properties of its surface. This

severely limits the use of polymers in medical applications.

In order to eradicate these limitations, various surface

modification techniques namely physico-chemical,

mechanical and biological methods were adopted. Among

those, radiation-induced modification of polymer surface

has gained widespread acceptance in recent days. So, in

this paper, we have reviewed few important works carried

out on polymers utilizing five common radiation

modalities.

J Mater Sci

123

From the results and remarks of various works sum-

marised, we can infer that most of the research has been

targeted on polymers coming under polyester, plastic,

thermoplastic, synthetic aromatic, silicone rubber, poly-

olefin, fluorocarbons and polyacrylate families. Almost all

polymers modified using radiation have been inferred to

exhibit similar changes like change in surface roughness,

wettability, surface energy and surface reactivity. These

changes satisfy tissue regeneration expectations such as

cell adhesion, spreading and proliferation which ended up

in improved biocompatibility of a polymer (Fig. 4). Hence,

the above-mentioned radiation treated polymers can be

utilized for cardiovascular application, tissue regeneration

and orthopaedic applications.

Even though most of the polymers attain desirable

changes after radiation treatment, some polymers like PET,

PU and PDMS were reported to have no significant chan-

ges after the treatment and also properties like wettability

decreased with increase in treatment time. PET when

treated with UV and Laser, PU when treated with UV and

PDMS when treated with laser showed decreased wetta-

bility, no significant surface roughness and poor cellular

adhesion. Although the above materials have decreased

wettability, their hydrophobic nature can be exploited for

applications like catheters where the cell interactions and

cell adhesion are not anticipated. But if we want to rectify

the above problem and intent to utilize the above materials

for cardiovascular applications like vascular grafts, it is

possible by subjecting it to some other radiation modalities.

It is inferred from the works documented that if PET, PU

and PDMS samples were subjected to Laser, Plasma and

Microwave, respectively, it attains better wettability, good

surface roughness and better cellular interactions which

can be utilized for cardiovascular application, tissue

regeneration and orthopaedic applications.

Acknowledgement This work was supported partly by the Ministry

of Higher Education Malaysia with the Grant Vot No.

R.J130000.7809.4F444 and the Ref No: PY/2014/03167.

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