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Materials Science and Engineering A 464 (2007) 93–100

The physical characterization of supermacroporous poly(N-isopropylacrylamide) cryogel: Mechanical strength

and swelling/de-swelling kinetics

Akshay Srivastava, Era Jain, Ashok Kumar ∗Department of Biological Sciences and Bioengineering, Indian Institute of Technology, Kanpur 208016, India

Received 29 September 2006; received in revised form 19 January 2007; accepted 15 March 2007

bstract

Poly(N-isopropylacrylamide) [poly(NiPAAm)] and poly(acrylamide) [poly(AAm)] cryogels were synthesized by radical polymerization at12 ◦C for 12 h using monomers of N-isopropylacrylamide (NiPAAm) and acrylamide (AAm) with N,N-methylene bisacrylamide (MBAAm) as

ross-linking agent, respectively. The cryogels synthesized in freezing conditions provided spongy, elastic and supermacroporous character asompared to the hydrogels synthesized at ambient temperatures. Our earlier observations revealed that the elastic deformation of cryogels either byxternal forces (mechanical deformation) or internal forces (shrinkage-swelling of poly(NiPAAm) cryogels) led to detachment of affinity boundioparticles to these gels, which promises great potential in understanding cell interactions on elastic matrices [M.B. Dainiak, A. Kumar, I.Y.alaev, B. Mattiasson, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 849–854]. The deformation characteristic of cryogels as measured by Young’sodulus indicates that the modulus of elasticity of poly(NiPAAm) cryogel (33–65 kPa) is comparatively lower than the Young’s modulus for

oly(AAm) cryogel (42–86 kPa). The Young’s modulus of both the cryogels was found to be dependent on monomer concentration in cryogels

nd increases with the increase in concentration. Thus, poly(AAm) cryogel are mechanically more rigid than poly(NiPAAm) cryogel. Further, thewelling/de-swelling kinetics study on poly(NiPAAm) cryogel and hydrogel showed, higher swelling ratios for cryogels in the range of 13–16 asompared to poly(NiPAAm) hydrogels which were in the range of 7–10. However, the extent of de-swelling is more in the case of poly(NiPAAm)ydrogels. 2007 Elsevier B.V. All rights reserved.

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eywords: Thermo-responsive cryogels; Swelling/de-swelling; Young’s modul

. Introduction

Polymeric gel is physically or chemically cross-linked net-ork of polymer chains, within which low molecular weight

iquid is immobilized and the amount of solvent present withinhe network is much higher than the amount of polymer con-tituting the network. Specifically, the xerogels which swelln aqueous medium are called as ‘hydrogels’. Based on theesponse to the surrounding medium conditions, hydrogels cane categorized into two classes: (a) conventional hydrogels (b)timuli-responsive hydrogels. The stimuli-responsive hydrogels

emonstrates sensitivity towards various external stimuli suchs pH, temperature, light, ions, electric field, etc. In such classf polymeric gels, the pH and temperature responsive gels have

∗ Corresponding author. Tel.: +91 512 259 4051; fax: +91 512 259 4010.E-mail address: ashokkum@iitk.ac.in (A. Kumar).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.03.057

ryogel elasticity; Mechanical strength of gels

hown great potential in various biotechnological applications1]. The poly(N-isopropylacrylamide) [poly(NiPAAm)] is a wellnown reversibly thermo-responsive polymer which exhibitslower critical solution temperature (LCST) in an aqueous

olution generally at 32 ◦C [2]. The swelling and de-swellingehavior of poly(NiPAAm) hydrogels is due to the change inemperature that causes changes in the fine balance betweenhe elastic force of chains and the interaction between waternd hydrophilic chains [3]. This volume transition from swolleno shrunken state of poly(NiPAAm) hydrogels occurs around4 ◦C [4]. However, such critical temperatures depend on vari-us factors, including molecular weight and chain tacticity forinear polymers, and on the degree of cross-linking and type ofross-linking agent for poly(NiPAAm) hydrogels.

Another category of gels, so called ‘cryogels’, are the gelshat are formed in moderately frozen media [5–7]. Cryogels areolymeric gel matrices that are formed as a result of cryogenicolymerization of low or high molecular weight precursors. The

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4 A. Srivastava et al. / Materials Scienc

ryogels have a continuous system of interconnected macrop-res. The pore size in cryogels is quite large and pore sizes up to00 �m have been obtained in spongy cryogels [8]. Due to pres-nce of such supermacroporus structure these gels exhibit veryow flow resistance and allow unhindered diffusion of solutes ofractically any size. The ‘supermacroporous cryogels’ synthe-ized from poly(acrylamide) or any other gel forming polymersr polymeric precursors have recently been used for variouspplications in biotechnology [8]. Poly(acrylamide) cryogelsave been used successfully in the area of bioseparation [9]pecifically for direct recovery of products from fermentationedia [10], separation of lymphocytes [11], and also human

umor cells [12], capture of enzyme from crude homogenate13] and chromatography of microbial cells by affinity and ion-xchange columns [14]. Also cryogels have demonstrated theirotential in cell [15] and enzyme [16] immobilization, and tissuengineering applications [17].

In one of our recent co-works [18] it was shown that elasticeformation of cryogel can be utilized in physical desorption offfinity bound bioparticles—the phenomenon of particle detach-ent upon elastic deformation was shown to be of a generic

ature, because it was applicable for a variety of bioparticlesf different sizes and nature. This detachment was believed toe caused when either external force like mechanical force ornternal forces within the gel caused deformation of the gel andhus the detachment of the particles by multivalent interactions18]. Using poly(NiPAAm) cryogel the shrinkage and swellingf thermosensitive, macroporous hydrogel upon an increase andecrease of the temperature resulted in the deformation of theel utilizing internal forces [18].

The aim of the present study was to optimize the synthesis ofryogel matrix based on poly(NiPAAm) and to characterize thehysical properties of the poly(NiPAAm) cryogel. The physicalroperties, such as porosity, mechanical strength and swellinginetics of poly(NiPAAm) cryogel were studied and comparedith the corresponding hydrogel. Here, we have studied theasic property of poly(NiPAAm), i.e., temperature responsive-ess in the form of a cryogel. Also the mechanical strengthf poly(NiPAAm) and poly(AAm) cryogel were studied andompared. These studies were done to characterize cryogeleformation through internal force like shrinkage of cryogelpon increase in temperature and by external force like mechan-cal pressure.

. Experimental

.1. Materials

N-isopropylacrylamide (NiPAAm) was purchased fromcros Organics (New Jersey, USA.). Acrylamide (AAm)as purchased from Merck (India). N,N′-Methylene-bis

acryalamide) (MBAAm), ammonium persulphate (APS),,N,N′,N′-tetramethylethylenediamine (TEMED) were bought

rom Sisco Research Laboratories (Mumbai, India). Ethyleneiamine tetraacetic acid (EDTA) was purchased from S.D. Finehemicals Ltd. (Boisar, India.) All other chemicals used weref analytical grade.

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Engineering A 464 (2007) 93–100

.2. Methods

.2.1. Preparation of NiPAAm hydrogelsThe cross-linked poly(NiPAAm) hydrogels of 6, 7 and 8%

otal monomer concentration were prepared by mixing 5, 5.84nd 6.67 g of NiPAAm and 1, 1.17 and 1.33 g MBAAm, respec-ively, in degassed deionized water to a total volume of 100 ml.he mixture was again degassed and TEMED (95 �l) and APS

110 mg) were then added into the reaction mixture. The reac-ion mixture was then poured into 2.5 ml syringe and kept atoom temperature.

.2.2. Preparation of poly(NiPAAm) cryogel (cryogenicolymerization of NiPAAm)

The poly(NiPAAm) cryogels were synthesized by mixing 5 g6%), 5.84 g (7%) and 6.67 g (8%) of NiPAAm monomer andg (6%), 1.17 g (7%) and 1.33 g (8%) of MBAAm in degassedeionized water and the mixture was again degassed and keptn ice for 30 min. TEMED (95 �l) and APS (110 mg) were thendded and mixed thoroughly. The mixture was then poured intoand 2.5 ml syringes and was immediately frozen at −12 ◦C and

ncubated at this temperature for 12 h. After thawing, the gelsere immediately washed with distilled water and were vacuumried and stored at room temperature. Cryogels were synthe-ized by varying monomer concentration of 6, 7 and 8%. Atach concentration the cross-linking agent ratio was also varied.hese were further used for mechanical strength determinationnd swelling/de-swelling studies.

.2.3. Swelling and de-swelling measurementThe kinetics of swelling was carried out following con-

entional gravimetric procedure [19,20]. Briefly, it involvedeasurement of water uptake by samples placed in deion-

zed water, kept in a thermostated water bath at 20 ◦C. Theoly(NiPAAm) hydrogels and cryogels of 6, 7 and 8% wereried at 60 ◦C for 3 days and then kept in vacuum desiccate tillurther use. The dried cryogels were swollen at 20 ◦C in deion-zed water and removed from swelling medium at regular timentervals. The excess water on surface was whipped off by fil-er paper and the weight of all the gels was taken after regularime intervals until the equilibrium was reached. The sampleshydrogels and cryogels) were of 2 cm in length and 1 cm iniameter. At least four samples with similar dimension of eachoncentration of the gel were used for the study.

The water uptake capacity (Wu) (%) is given by:

u = 100 × (Mt − Mg)

Me

here Wu is the water uptake capacity, Mt the weight at regularime interval, Mg the weight of the xerogel, and Me is the weightf water in swollen hydrogels or cryogels at swelling equilibriumt a particular temperature.

The weight-swelling ratio (qw) can also be calculated as:

w = weight of swollen gel (Ms)

weight of xerogel (Mg)

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W

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A. Srivastava et al. / Materials Scie

Similarly the de-swelling kinetics of hydrogels and cryogelsere also performed by the gravimetric method at a constant

emperature of 40 ◦C maintained in a thermostated water bath.he swollen hydrogels equilibrated at 20 ◦C were transferred toater bath maintained at 40 ◦C and weight of all the swollenels were taken before it was transferred to 40 ◦C. After regularime intervals the gels were removed and water was whipped offrom the surface by filter paper. The weight changes of the gelsere recorded during the course of de-swelling at regular time

ntervals. The percentage of water retention (Wr) is given by:

r = 100 × Mt − Mg

Me

here Mt is the weight at regular time interval, Mg the weightf the xerogel, and Me is the weight of water in hydrogels orryogel at de-swelling equilibrium at a particular temperature.

Poly(NiPAAM) cryogels with varying cross-linking agentoncentration ratios were further selected to study swelling/de-welling kinetics by above-mentioned procedure to determinehe effect of cross-linking agent on swelling/de-swelling kineticsf the cryogels.

The physical change in dimension of cryogels was also deter-ined by increasing and decreasing the temperatures above and

elow LCST of poly(NiPAAm). Initial diameter of the cryogelas determined at room temperature that is at 25 ◦C and then the

ryogels were placed in water bath at 40 ◦C for a definite time.he decrease in diameter of the cryogel (T) was determined as

ollows:

= D25◦ − D40◦

here D25◦ is diameter of gel at 25 ◦C and D40◦ is diameter at0 ◦C.

The (T) value, i.e., decrease in diameter of gels caused due tohrinkage of gels as a result of increase in temperature also deter-ines the thermoresponse of the gels. The greater the change in

iameter (T) by increasing the temperature, better the thermore-ponse of the gel.

.2.4. Mechanical strengthThe compression test on poly(NiPAAm) cryogel and

oly(AAm) cryogel was performed using uniaxial compressionest. The samples were tested by mechanical tester (NI DAQard USB 6009 with labview software and load cell from Eltek),here the samples were placed between two arms of load frame

nd then compressed up to 80% of the total length, from wherehe compressed cryogel can regain its original shape on additionf liquid. The applied force was recorded and change in columnength due to compression was measured.

The compression modulus of cryogel monolith was estimatedsing the equation:

= F/A

�l/lkPa

here E is the elastic modulus, F the applied force, A the cross-ectional area of the test sample, l the initial length of the testample and �l is the change in length under the compressiveorce.

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nd Engineering A 464 (2007) 93–100 95

.2.5. Measurement of flow resistance of cryogel columnThe flow resistance of the cryogel columns (5 ml) evaluated at

ow rates of 1–5 ml/min was determined using peristaltic pump,egistering the flow rate at given pump settings. In a separatexperiment, the pump settings were calibrated against flow rateith no column connected according to Adrados et al. [21].

.2.6. Scanning electron microscopic (SEM) analysisPoly(NiPAAm) cryogel and hydrogels of different concen-

rations were subjected to SEM analysis. All the samples werethanol dried [12]. The samples were put consecutively inncreasing concentration of ethanol that is 20% (v/v), 40% (v/v),0% (v/v), 80% (v/v) and finally in 100% (v/v) ethanol. The sam-les were then vacuum dried overnight before gold coating. TheEM pictures were taken using FEI Quanta 200 and the poreiameters of cryogel column were measured arbitrarily.

. Results and discussion

.1. Synthesis and optimization of poly(NiPAAm) cryogel

The process of poly(NiPAAm) cryogel formation is same ashat for polyacrylamide cryogels. The poly(NiPAAm) cryogel

atrices were synthesized by co-polymerization of monomersf NiPAAm and MBAAm as cross-linking agent. The monomersere mixed under chilled conditions and the polymerization was

llowed to proceed at sub-zero temperature until completion.he poly(NiPAAm) cryogel columns were made at −12 ◦C.he gels were formed completely only after 12 h. The condi-

ions employed to make the poly(NiPAAm) cryogel ensuredinimum competition between the factors facilitating gelation

nd factor decelerating it (low temperature, high viscosity innfrozen liquid microphase). A temperature regime lower than12 ◦C will cause the formation of smaller and numerous sol-

ent crystals and hence smaller pore sizes. This has been wellstablished from earlier works published on poly(AAm) cryogel,hich defines the optimum range of temperature to lie between10 and −15 ◦C for formation of supermacroporous structure

5].The synthesis of poly(NiPAAm) cryogel involves cryotropic

elation and polymerization of (NiPAAm) via free radical poly-erization. The principle and mechanism of cryogelation is

iscussed elsewhere [9]. The different concentration range ofoly(NiPAAm) cryogel with varying weight ratio (w/w) ofiPAAm to MBAAm from 5:1 to 20:1 were synthesized inrder to optimize and determine the effect of cross-linking onwelling-shrinkage behavior of poly(NiPAAm) cryogels. Theryogel were selected such that they have large contractionesponse to temperature and a good flow rate. Hence, the ther-al shrinkage and flow rate of all the synthesized gels were

etermined and compared. The comparative study of thermalhrinkage, morphology and flow rates of poly(NiPAAm) cryo-el made at different ratio of NiPAAm to MBAAm are shown

n Table 1 and visually can be seen in Fig. 1. From the physicalbservation of the gels it was shown that as the concentra-ion of the monomer increases from 6 to 8%, the cryogelecomes more rigid and less spongy. This may be due to forma-

96 A. Srivastava et al. / Materials Science and Engineering A 464 (2007) 93–100

Table 1Physical properties of poly(NiPAAm) cryogels

Total monomer concentration (%) Ratioa Flow rate (ml/min) Shrinkage at 40 ◦C (mm) Physical characteristic

6

5:1 2 1 Less spongy10:1 1.4 2 Spongy15:1 1.3 4 Spongy20:1 1.1 5 Very spongy

7

5:1 4.7 1 Less spongy10:1 4 2 Spongy15:1 3.2 2 Spongy20:1 2.5 5 Very spongy

8

5:1 1.8 1 Less spongy10:1 1.3 1 Spongy

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a Concentration of NiPAAm/concentration of MBAAm.

ion of thicker walls. The thermal shrinkage of cryogel variesithin each concentration and increases as the concentrationf cross-linking agent decreases. This is shown by the facthat 6 and 7% poly(NiPAAm) cryogel shows thermal shrink-ge in the range of 1–5 mm in diameter for the ratio of 5:1–20:1NiPAAm:MBAAm) but 8% cryogel at 20:1 ratio showed lesshrinkage up to 3 mm in diameter due to thicker walls of cryogel.he 7% cryogel shows better morphology and flow rate in the

ange of 2.5–4.7 ml/min than 6 and 8% cryogel. Fig. 1 demon-trates the shrinkage of poly(NiPAAm) cryogel in response toncrease in temperature above its LCST.

On the other hand, the control gel (hydrogel) was allowedo polymerize at room temperature and it has been foundhat control gel polymerized within 1–2 h as compared tooly(NiPAAM) cryogels which need approximately 12 h orbove to polymerize at −12 ◦C. The hydrogel adopted transpar-nt, glassy and rigid morphology and all the solvent was boundithin polymer network. Contrarily, poly(NiPAAm) cryogelsaving exactly the same chemical composition had heterophasic

on-transparent morphology with spongy and elastic character.n poly(NiPAAm) cryogels solvent is retained within the gel bothue to binding by polymer network and entrapment within cap-llary. The binding of solvent molecules on the poly(NiPAAm)

ig. 1. Digital photographs of 7% supermacroporous poly(NiPAAm) cryogel:A) dried poly(NiPAAm) cryogel, (B) water swollen poly(NiPAAm) cryogel,C) de-swollen poly(NiPAAm) cryogel after keeping in water at 40 ◦C.

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dtdwoto3v

2 Spongy3 Very spongy

ackbone is through hydrogen bonding and single NiPAAmonomer has at least four sites for hydrogen bonding withater (two lone pairs from carbonyl oxygen and one each fromitrogen and hydrogen amide atoms).

.2. Poly(NiPAAm) cryogel morphology

The mechanism for formation of poly(NiPAAm) cryogel isimilar to that of polyacrylamide cryogel. When the polymer-zation of monomer solution is allowed to proceed at sub-zeroemperature, cryoconcentration of monomers takes place in non-rozen solvent phase where they polymerize and results in ahemically cross-linked poly(NiPAAm) cryogel [9].

The morphology of poly(NiPAAm) cryogel formed at12 ◦C is clearly reflected in the SEM picture taken at low

acuum (Fig. 2). The poly(NiPAAm) cryogel prepared at 7%otal monomer concentration have interconnected and largeore size in the range of 30–99 �m. The interconnectivity ofores determine the convective flow of solvent while large poreize demonstrates the potential of cryogel for particle process-ng, like cells, cell organelles and inclusion bodies [9–12,22].ighly porous structure and sufficiently large pore size ofoly(NiPAAm) cryogel provides non-hindered diffusion of allolutes including macromolecules. The porosity, interconnectiv-ty and convective liquid flow of all these poly(NiPAAm) cryogelormed at different monomer concentration deciphers the flowate of the corresponding poly(NiPAAm) cryogel (Table 1).

.3. Compression analysis of cryogel monolith

The elastic and compression properties of the cryogel wasetermined by exerting physical stress on the gel which was inurn used to calculate Young’s modulus, which is a mathematicalescription of an object or substance’s tendency to be deformedhen a force is applied to it. Fig. 3 shows the Young’s modulusf each cryogel sample of same size as calculated by analyzing

he stress and strain values of each cryogel. The Young’s modulif poly(NiPAAm) cryogel were calculated to be in the range of3–65 kPa which was less than poly(AAm) cryogel which showalues in the range of 42–86 kPa. The poly(NiPAAm) cryogel

A. Srivastava et al. / Materials Science and Engineering A 464 (2007) 93–100 97

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ig. 2. Scanning electron microscopy pictures of: (A) supermacroporus poly(Nias 7% and NiPAAM:MBAAm ratio was 5:1.

ndergoes more strain up to 90% as compared to 80% com-ression in poly(AAm). It is well demonstrated that the stressequired by the poly(NiPAAm) cryogel to undergo compres-ion is less than the poly(AAm) cryogel which infers that theoly(NiPAAm) cryogel is more elastic and soft. It was also seenhat with the change in total monomer concentration from 6 to%, the rigidity of cryogels is also altered. As the concentra-ion increases the sponginess and elasticity decreases which inurn decreases the compressibility and squeezability of cryogel.his is due to the fact that at high concentration of monomer,

he cross-linking increases the rigidity of cryogel and in turnhe elastic behavior decreases. It can be assumed that as theross-linking agent concentration increases in total monomeroncentration, it causes the formation of more compact and

ig. 3. Comparative study of mechanical strength of poly(NiPAAm) andoly(AAm) cryogel. The Young’s modulus of 6, 7 and 8% poly(NiPAAm) (�)nd poly(AAm) (�) cryogel. The parameters were determined at 80% compres-ion from where the cryogel regains its original shape after swelling in water.or details see Section 2.

mcikio

3

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) cryogels and (B) poly(NiPAAm) hydrogel. The total monomer concentration

igid cryogels. If the compression force applied on cryogel isncreased further, the gel gets deformed and the original lengthannot be regained. In contrast when hydrogels were tested forompression, it was not possible to apply the compressing forces the gel could not withstand the applied force and broke down.

It is observed that poly(NiPAAm) cryogel is more spongyhan poly(AAm) cryogel and undergoes greater change in length.ne of the potential applications of these cryogels, that has

ecently been established, is in detachment of bioparticles whichre attached/adsorbed to the surface of cryogel [18]. This detach-ent of bioparticles is facilitated by elastic deformation of

ryogels. Thus, it can be said that elasticity of cryogel is anmportant factor for such applications. It would be beneficial tonow elasticity of cryogels in mathematical terms and the max-mum force up to which they can regain their shape, which isne of the aims of the present study.

.4. Swelling/de-swelling kinetics

The swelling kinetics of poly(NiPAAm) (5:1) cryogels andydrogels of different monomer concentration studied by gravi-etric method is shown in Fig. 4. Both the gel systems were

ompared on the basis of the time required to reach a particu-ar value of Wu (or Wr). It is clearly evident from the obtainedata that poly(NiPAAm) cryogel irrespective of the monomeroncentration attains swelling equilibrium within 20 min, whileydrogels of similar monomer concentration took more than 2ays to reach equilibrium. This difference in swelling kineticsf poly(NiPAAm) cryogels and hydrogels is due to the basicifference in their pore morphology and wall thickness. Thoughhe conventional hydrogels consist of an interconnected networkf pores (the pore size is rather small and the distance betweenores is long) made up of thick walls. In comparison to this,

ryogels have pores that are quite large (up to 200 �m) and arenterconnected via thin walls. This allows fast transport of sol-ent molecules within thin walls over short distances across theacroporous structure. This phenomenon is quite useful specif-

98 A. Srivastava et al. / Materials Science and Engineering A 464 (2007) 93–100

Fig. 4. Swelling kinetics of poly(NiPAAm) gels. Water uptake capacity wasdp(

iitmtaopgmttpirtrpcwtar

Fig. 5. Swelling ratio of 6, 7 and 8% poly(NiPAAm) hydrogel and cryogel.Swelling ratio of poly(NiPAAm) hydrogel (filled bar) and cryogel (wide upwarddw

eoctdmp

fTchrmmrriCsgobip

c(ml

etermined at increasing time interval for (A) poly(NiPAAm) cryogel and (B)oly(NiPAAm) hydrogel at three different monomer concentrations: (�) 6%,�) 7% and (�) 8%. For details see Section 2.

cally in case of responsive gels like that of poly(NiPAAm). Onemportant performance criteria for stimuli-responsive systems ishe rate at which they respond to any change in their environ-

ent. It is obvious that response time depends upon the size ofhe system, larger the thermo-sensitive gel slower is its swellingnd shrinkage as the response time required depends upon ratef heat and mass transfer process as well as the distance of theeriphery of the gel to the center of gel. This response time isreatly reduced in macroporous gel structure as the heat andass transfer process takes place only at short distance in the

hin walls of the macropores contrary to the long distances ofhe conventional gels [9]. One of the characteristic feature ofoly(NiPAAm) cryogels is their rapid response to any changen temperature. In response to change in temperature of its envi-onment poly(NiPAAm) changes its property from hydrophilico hydrophobic or vice versa above or below its LCST (32 ◦C),espectively [23]. The rapidity of response in such supermacro-orous structure depends upon total monomer concentration,ross-linking density, pore wall thickness, temperature, etc. at

hich the gels are prepared. The response time is also affected by

he thickness of gel, i.e., the distance between the outer bound-ries to central parts of cryogel, larger the distance slower theate of swelling and shrinkage due to slow rate of mass and heat

ttcg

iagonal bar). Swelling ratio was determined as the ratio of wet weight to dryeight of hydrogel and cryogel. For details see Section 2.

xchange due to increased distance [24–26]. Interconnectivityf pores plays a crucial role in fast swelling and de-swelling ofryogels as solvent molecule could move by convection acrosshis network, while in conventional hydrogels this process isiffusion dependent and thus slower. This difference in poreorphology leads to a faster swelling/de-swelling kinetics in

oly(NiPAAm) cryogels.The swelling ratio (qw) of poly(NiPAAm) cryogels ranges

rom 13.5 to 16, while that of hydrogels is in the range of 7–10.he swelling ratio in cryogels decreases slightly as the monomeroncentration increases from 6 to 8% with 6% gels having theighest swelling ratio of 16 (Fig. 5). This decrease in swellingatio with concentration can be explained on the basis that as theonomer concentration increases, wall thickness increases andore rigid and less porous cryogel is formed, thus exhibiting

educed swelling. As the porosity of gel increases the swellingatio increases because large amount of water molecule diffusesnside the gel with high porosity than the low porous gel system.onnectivity of pores plays a crucial role and leads to faster

welling rate of the gels. Water can enter or leave the cryo-el through interconnected pores by convection. Similar studiesn swelling/de-swelling kinetics of ionic poly(AAm) cryogelased on the volume changes of the gel have demonstrated thatonic poly(AAm) cryogel swells and de-swells much faster thanoly(AAm) hydrogel [27].

The swelling/de-swelling weight ratio of poly(NiPAAm)ryogel with varying concentration of cross-linking agentMBAAm) was also studied and it was seen that, there is notuch difference in swelling/de-swelling ratio on varying cross-

inking agent concentration (Fig. 6). The swelling ratio of allhe cryogel at different cross-linking agent concentration are in

he range of 17–21, the cryogel with 1% cross-linking agentoncentration have little higher swelling ratio than other cryo-els with a higher cross-linking agent concentration. It may be

A. Srivastava et al. / Materials Science and Engineering A 464 (2007) 93–100 99

Fig. 6. The swelling/de-swelling ratio of poly(NiPAAm) cryogel at differentconcentration of cross-linking agent. Swelling ratio (A) and de-swelling ratio (B)o(d

ptmnbcscgac

gcIovthd

Fig. 7. De-swelling kinetics of poly(NiPAAm) gels. Water retention capacityw(a

adtpmsmbIsstotocwp

f poly(NiPAAm) cryogel at cross-linking agent concentration of: (�) 1 wt%,�) 1.25 wt% and (�) 1.5 wt% at different time interval during swelling. Foretails see Section 2.

robably because the concentration of cross-linking agent effecthe cross-linking of polymer at localized concentration which

akes the pore wall of the cryogel loose or rigid but it wouldot affect much on the interconnectivity of pores and pore distri-ution. The solvent molecules still travels through the walls byonvection and moves in and out of the gel structure resulting inwelling and de-swelling of cryogel which can be independent ofross-linking agent concentration. These poly(NiPAAm) cryo-els with different cross-linking agent concentration undergoesbout 25% de-swelling, which was found to be independent ofross-linking agent concentration.

De-swelling kinetics of poly(NiPAAm) cryogel and hydro-el was determined at 40 ◦C for three different initial monomeroncentration. A graph of Wr versus time was plotted (Fig. 7).t can be clearly seen from these results that de-swelling ratef poly(NiPAAm) cryogels is 10–15 times faster than the con-

entional hydrogels. Also it can be seen that the cryogels attainheir de-swelling equilibrium almost instantaneously while theydrogels show a two phase response curve, in which maximume-swelling occurs in the first 30–50 min while the equilibria is

aww

as determined at increasing time intervals for: (A) poly(NiPAAm) cryogel andB) poly(NiPAAm) hydrogel for three different concentrations (�) 6%, (�) 7%nd (�) 8%. For details see Section 2.

chieved slowly over a period of 350 min. This difference in thee-swelling kinetics of cryogel and hydrogel can be attributedo the mechanism by which the solvent transport occurs in theseolymer networks. In conventional hydrogel systems the solventoves in and out of the polymer network by diffusion while in

upermacroporus like structures as found in cryogels the solventoves by convection through the thin walls around the pore. The

iphasic response seen in hydrogels can be explained as follows.nitially as the gel samples are placed above LCST only the outerurface chains attain the surrounding temperature and begun tohrink while due to slow heat transfer by diffusion in hydrogelshe inner surface comes in equilibrium with the surroundingsnly after a lag period. This difference in response time leadso formation of two layers within the hydrogel consisting of anuter layer largely in de-swollen state while the inner layer isonstantly decreasing as more and more de-swelling takes place,hich is a slow process and thus leads to an extended de-swellinghase after initial rapid response.

Further the de-swelling kinetics of poly(NiPAAm) cryogels shown in Fig. 7 demonstrates about 25% de-swelling or 75%ater retention capacity. Thermo-induced de-swelling ratio orater retention capacity of the cryogel is found to be smaller

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han that for conventional hydrogels. This may be due to theact that the polymeric chains in cryogels are more rigid and doot collapse to the same extent as in hydrogels. High rigidity ofryogel walls is also evident by the fact that the pores in cryo-els do not collapse even when the gels is in dried state. Thiseature of thermal shrinkage of poly(NiPAAm) cryogel whileaintaining the pore morphology combined with elastic defor-ation of smart cryogel by internal force can give vent to its

ovel applications [18].

. Conclusion

Here the physical properties of poly(NiPAAm) cryogel wasompared with poly(NiPAAm) hydrogel. The study demon-trates that poly(NiPAAm) hydrogel is less elastic thanorresponding cryogel which is much more elastic and canegain their shape as the load is removed. The elasticity ofryogel is found to be monomer concentration dependent andryogel with higher initial monomer concentration is less elas-ic. The swelling/de-swelling kinetics demonstrates that the ratef swelling and water uptake capacity of poly(NiPAAm) cryo-el is greater than the corresponding hydrogel. The swellingatio of poly(NiPAAm) cryogel is much more than smart hydro-el but the percentage of water retention is greater in hydrogel.owever, the rate of thermo-response of poly(NiPAAM) cryo-el is greater than hydrogel but the de-swelling ratio is highor poly(NiPAAm) hydrogel. The amount of cross-linking agentlso affects the swelling kinetics of poly(NiPAAm) cryogel andhe swelling ratio decreases as the cross-linking agent concen-ration increases.

The physical characterizations done in this study will beffective in designing of these cryogels suitably for biosepa-ation, drug delivery, biosensor, immobilization of bioparticlesnd tissue engineering.

cknowledgements

The work was financially supported from the grants from

ndian Institute of Technology, Kanpur, India. AS and EJ grate-ully acknowledges the fellowships received from Universityrants Commission and Department of Biotechnology, Gov-

rnment of India organizations, respectively.

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Engineering A 464 (2007) 93–100

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