poly(1-vinyl-2-pyrrolidinone) hydrogels as vitreous substitutes: a rheological study

Polymer International 46 (1998) 183È195

Poly(1-vinyl-2-pyrrolidinone) Hydrogelsas Vitreous Substitutes: a Rheological

Study

Traian V. Chirila* & Ye Hong

Lions Eye Institute, Department of Biomaterials and Polymer Research, 2 Verdun Street, Block A, Nedlands,Western Australia 6009, Australia

(Received 3 June 1997 ; accepted 16 October 1997)

Abstract : In order to develop an artiÐcial vitreous, a large series of hydrogelshave been previously produced by polymerization of 1-vinyl-2-pyrrolidinone,with or without crosslinking. Based on the assumption that a functional vitreoussubstitute should possess viscoelastic properties after its delivery, a number ofselected gels were characterized rheologically by both oscillatory shear stressanalysis and shear creep analysis, using a controlled stress rheometer in the cone/plate conÐguration. The experiments demonstrated a dramatic e†ect of injectingthe gels through small-gauge needles, as many lost their viscoelasticity to becomefree-Ñowing Ñuids, probably because of the cleavage of chains and crosslinks. Itwas also found that the increase of comonomer content (2-hydroxyethylmethacrylate) and of crosslinking level generally had a strengthening e†ect.However, the e†ects of hydrophilic crosslinking agents (diallyl ether and divinylglycol) were irregular. Eventually, only four hydrogels in this series showed visco-elastic characteristics after injection through a 30-gauge (0É13 mm diameter)needle, maintaining behaviour typical of crosslinked networks and warrantingfurther assessment as potential vitreous substitutes. 1998 SCI.(

Polym. Int. 46, 183È195 (1998)

Key words : artiÐcial vitreous ; poly(1-vinyl-2-pyrrolidinone) ; hydrogels ; injec-tion ; oscillatory shear ; shear modulus ; creep compliance

INTRODUCTION

The vitreous body of the eye, also termed vitreoushumour or, simply, vitreous, is a transparent jelly-likesubstance occupying two-thirds of the ocular volume.Although water is quantitatively its predominant com-ponent, the vitreous is an evolutionary product with a

* To whom all correspondence should be addressed.Contract/grant sponsor : National Health and MedicalResearch Council of Australia.Contract/grant number : 940707.Contract/grant sponsor : Australian Foundation for the Pre-vention of Blindness (Western Australia), Inc.Contract/grant sponsor : Lions Clubs International Founda-tion, Oak Brook, IL, USA.

unique macromolecular organization made up of aprotein (collagen), a polysaccharide (hyaluronan), andproteoglycans, structured in such a manner that theresulting material has sufficient strength and elasticityto fulÐl all functional demands. The vitreous providesan adequate support for the retina, protects the sur-rounding ocular tissues from external adversities, allowslight to reach the sensory elements at the back of theeye, and allows the circulation of metabolic solutes. Tosum up, the vitreous contributes essentially to the main-tenance of the eye itself as an independent globularorgan in vertebrates.

There are two circumstances in which the availabilityof an artiÐcial substitute for the vitreous, either as asupplement or as a total replacement, would reduce the

1831998 SCI. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

184 T . V . Chirila, Y . Hong

TABLE 1. Code names and composition of PVP

synthetic hydrogelsa

Code nameb Comonomer Crosslink agentc

(HEMA) (wt%)

(wt%)

*E2 – EGDMA (0·25)

*E9 1 EGDMA (0·1)

E10 1 EGDMA (0·25)

E12 1 EGDMA (0·5)

E16 2 EGDMA (0·1)

E21 3 EGDMA (0·1)

*D11 1 DEGDMA (0·25)

D13 1 DEGDMA (0·5)

D22 2 DEGDMA (0·25)

T12 1 TEGDMA (0·25)

T13 1 TEGDMA (0·5)

T14 1 TEGDMA (0·75)

T22 2 TEGDMA (0·25)

A10 5 DAE (0·25)

*A11 5 DAE (0·5)

*A12 5 DAE (0·75)

A14 7 DAE (0·25)

*G12 1 DVG (0·5)

*G13 1 DVG (0·75)

*G14 1 DVG (1·0)

*G15 1 DVG (1·5)

*G21 2 DVG (1·0)

*G24 3 DVG (0·1)

G25 3 DVG (0·25)

G26 3 DVG (0·5)

G27 3 DVG (0·75)

G28 3 DVG (1·0)

*G29 3 DVG (1·5)

G30 3 DVG (2·0)

*P4 3 – –

a All polymers were synthesized in the presence of 2,2¾-

azo-bis-(2,4-dimethyl valeronitrile) as initiator (0·1 wt% of

monomers).

b The polymers marked by asterisk were selected as a result

of previous studies ;10 the other polymers are included in

this study for comparison.

c For acronyms see text.

occurrence of poor vision or blindness in humanpatients. The vitreous body itself can become dys-functional due to disease or trauma. A damaged vitre-ous leads to opaciÐcation, or to its detachment from theretina, both conditions causing poor vision or blindness.However, it is the surgical treatment of retinal detach-ment (a pathological condition caused by the separationof the neural retina from the underlying pigmentepithelium) that primarily requires the availability of avitreous substitute, ideally to be injected into the vitre-ous humour at the time of surgery. The purpose ofinjecting a Ñuid into the vitreous cavity is the resto-ration of the volume and internal pressure of the ocular

globe, because the natural vitreous usually collapsesduring surgical intervention. The injected materialshould be able to exert a hydraulic pressure so as toprovide tamponade to the retina against the choroid,and also to alter the characteristics of the vitreous inorder to relax and prevent vitreoretinal tractions.

The substitution of the vitreous body is a complexissue. Because an ideal permanent vitreous substitutemust fulÐl a set of rather stringent requisites,1h3 it is notsurprising that no such material is currently available inspite of a century of recorded attempts. The success, astemporary substitutes of some materials (saline solu-tions, gases, perÑuorinated compounds, silicone oils) isalso very limited. Historically, the research on artiÐcialvitreous is distinguished by an ambiguous method-ology : generally, the materials were chosen and testedbecause of their transparency and ease of injection, andno systematic investigations of toxicity, retention in thevitreous humour, rheological characteristics andinjection-induced e†ects were done on them before invivo experiment. The existing literature is full of incom-plete research and lack of details, which eventually pre-clude valid conclusions on most of the materials testedas vitreous substitutes. The introduction of syntheticpolymers as candidates for vitreous substitution did notchange the methodology signiÐcantly, at least not untilrecently (see below).

In our own quest for an artiÐcial vitreous substitute,we produced a large number of polymers of 1-vinyl-2-pyrrolidinone (VP) including crosslinked homo-polymers, together with copolymers of VP with2-hydroxyethyl methacrylate (HEMA), both cross-linked and uncrosslinked. Five di†erent crosslinkingagents were employed : ethyleneglycol dimetha-crylate (EGDMA), diethyleneglycol dimethacrylate(DEGDMA), tetraethyleneglycol dimethacrylate(TEGDMA), diallyl ether (DAE), and 1,5-hexadiene-3,4-diol also known as divinyl glycol (DVG).

Following hydration to equilibrium, all these poly-mers became hydrogels with high or very high watercontents. A number of these materials were character-ized by oscillatory shear stress analysis,4,5 and by arapid immunocytochemical toxicity assay.5,6 Theirretention in animal vitreous humour was evaluated byvarious methods, including Fourier transform infraredspectrometry,7 histopathological analysis,8 and by mon-itoring the residual radioactivity of radiolabelledmaterials.9 We have suggested that most of these assess-ments, together with in vitro biodegradation tests andexamination of physical and optical properties, shouldbe done before any in vivo assessment, thus avoidingexpensive and time-consuming surgery and unnecessaryanimal sacriÐces. In a major study,10 342 di†erenthydrogels produced by polymerization of VP were sub-jected to an extensive analysis of their physical appear-ance, mechanical behaviour upon handling, waterabsorption at equilibrium, injectability, and optical

POLYMER INTERNATIONAL VOL. 46, NO. 3, 1998

Rheological study of PV P hydrogels as vitreous substitutes 185

properties (visual acuity, total light transmission, trans-mittance at 550 nm wavelength, refractive index). Thisthorough investigation indicated that no more than 13hydrogels in this series satisÐed the physical and opticalprerequisites for a potential vitreous substitute.

According to principles of rheology, an ideal viscousÑuid should obey NewtonÏs law, that the stress (p) isproportional to the rate of strain (dc/dt) but indepen-dent of strain (c). The proportionality coefficient isdeÐned as the viscosity (g). All the mechanical workexpended in producing the deformation is dissipatedinstantaneously. A purely elastic material should obeyHookeÏs law, that the stress is proportional to theresulting strain but independent of the rate of strain,and the proportionality coefficient in this case is deÐnedas either shear (G) or tensile (E) modulus. If the stress isremoved, the strain returns instantaneously to zero ; inother words, the elastic material has undergone a fullyrecoverable deformation and no Ñow has occurred. Allthe mechanical work is stored as elastic energy. Inreality, there is no material behaving either as an idealNewtonian Ñuid, or as an ideal elastic solid. Mostmaterials possess elements of viscous Ñow and elasticity,i.e. they are viscoelastic. In order to ideally reproducethe properties of the natural vitreous body,11h17 webelieve that an artiÐcial vitreous should be a viscoelasticmaterial which should maintain this quality after injec-tion through small-gauge needles. Injection is the pre-ferred method for delivering a vitreous substitute.Having taken this into consideration, the aim of thepresent study is to further characterize the remainingselected hydrogels10 (and a few more for comparison)by rheological procedures including a dynamic tech-nique (oscillatory shear stress analysis) and also a statictechnique (shear creep analysis). The study is focused ondetermining the linear viscoelastic response of hydro-gels.

EXPERIMENTAL

Materials

The polymers investigated in this study are presented inTable 1. Their synthesis has been described in detail inmany of our previous reports.4,5,8,10 BrieÑy, bulk poly-merization was carried out in a unit designed by us,under nitrogen, for 30 h at 45¡C and 8 h at 70¡C. At theend of polymerization, the polymer specimens wereremoved from the moulds and further cured for 6 h at100¡C. The polymers were hydrated in deionized/distilled water for 2 weeks, when the weight of speci-mens usually became constant. The fully hydrated gelswere stored in deionized water and not allowed to dryout before measurements.

Rheological measurements

A controlled stress rheometer (Bohlin CS-10, Sweden)was used for all rheological measurements (yield stress,oscillatory shear, and shear creep), which were per-formed at room temperature. Fully hydrated gelsamples of approximately 1É2 ml were delivered througha large diameter cylinder, or injected through 16-gaugeand 30-gauge needles. The gels were placed between a60 mm diameter lower plate and a 4¡/40 mm diametercone truncated to give a gap of 0É15 mm.

Yield stress analysis

The yield stress of a material is usually deÐned as thestress below which no Ñow occurs. Consequently, wecan be sure that measurements using any stress (orresponding strain) before the yield point are in thelinear region. There are three common methods usedfor determining the yield value : (1) Ñow curve method,(2) step stress test and (3) ramp stress test. The lastmethod was employed in this study, which involvesapplying a gradually increasing stress and monitoringthe instantaneous viscosity for an inÑexion of the curve,i.e. the onset of Ñow. The shear stress applied to thesamples was varied between 0É06 and 10 Pa. The yieldstress (strain) was determined before the oscillatoryshear experiments.

Oscillatory shear stress analysis

Oscillatory shear experiments and the resulting plots ofreal (G@, the storage modulus) and imaginary (GA, theloss modulus) parts of the complex dynamic shearmodulus versus the frequency (l) or angular velocity (u)of oscillatory stress, sometimes termed “mechanicalspectraÏ, or “dynamic mechanical spectraÏ, are currentlyused to demonstrate the gel character, and to discrimi-nate between di†erent classes of gels such as entangle-ment networks or covalently crosslinked gels.18h20 Thelatter gels exhibit a storage modulus (representing therecoverable energy stored as elastic energy) which isalways higher than the loss modulus (representing theenergy dissipated upon deformation), and the G@(l) andGA(l) plots are parallel and relatively frequency insensi-tive, i.e. the slopes are approaching zero. In the case ofentanglement gel networks, there is a crossover in G@(l)and GA(l) plots as the frequency decreases. At low fre-quencies GA [ G@, and the material is a free-Ñowing Ñuidrather than a resilient gel.

The oscillatory technique used is to apply a stresswhich changes according to a sine wave equation. Thus,the induced response (strain) will also follow a sinewave. In these conditions, the sample is continuouslyexcited, but its structure will not be destroyed if thestrain is maintained at a low level, well below the yield



Fig. 1. Mechanical spectra and loss tangent plots of hydrogels E9, A11, G12, G13, G14 and G29 after injection through a 30-gaugeneedle. Code names are explained in Table 1.

strain as determined by the yield stress analysis. If theoscillation frequency is set too high then the raw phaseangle, which the rheometer uses to compute G@ and GA,approaches 90¡ and the instrument error becomes high.Therefore, the oscillation frequency was varied from0É001 to 0É1 Hz, to be applicable to all gels tested in thisstudy.

Because sufficient resilience of the material, when sub-jected to shear stress, is also desirable for vitreous sub-stitute, this parameter was estimated by recording thedissipation factor, also known as the loss tangent(tan d \ GA/G@). Resilience (R) is an inverse measure of

the damping property and usually estimated21 asRB 1 [ 2n tan d.

Shear creep analysis

In a shear creep experiment, a constant shear stress (p0)is imposed for a speciÐed time. While in NewtonianÑuids and Hookean solids, strain (c) and rate of strain(dc/dt) will, respectively, become zero, in viscoelasticmaterials the strain response is linear (provided thatshear stress is small enough). The ratio is calledc(t)/p0shear creep compliance (J(t)). In our study, J(t) was



Fig. 2. Mechanical spectra of hydrogels E2, E9, D11, A12, G14 and G15 after delivery through di†erent systems. Code names areexplained in Table 1.

recorded using a Bohlin rheometer. After some prede-termined time, the stress was removed and the recover-able compliance was also monitored. A shear stress(Jr)of 0É1 Pa was applied to the gel samples with bothnominal creep and creep recovery times of 500 s, respec-tively.

RESULTS

Oscillatory shear stress analysis

The gels selected for Ðnal assessment (marked by anasterisk in Table 1) were delivered to the rheometerÏs



Fig. 3. E†ect of monomer composition on the shear modulus. The amount of crosslinking agent is the same in each group, butthe amount of comonomer (HEMA) is variable. Code names are explained in Table 1. Crosslinking agents : (a) EGDMA;

(b) DEGDMA; (c) TEGDMA; (d) DAE; (e) DVG.



Fig. 4. E†ect of hydrophobic crosslinking agents ((a) EGDMA; (b) DEGDMA; (c) TEGDMA) on the shear modulus. Code namesare explained in Table 1.

Fig. 5. E†ect of hydrophilic crosslinking agents ((a) DAE; (b) DVG) on the shear modulus. Code names are explained in Table 1.For hydrogels crosslinked with DVG, only the storage modulus is presented. The uncrosslinked hydrogel P4 is included in (b).



Fig. 6. Creep compliance plots of representative hydrogels E2, A11, G29 and P4 after injection through a 30-gauge needle. Codenames are explained in Table 1.

cone/plate system by injection through a 30-gaugeneedle, and plots of G@, GA and tan d were recorded as afunction of frequency. Representative mechanicalspectra are shown in Fig. 1.

In another experiment, the same hydrogels were eachdelivered to the instrument through a cylinder(d \ 8É7 mm), a 16-gauge needle (d \ 1É3 mm), and a 30-gauge needle (d \ 0É13 mm), respectively, in order toinvestigate the e†ect of shearing history on the shearmodulus. Because the diameter of the cylinder is muchlarger than the diameters of the needles, we assumedthat gels can pass almost freely through the cylinderchannel without signiÐcant change in their structure.When passing through the cylinder, the stretching Ñowcomponent is predominant,22 while through the barrelof a syringe to a needle there will be very little stretch,but the shear Ñow component will be high. Thedynamic mechanical spectra of the same specimen sub-jected to the three di†erent delivery systems wererecorded and compared. Some polymers, too rigid afterpassing through the cylinder to be sheared by oscil-lation in the cone/plate system, were only passedthrough the needles. Mechanical spectra of representa-tive hydrogels are shown in Fig. 2.

To investigate the e†ect of polymer composition onthe shear modulus, additional gels were included in thisstudy. Mechanical spectra of Ðve groups of gels, corre-sponding to Ðve di†erent crosslinking agents, are shownin Fig. 3. In each series, the amount of crosslinkingagent was maintained constant, while the amount ofcomonomer (HEMA) varied. The e†ects of nature andamount of crosslinking agents were also studied in eachseries by maintaining the same monomer/comonomerratio, but varying the amount of crosslinking agent. Theresults for hydrophobic crosslinking agents (EGDMA,DEGDMA, TEGDMA) and for hydrophilic cross-linking agents (DAE, DVG) are shown in Figs 4 and 5,respectively.

Shear creep analysis

The preselected gels (asterisk, Table 1) were all sub-jected to creep analysis, being delivered by injectionthrough a 30-gauge needle. Representative plots of thecreep compliance and recoverable compliance areshown in Fig. 6. The e†ect of shearing history on creepbehaviour was studied only in gel E2 (Fig. 7).



Fig. 7. Creep compliance of hydrogel E2 after deliverythrough di†erent systems : (a) natural ; (b) logarithmic.

DISCUSSION

Oscillatory shear behaviour

Four types of behaviour can be surmised on the basis ofour results. They are schematically presented in Fig. 8.

Type (I). Figure 8(a) shows that GA is always largerthan G@ over the range of frequencies used, i.e. theviscous component of the material behaviour is pre-dominant and the polymer behaves as a viscous Ñuid.The polymers D11, G12 and G15 may be included here.

Type (II). Figures 8(b,c) show that GA is generallyhigher than G@, but there is a G@ÈGA crossover either inthe low or in the high frequency range. The gels E2 andG13 can be included into the former category, while P4belongs to the latter. Both spectra suggest that the gelsbehave as entanglement networks, and the samples aremainly viscous rather than elastic.

Type (III). Figures 8(d,e) reveal that GA is generallylower than G@ ; therefore the elastic component is pre-dominant. The G@ÈGA crossover occurs at one of the twoextreme frequency ranges. The spectra of E9 displayed acrossover at low frequency, while those of G24 and G29displayed it at high frequency. As in the case of type II,both spectra suggest a behaviour typical of entangle-

Fig. 8. Types of mechanical spectra of hydrogels after injec-tion through a 30-gauge needle.

ment networks ; however, the samples behave as elasticmaterials rather than viscous Ñuids.

Type (IV). In contrast to type I, G@ is higher than GAat all frequencies (Fig. 8f). The polymers A11, A12, G14and G21 can be included here. They behave as cova-lently crosslinked networks. As mentioned before, thisclass of gels would be most suitable for vitreous substi-tution.

Resilience of the hydrogels which belong to type IVwas also compared (Fig. 9). The lower the loss tangent,the higher the resilience. According to their increasingresilience, the gels in this group can be arranged in thefollowing order : A12\ A11\ G14 \ G21. Becausematerials will be subjected to only one injection cyclewhen used as vitreous substitutes, it is unlikely thatthese gels will later exhibit Ñow under their own weight.A material with high resilience may perform bettermechanically in a vitreous cavity ; therefore, G14 andG21 should be the best candidates amongst all the othergels.

Shearing history

G@ and GA of the gels delivered by injection were lowerthan of those delivered through a cylinder (Fig. 2). Thedrop in G@ was always larger after passing the samplethrough a 30-gauge needle as compared with a 16-gaugeneedle. In other words, a signiÐcant change in the gelstructure has occurred after injection : non-injectedpolymers behaved as crosslinked networks (i.e. G@[ GA),but some of the gels after injection behaved either as



Fig. 9. Loss tangent plotted against frequency for the hydro-gels displaying type IV shear behaviour (as deÐned in Fig. 8).

Code names are explained in Table 1.

entanglement networks (there is a G@ÈGA crossover), oralmost as free-Ñowing Ñuids (i.e. GA [ G@). Althoughsome gels after injection still maintained a behavioursuggesting covalently crosslinked networks (G@[ GA), Rdecreased signiÐcantly (ratio GA/G@ increased), and theyalso became much thinner.

The process of injection resulted in signiÐcantchanges in the mechanical properties of the crosslinkedhydrogels. The narrower the needle used the bigger thedecrease in G@ and R, and the thinner the gels. Duringinjection, the molecular chains of polymer are subjectedto an external shear stress, which forces the extensivelyoverlapped chains to pass through a very small channel.According to the molecular theories of entangled chaindynamics,23h26 overlapped molecular chains create,hypothetically, a three-dimensional “tubeÏ or “tunnelÏ.Each chain lies through its own tube in which themotion of the chain is restricted. A linear chain may,however, move along its own tunnel, wriggle and twistto change its position and conformation in time, andeventually it may be able to disengage itself from thetube. This snakelike motion, known as “reptationÏ, pro-vides a mechanism for stress relaxation and di†usion inpolymers. Unlike entangled polymer networks, cross-linked polymer chains are linked to each other at multi-ple points, forming one giant covalently bondedmolecule. The reptation of such a structure, therefore, issigniÐcantly suppressed, and the di†usion in polymers islimited. A crosslinked polymer which is forced to passthrough a narrow channel (e.g. a small-gauge needle),experiences a dramatic structure deformation, in whichcovalent bonds may have to be broken to allow suchmovement. Once the breakage of a covalent bondoccurs, it is improbable that the broken bond can bereformed. In other words, the shear stress exerted

during injection may result in a massive mechanicalbreakage of crosslinks, inducing signiÐcant loss of elas-ticity (G@ decreases), and an increase in the dissipationfactor (i.e. a decrease of resilience). After being injectedthrough a 30-gauge needle, some of the hydrogelsbehaved as entanglement networks (P4, E2, E9, G13,G24, G29), or even as viscous Ñuids (D11, G12, G15),rather than as covalently crosslinked networks. Onlythe gels A11, A12, G14 and G21 had the storagemodulus higher than the loss modulus and the plotswere parallel after injection, although there was someloss of elasticity and resilience.

Effect of polymer composition

As seen in Fig. 3, the gels became thicker (i.e. G@increased), and also the resilience of materials increased(i.e. the ratio GA/G@ decreased), when the amount of thecomonomer HEMA increased. The results suggest thatHEMA has a strengthening e†ect as a comonomer.

Hydrophobic crosslinking agents (EGDMA,DEGDMA, TEGDMA) also have a strengthening e†ectupon the polymers. The hydrogels (Fig. 4) becamethicker (and R increased) with the increase of cross-linking density. This is likely because the crosslinkingprocess suppresses the Ñow of one chain past another.

However, the hydrogels crosslinked with hydrophiliccrosslink agents (DAE, DVG) displayed irregular e†ects(Fig. 5), as noticed previously10 in the case of theirswelling behaviour. With increasing crosslinking agentconcentration, G@ and R increased initially. When theamount of crosslinking agent increased beyond acertain extent, G@ and R began to decrease. Thematerials became very thin, eventually “wateryÏ (i.e. free-Ñowing Ñuids rather than gels). As suggested pre-viously,10 this may be a complex result of thecompetition and/or cooperative interaction betweenvarious factors, including the hydrophilic character ofthe crosslinking agents, the level of crosslinking, thecompositional heterogeneity of the network, and theformation of branched, water-soluble polymers, ratherthan crosslinked ones.

Creep behaviour

The creep behaviour of materials is often described andimitated by models (elements).21,24 The ideal viscouselement is a dashpot Ðlled with a Newtonian Ñuid. Theideal elastic element is a spring which obeys HookeÏslaw. These two basic elements can be combined in seriesto give the Maxwell element, which imitates a materialthat can respond elastically to stress but also undergoesviscous Ñow. When combined in parallel, the result isthe Voigt (also known as Kelvin or VoigtÈKelvin)element which imitates a material that responds elasti-cally to stress, but not instantaneously ; the response isretarded by a viscous resistance. The simplest modelthat can be used for describing the creep behaviour of



Fig. 10. Creep compliance plotted logarithmically againsttime in the three groups of preselected hydrogels : (a) D11,G12, G13, G15, P4 ; (b) E2, E9, G24, G29 ; (c) A11, A12, G14,

G21. Code names are explained in Table 1.

viscoelastic materials is the Burgers model consisting ofa Maxwell element and a Voigt element, combined inseries.

In this study, the creep behaviour of the hydrogels(Fig. 6) can be described by three di†erent mechanicalmodels : Newton, Voigt and Burgers.

(i) The plots of P4, D11, G12, G13 and G15 showthat the compliance curve J(t) is approximately pro-portional to the time under constant shear stress, andthe recoverable compliance curve approaches aJr(t)horizontal line independent of time. There were noinstantaneous or retarded elastic responses observablein this experiment. Neither could signiÐcant recoverabledeformation be observed. The material may henceexhibit Newtonian Ñow (i.e. the dashpot model).

The creep compliance curve of hydrogel E2 does notproceed to an equilibrium value, but to a limiting slope.Unlike in other gels, there is a retarded elastic recoverycompliance after removing the shear stress. However,the material may still manifest mainly Newtonian Ñowbecause its permanent unrecoverable compliance (equalto t/g) is almost linear with time, in addition to a minorregion of time-dependent elasticity.

(ii) The curves of G24 and G29 show that thematerials have no signiÐcant instantaneous elasticresponse, but respond slowly to stress, bearing all of itinitially as in a dashpot model (viscous Ñow contribu-tion, t/g) and gradually transferring it to the spring, andthat the response becomes extended (i.e. retarded elasticresponse). When the spring bears all the stress, thespecimen stops deforming, and creep ceases. The plotssuggest that G24 and G29 have asymptotic behaviours,as in a Voigt model.

(iii) The plots of E9, A11, A12, G14 and G21 indicatethat the materials undergo an instantaneous elasticdeformation followed by Voigt model creep which(Jg),includes a retarded elastic response and a true(Jd)viscous Ñow. After removing the stress, the specimensundergo a partial recovery, including instantaneouselastic recovery and retarded elastic recovery (J0\ Jg

The deformation resulting from the true Ñow is] Jd).non-recoverable. The behaviour of the materials can bedescribed by the Burgers model. The presence of aninstantaneous elastic response in the network of E9 isnot as signiÐcant as in other gels, and its retardedelastic response is more substantial. The instantaneouselastic response is mainly because of an increase ininternal energy, and is probably associated with van derWaals forces between polymer chains.27 In this region,the conformational rearrangement of chain backbonesis almost immobilized during experiments. The retardedelastic response corresponds to the gradual uncurling,or curling up, of randomly kinked and entangledpolymer chains. Therefore, the plot may indicate thatdangling entangled chains are predominant in themolecular structure of E9, and there are only a few wellseparated linkage points present in the work.

In his fundamental work, Ferry28 gave a classiÐcationof polymers based on their viscoelastic behaviour. Hedi†erentiated eight structural types of polymer which



were conÐrmed by the same number of di†erent J(t)logarithmic plots. In order to include each preselectedhydrogel in one of FerryÏs types, the experimental com-pliances were plotted logarithmically against time (Fig.10), resulting in three distinctive groups.

The compliance of P4, D11, G12, G13 and G15,which may be due to viscous Ñow, increases with nolimit during the duration of the experiment. These poly-mers may be either related to type I deÐned by Ferry28as “dilute polymer solutionsÏ, in which viscoelasticitydoes not perturb a Newtonian Ñow greatly, or to type II(deÐned as uncrosslinked “amorphous polymers of lowmolecular weightÏ), in which their viscoelasticity arisesfrom local frictional forces induced by the movement ofshort segments in a chain. Unlike other types ofmaterial, the chains in this case are not signiÐcantlyintertwined, are possibly separated, and can move inde-pendently with little interaction. This class of gels isprobably not suitable as material for vitreous replace-ment.

The compliance of E2, E9, G24 and G29 increasesgradually, and there is a weak plateau compliance (JN)at intermediate times. J(t) then rises from to anJNunlimiting value. In this case, the behaviour of gels issimilar to that of viscoelastic liquids, especiallyuncrosslinked “amorphous polymers of high molecularweightÏ (type III in FerryÏs classiÐcation28). The visco-elasticity may be described in terms of local frictionalforces encountered by a short segment of a movingchain, together with additional entanglement couplingto other chains. The entanglements profoundly inhibitlong-range conformational rearrangement and separa-tion of chains from each other, as if the network wouldbe crosslinked.

The compliance of A11, A12, G14 and G21 increasesinitially, and tends to level o† to approach a plateaucompliance at intermediate times. J(t) then rises toJNapproach closely a limiting value the elastic equi-Je ,librium compliance. Accordingly, the creep behaviour ofthese hydrogels resembles that of “very lightly cross-linked amorphous polymersÏ deÐned by Ferry28 as typeVII. This type of material has few crosslinking pointspresent in the network. As suggested by Ferry,28 the risefrom to may be due to possible slippage of chains,JN Jewhich are trapped and kinked on branched structures,and are incompletely attached to the network.

Considering that hydrogels A11, A12, G14 and G21still behaved as slightly crosslinked networks after injec-tion through a 30-gauge needle, their creep compliancewas further compared in Fig. 11. In their networks, asmentioned before, there are prominent instantaneouselastic responses followed by substantial retarded elasticresponses during the time under constant shear stress.The values of (which represent the instantaneousJgelastic response) of G14 and G21 are higher than thoseof A11 and A12, indicating that the former are moreelastic. However, the retardation time of gels increases

in the following order : G21\ G14 \ A11\ A12: thelower the retardation time, the higher the Ñexibility. Asa vitreous substitute, a material should possess not onlyhigh elasticity in order to support and push a detachedretina, but also sufficient Ñexibility (i.e. resilience) undersudden external forces. Hydrogels G21 and G14 may bethe optimum candidates for this task, a conclusionwhich corresponds to the results obtained from oscil-latory shear experiments.

The J(t) plots of hydrogel E2 after passing through acylinder, a 16-gauge and a 30-gauge needle were com-pared in Fig. 7. The retardation time of the gel signiÐ-cantly decreases as the channel of delivery systembecomes narrower. The retarded elastic response (Jd)became more predominant, and the instantaneouselastic response became less evident. J(t) increased ini-tially after passing through a cylinder, and then tendedto closely approach an elastic equilibrium value Je ,without a transitional zone (Fig. 7b). Its creep behav-JNiour is more similar to that of type VI (“lightly cross-linked amorphous polymersÏ, as deÐned by Ferry28).After injection through a 16-gauge needle, J(t) increasedinitially and passed through a plateau compliance zone

and then rose slightly to a value The spectraJN , Je .suggest that the number of crosslinking points is lowerin the gel network after the 16-gauge needle injection,and it behaved more like type VII (very lightly cross-linked polymers). Furthermore, after injection through a30-gauge needle, the hydrogel E2 behaved more as anuncrosslinked amorphous polymer. Its viscoelasticity ismainly because of the interaction between short seg-ments of moving chains (which results in local frictionalforces), in addition to entangled coupling to neighbour-ing chains. Obviously, the injection process signiÐcantlychanged the structure of the gel, and the smaller the

Fig. 11. Creep compliance plotted against time in the hydro-gels displaying type IV shear behaviour (as deÐned in Fig. 8).

Code names are explained in Table 1.



gauge of the needle, the larger the number of brokencrosslinks. This conÐrms the results of oscillatory shearexperiments.

CONCLUSION

There is experimental evidence that the natural vitreousbody in the vertebrate eye has viscoelastic properties.Based on the assumption that any biomaterial to func-tion as an artiÐcial vitreous should also be viscoelastic,hydrogels with high water contents were produced bypolymerization of VP. After a selection based on physi-cal and optical characteristics, which reduced dramat-ically the number of valid candidates for vitreoussubstitution, the rheological characterization of theremaining hydrogels by oscillatory shear stress andshear creep analysis indicated that only a few hydrogelsmaintained viscoelastic properties after being injectedthrough small-gauge needles. Both copolymerizationwith HEMA and crosslinking with hydrophobic orhydrophilic agents, are beneÐcial within certain limits.This study indicated that copolymers of VP with 5%HEMA, crosslinked with 0É5 or 0É75% DAE, andcopolymers with 1% and 2% HEMA, crosslinked with1% DVG displayed sufficient post-injection visco-elasticity in order to fulÐl the requirements for vitreoussubstitution.

ACKNOWLEDGEMENTS

This research was supported by a grant from theNational Health and Medical Research Council of Aus-tralia The support from the Australian(d940707).Foundation for the Prevention of Blindness (WesternAustralia), Inc., is also acknowledged. The Lions ClubsInternational Foundation (Oak Brook, IL, USA) kindlyprovided a grant for the Bohlin CS 10 rheometer. Wethank BASF Australia Ltd. (Mrs L. Valente) for gifts ofVP. We thank Paul D. Dalton for expert advice andhelp.

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poly(1-vinyl-2-pyrrolidinone) hydrogels as vitreous substitutes: a rheological study

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