Elsevier

Biomaterials

Volume 22, Issue 7, April 2001, Pages 667-676
Biomaterials

Water absorption and surface properties of novel poly(ethylmethacrylate) polymer systems for use in bone and cartilage repair

https://doi.org/10.1016/S0142-9612(00)00229-5Get rights and content

Abstract

The surface and bulk properties of novel methacrylate polymers prepared by gelling poly(ethyl methacrylate) (PEMA) powder with different ratios of tetrahydrofurfuryl methacrylate (THFMA) and hydroxyethyl methacrylate (HEMA) monomers were investigated. The water adsorption and desorption characteristics of these polymers were measured in water and phosphate buffered saline (PBS). The desorption diffusion coefficients were higher than the adsorption coefficients in both water and PBS. Linear relationships between the equilibrium mass of water taken up and the mass of water desorbed with the concentration of HEMA in the polymer were established. Polymer surfaces were analysed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Surface features varied with polymer composition; during hydration only selective areas of the surface hydrated indicating a heterogeneous surface. Contact angle data showed no trend between the different polymers indicating that contact angles are not an acceptable method of assessing hydrophobicity/wettability of a material which does not have a homogeneous surface. The effect of these bulk and surface characteristics on biological interactions were examined using bovine chondrocytes and human osteoblast (HOB) cell cultures. Cell attachment decreased when HEMA was present in the copolymer.

Introduction

Polymeric materials play an important role in, for example, drug release [1], [2] joint replacement [2] or sutures and biosensors [3], [4]. A polymer system consisting of PEMA powder gelled with THFMA monomer has shown potential for use in dental applications [5], drug release [6] and for bone and cartilage repair [2], [7], [17]. The surface properties, water adsorption characteristics, biocompatibility and the ability of this polymer to support cell attachment and growth have been examined [4], [8], [9], [10].

The surface properties of biomaterials are important in determining the interfacial response in a biological environment. Surface topography, chemistry and wettability influence cell attachment and growth on the biomaterial surface [11], [12]. The water absorption characteristics of methacrylate polymers have been monitored to investigate effects such as distortion and reduced strength [13], the possibility of ingress of microorganisms and leaching of residual monomer and other materials which may be toxic [14].

For poly(THFMA), a heterocyclic methacrylate, the water absorption values were high; after a period of 33 months equilibrium had not been reached and Fickian diffusion did not apply [10]. Polymerisation with PEMA limited water absorption. Consequently, diffusion theory was obeyed and equilibrium was reached, although the water absorption of the system was still relatively high, being 34%w/w in 2 years. However, when water absorption from a range of biological fluids, (phosphate buffered saline (PBS), artificial saliva, foetal calf serum and cell culture media), was measured the water uptake reduced to 1.5–3% suggesting that water uptake is related to the osmolarity of the external solution and the presence of osmotic sites in the polymer [10]. No clear trend has been found between equilibrium/diffusion behaviour and the PEMA/THFMA ratio. This may be due to the THFMA in the copolymer having two roles. Whilst it could be expected to increase water uptake, THFMA is also a cross-linking agent [15], and increasing cross-linking reduces water uptake [1], [10], [16], [17]. The PEMA/THFMA system is able to swell in situ, remain rigid and does not change structurally.

The specific water uptake properties of polymers allows their classification as hydrogels which are defined as polymeric materials with the special characteristics of being hydrophilic, soft, elastic, able to swell in water and to imbibe large quantities of water (>20%) without the dissolution of the inter penetrating network (IPN). Currently, poly[HEMA] is utilised as a hydrophilic polymer in contact lenses [18]. Modification of the PEMA/THFMA polymer system by gelling PEMA with the hydrophilic monomer HEMA results in an increase in hydrophilicity [2].

Although it has previously been suggested that the hydrophilicity of a polymer influences water absorption, other factors such as the number and distribution of hydroxyl groups on the surface, water organisation and swelling of the polymer also play a role [12]. Therefore, the surface wettability of a material is not necessarily related to water uptake when dealing with a heterogeneous system.

HEMA was incorporated into the PEMA/THFMA polymer system to examine the effect of a more hydrophilic monomer on water adsorption characteristics and surface properties. A series of polymers was prepared by varying the monomer ratio of THFMA/HEMA gelled with PEMA and the water absorption characteristics of this system studied in water and PBS. PBS has an osmolarity closer to that of the body.

The contact angles and surface topography of both dry and hydrated polymers were examined to observe if differences in the surface topography of the different polymer systems led to changes in topography upon hydration.

Although the water adsorption characteristics of materials are an important consideration when designing a novel biomaterial, the initial interfacial interaction of the surface with the biological environment is also crucial. Therefore, the PEMA polymer was modified to control water uptake. Since the PEMA/THFMA polymer system has shown potential as a biomaterial for bone and cartilage repair [2], [7], the subsequent effect of these changes on cell attachment was studied using bovine chondrocytes and human osteoblasts (HOBs).

Section snippets

Polymer preparation

Polymer discs were prepared by mixing 3 g PEMA powder (240 mesh, standard commercial PEMA powder, Reference TS 1364 ex Bonar Polymers Ltd., Newton Aycliffe, Co. Durham, UK) and 1.8 ml of the monomer liquid containing 2.5% v/v N,N-dimethyl-p-toluidine. The monomer liquid contained HEMA (Aldrich Chemical Co., Gillingham, UK) and/or THFMA (Rohm Chemie, Darmstadt, Germany). Different monomer ratios of HEMA : THFMA (100 : 0 to 0 : 100) were prepared and mixed with the PEMA powder. The polymer mixtures were

Water absorption and desorption

The absorption and desorption data for the polymer samples incubated in water and PBS are summarised in Table 1, Table 2, respectively. In both PBS and water the mass gain was significantly less than the mass loss, (p<0.0001, students t-test). The samples incubated in water had a significantly greater equilibrium percent gain and loss of water compared to those incubated in PBS (p<0.0001, students t-test). An increase in the equilibrium gain in mass with increasing HEMA content was observed in

Discussion

This paper presents an investigation of the water adsorption and desorption characteristics, the wettability, the surface topography and the effect of hydration for a series of methacrylate copolymers prepared by gelling PEMA powder with liquid monomer consisting of THFMA and/or HEMA. By varying the ratio of the THFMA/HEMA present in the monomer component incremental changes were made in the polymer chemistry. The effect of this on cell attachment and activity was also explored.

In general, the

Conclusions

This study has shown that incorporation of HEMA into a PEMA/THFMA polymer system alters the water absorption properties by increasing the equilibrium water uptake of the material and decreasing the absorption diffusion coefficient. The contact angle measurements suggest that the wettability of the system is not affected by the presence of HEMA although this may be due to the implementation of contact angle studies suitable for model systems into a real polymer system. We have shown that

Acknowledgements

This work was funded by the University of Nottingham, Action Research and SPARKS. The authors would like to thank Susan Anderson for her help with the SEM analysis and photography, X. Chen for his assistance with the AFM measurements and Colin Scotchford for his critical reading of the manuscript.

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