Hydrophilic sponges based on 2-hydroxyethyl methacrylate. I. effect of monomer mixture composition on the pore size

Morphological and topographic effects on calcification tendency of pHEMA hydrogels

Poly(2-hydroxyethyl methacrylate) hydrogels were prepared in the presence of varying concentrations of water, or a co-monomer ethoxyethyl methacrylate at different strengths of crosslinking agent ethylene glycol dimethacrylate. Calcification tendency and its correlation with monomer mixture composition, topography and porosity of these materials were investigated. Scanning (SEM) and transmission electron microscopy (TEM) was used to study topography and porosity respectively. Calcification and calcium diffusion ability in to the hydrogels were investigated by light microscopy, SEM and energy dispersive analysis of X-rays (EDAX) after incubation of the materials in a metastable calcifying solution for 48 days. Polymer and solvent volume fractions were also studied to determine if a correlation existed between porosity and calcification. Most of the series of hydrogels showed surface irregularities. Internal structure showed evidence of a porous structure in one of the series. Calcification studies indicated diffusion of calcium ions in some of the series. The diffusion of calcium is limited to 30-40 microm in most calcified specimens. For hydrogels that exhibited substantial surface irregularities and micro channels, the infiltration of calcium up to 200 microm was observed. Attempts to detect porosity by electron microscopy failed in some of the hydrogels due to difficulty in sample processing and sectioning. However, collaboration of the results with different techniques used, indicated that surface defects are the major contributors to calcium deposition. Decrease in porosity reduces the amount of calcium deposits and infiltration with decreasing solvent volume fraction which is associated with crosslinking concentration and initial water content of the polymer.

Interaction of resin-modified glass-ionomer cements with moist dentine

OBJECTIVES

The objective of this study was to report on a novel phenomenon that occurs when resin-modified glass-ionomer cements (RMGICs) are bonded to moist human dentine.

METHODS

Dentine surfaces from extracted third molars were abraded with 180-grit SiC paper. Ten teeth were prepared for each of the two RMGICs tested (Fuji II LC, GC Corp. and Photac-Fil Quick, 3M ESPE). RMGIC buildups were made according to the manufacturers' instructions. After storage at 37 degrees C, 100% humidity for 24 h, the bonded specimens were cut occlusogingivally into 0.9 x 0.9 mm beams. Dentine surfaces bonded with the two RMGICs were examined along the fractured RMGIC/dentine interfaces. Additional beams fractured within the RMGICS and at 3 mm away from the interfaces were used as controls. The fractured beams were examined using scanning electron microscopy (SEM), field emission-environmental SEM (FE-ESEM) and transmission electron microscopy (TEM).

RESULTS

SEM and FE-ESEM revealed numerous solid spherical bodies along the RMGIC/dentine interfaces. By contrast, no spherical bodies could be identified within the RMGIC fractured 3 mm distant from the bonded interface. TEM and energy dispersive X-ray analyses performed on carbon-coated ultrathin sections showed that these solid spherical bodies consisted of a thin aluminum and silicon-rich periphery and an amorphous hydrocarbon core within the air voids of the original resin matrix.

CONCLUSION

The spherical bodies probably represent a continuation of GI reaction and poly(HEMA) hydrogel formation that results from water diffusion from the underlying moist dentine. Their existence provides evidence for the permeation of water through RMGIC/dentine interfaces.

Drug release characteristics of phase separation pHEMA sponge materials

A number of phase separation pHEMA sponge hydrogels have been prepared based on variations in monomer contents, concentration of cross-linking agent, solvent mixture and temperature of polymerization. The loading levels and release profiles of the anti-inflammatory drug prednisolone were examined for each of the pHEMA sponge materials. An effective diffusion coefficient determined by an optimization approach based on the experimental data was used to measure their release characteristics. The effect of morphological variations, revealed by the environmental scanning electron microscopy, and polymer/solvent volume fractions on these properties were discussed.

Chemically-bound nerve growth factor for neural tissue engineering applications

In order to promote regeneration after spinal cord injury, growth factors have been applied in vivo to rescue ailing neurons and provide a path finding signal for regenerating neurites. We previously demonstrated that soluble growth factor concentration gradients can guide axons over long distances, but this model is inherently limited to in vitro applications. To translate the use of growth factor gradients to an implantible device for in vivo studies, we developed a photochemical method to bind nerve growth factor (NGF) to microporous poly(2-hydroxyethylmethacrylate) (PHEMA) gels and tested bioactivity in vitro. A cell adhesive photoreactive poly(allylamine) (PAA) was synthesized and characterized. This photoreactive PAA was applied to the surface of the PHEMA gels to provide both a cell adhesive layer and a photoreactive handle for further NGF immobilization. Using a direct ELISA technique, the amount of NGF immobilized on the surface of PHEMA after UV exposure was determined to be 5.65 +/- 0.82 ng/cm2 or 3.4% of the originally applied NGF. A cell-based assay was performed to determine the bioactivity of the immobilized NGF. Using pheochromocytoma (PC-12) cells, 30 +/- 7% of the cell population responded to bound NGF, a response statistically similar to that of cells cultured on collagen in the presence of 40 ng/ml soluble NGF of 39 +/- 12%. These results demonstrate that PHEMA with photochemically bound NGF is bioactive. This photochemical technique may be useful to spatially control the amount of NGF bound to PHEMA using light and thus build a stable concentration gradient.

Overestimating hybrid layer quality in polished adhesive/dentin interfaces

The most popular techniques for determining the quality of the hybrid layer (HL) have relied on morphologic characterization of the polished adhesive/dentin (a/d) interfaces before and after acid-bleach chemical treatment. Using these techniques, the existence of smooth, acid-resistant layers has been consistently reported for most adhesive systems. The purpose of this study was to determine whether popular specimen preparation techniques that include polishing and acid-bleach treatment modify the a/d interface, mask the complexity of the HL, and lead to inaccurate assessment of the quality of the HL. To understand the impact of specimen preparation techniques on the morphology of the resin-dentin interface, polished and unpolished specimens from the same tooth were closely compared after different acid-bleach chemical treatment procedures. Two one-bottle adhesives, that is, 3M Single Bond and Pulpdent UNO, exhibiting distinct differences in hydrophilic/hydrophobic composition, were used in this investigation. Using specimens from the same tooth, the effect of chemical treatments on the morphology of the resin-dentin interdiffusion zone and the differences in the morphology of polished and unpolished specimens after these same treatments were studied with scanning electron microscopy. It was shown that conventional specimen preparation techniques that include polishing and acid-bleach treatment can adversely affect and even obscure the structural detail of the a/d interface in specimens that possess a porous HL. The results indicated that the Pulpdent UNO/dentin interface had better quality than the 3M Single Bond/dentin interface. The difference in the quality of HL can be attributed to factors such as compositional differences that impact the adhesive interaction with water, that is present within the substrate during wet bonding. The inability of the conventional acid-bleach procedure to reveal the differences in the scanning electron microscopy interfacial morphology was overcome in this investigation by using a multistep technique.

Water sorption dynamics of a binary copolymeric hydrogel of 2-hydroxyethyl methacrylate (HEMA)

The water imbibing property of poly(2-hydroxyethyl methacrylate) (poly HEMA) has been improved by copolymerizing HEMA with acrylamide in the presence of a hydrophilic polymer such as polyethylene glycol (PEG). The hydrogel was characterized by IR spectral analysis and several network parameters such as average molecular weight between crosslinks (Mc), crosslink density (q) and number of elastically effective chains were evaluated. The swelling ratio of the hydrogel was found to be influenced by varying the chemical architecture of the hydrogel, i.e. by changing the proportions of PEG, HEMA, acrylamide and crosslinking agent in the feed mixture of the hydrogel. The degree of water sorption was studied as a function of the experimental conditions such as the pH and temperature of the swelling medium and presence of salt ions in the outer solution. The dynamics of the swelling process was studied quantitatively and kinetic constants such as the swelling exponent (n) and diffusion constant (D) were also evaluated. The hydrogels prepared of varying composition were judged for antithrombogenic nature of their surfaces by blood-clot formation test.

Manufacture of poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) hydrogel tubes for use as nerve guidance channels

Hydrogel tubes of poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (p(HEMA-co-MMA)) made by liquid-liquid centrifugal casting are being investigated as potential nerve guidance channels in the central nervous system. An important criterion for the nerve guidance channel is that its mechanical properties are similar to those of the spinal cord, where it will be implanted. The formulated p(HEMA-co-MMA) tubes are soft and flexible, consisting of a gel-like outer layer, and an interconnected macroporous, inner layer. The relative thickness of the gel phase to macroporous phase is controlled by the formulation chemistry, and specifically by the ratio of co-monomers, HEMA and MMA. By varying the surface chemistry of the mold within which the tubes are synthesized, tubes were prepared with either a "cracked" or a smooth outer morphology. Tubes with the cracked outer morphology had periodic channels that traversed the wall of the tube, which resulted in a lower modulus than smooth outer morphology tubes, yet likely greater diffusive permeability. For tubes (and not rods) to be formed, phase separation must precede gelation as is detailed in a formulation phase diagram for HEMA, MMA and water. The tensile elastic modulus of p(HEMA-co-MMA) tubes reflected the formulation chemistry, with greater moduli (up to 400 kPa) recorded for tubes having 10 wt% MMA. The p(HEMA-co-MMA) tubes therefore had similar mechanical properties to those of the spinal cord, which has a reported elastic modulus range between 200 and 600 kPa.

Single-step adhesives are permeable membranes

OBJECTIVES

This study tested the hypotheses that micro-tensile bond strengths of all currently available single-step adhesives to dentine are adversely affected by delayed activation of a light-cured composite, and that such a phenomenon only occurs in the presence of water from the substrate side of the bonded interface.

METHODS

In experiment I, a control three-step adhesive (All-Bond 2, Bisco) and six single-step adhesives (One-Up Bond F, Tokuyama; Etch&Prime 3.0, Degussa; Xeno CF Bond, Sankin; AQ Bond, Sun Medical; Reactmer Bond, Shofu and Prompt L-Pop, 3M ESPE) were bonded to sound, hydrated dentine. A microfilled composite was placed over the cured adhesive and was either light-activated immediately, or after leaving the composite in the dark for 20 min. In experiment II, three single-step adhesives (Etch&Prime 3.0, Xeno CF Bond and AQ Bond) were similarly bonded to completely dehydrated dentine using the same delayed light-activation protocol. In experiment III, a piece of processed composite was used as the bonding substrate for the same three single-step adhesives. The microfilled composite was applied to the cured adhesives using the same immediate and delayed light-activation protocols. Bonded specimens were sectioned for micro-tensile bond strength evaluation. Fractographic analysis of the specimens was performed using SEM. Stained, undemineralised sections of unstressed, bonded specimens were also examined by TEM.

RESULTS

When bonded to hydrated dentine, delayed light-activation had no effect on the control three-step adhesive, but significantly lowered the bond strengths of all the single-step adhesives (p < 0.05). This adverse effect of delayed light-activation was not observed in the three single-step adhesives that were bonded to either dehydrated dentine or processed composite. Morphological manifestations of delayed light-activation of composite in the hydrated dentine bonding substrate were exclusively located along the composite-adhesive interface, and were present as large voids, resin globules and honeycomb structures that formed partitions around a myriad of small blisters along the fractured interfaces.

CONCLUSION

These features resembled the 'overwet phenomenon' that was previously reported along the dentine-adhesive interfaces of some acetone-based three-step adhesives. The cured adhesive layer in single-step adhesives may act as semi-permeable membranes that allow water diffusion from the bonded hydrated dentine to the intermixed zone between the adhesive and the uncured composite. Osmotic blistering of water droplets along the surface of the cured adhesive layer and emulsion polymerisation of immiscible resin components probably account for the compromised bond strength in single-step adhesives after delayed activation of light-cured composites.

An overview of the development of artificial corneas with porous skirts and the use of PHEMA for such an application

An overview of the efforts to develop functional polymeric artificial corneas (keratoprostheses) by incorporating a porous skirt is presented. The development of such a device by the author's group using poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels, as a combination of their homogeneous and heterogeneous states, and the rationale of this choice are also discussed. The latest results of the clinical trials with the PHEMA keratoprosthesis in human patients indicate a lower risk of the complications traditionally associated with the implantation of artificial corneas.

Self-assembled molecular structures as ultrasonically-responsive barrier membranes for pulsatile drug delivery

Noninvasive ultrasound has been shown to increase the release rate on demand from drug delivery systems; however, such systems generally suffer from background drug leaching. To address this issue, a drug-containing polymeric monolith coated with a novel ultrasound-responsive coating was developed. A self-assembled molecular structure coating based on relatively impermeable, ordered methylene chains forms an ultrasound-activated on-off switch in controlling drug release on demand, while keeping the drug inside the polymer carrier in the absence of ultrasound. The orderly structure and molecular orientation of these C12 n-alkyl methylene chains on polymeric surfaces resemble self-assembled monolayers on gold. Their preparation and characterization have been published recently (Kwok et al. [Biomacromolecules 2000;1(1):139-148]). Ultrasound release studies showed that a copolymer of 2-hydroxyethyl methacrylate and ethylene glycol dimethacrylate (MW 400) coated with such an ultrasound-responsive membrane maintained sufficient insulin for multiple insulin delivery, compared with a substantial burst release during the first 2 h from uncoated samples. With appropriate surface coating coverage, the background leach rate can be precisely controlled. The biological activity of the insulin releasate was tested by assessing its ability to regulate [C14]-deoxyglucose uptake in 3T3-L1 adipocyte cells in a controlled cell culture environment. Uptake triggered by released insulin was comparable to that of the positive insulin control. The data demonstrate that the released insulin remains active even after the insulin had been exposed to matrix synthesis and the methylene chain coating process.

Acrylic hydrogel implants after spinal cord lesion in the adult rat

Acrylic hydrogels, like the polymer of 2-hydroxyethyl methacrylate, are biocompatible, mechanically stable, porous materials that can be coated with collagen or laminin acting as bioadhesive substrates. Poly-2-hydroxyethyl methacrylate sponges have been proposed for restoring the anatomical continuity of damaged neural structures. In the present work, the ability of poly-2-hydroxyethyl methacrylate sponges to provide the injured spinal cord neurons with a conductive substrate for their regenerating axons was investigated in 32 adult Wistar rats. Collagen impregnated poly-2-hydroxyethyl methacrylate sponges were implanted into suction cavities of the dorsal funiculus of the spinal cord. Two to four months after implantation, the spinal cord was removed and processed for histology, and S100 and GFAP immunohistochemistry. To study axonal regeneration into the sponge, the spinal cord or the sensorimotor cortex were injected with 0.05-0.1 microl of an 8% solution of lectin-conjugated horseradish peroxidase or 10% dextran tetramethylrhodamine. The fibroglial reaction, accumulation of mononuclear cells, and angiogenesis at the interface between the spinal cord and the sponge were minimal. Cystic cavitation in the spinal cord was virtually absent. Anterograde labeled axons were seen to penetrate and to elongate the full length of the sponge. These results demonstrate that poly-2-hydroxyethyl methacrylate sponges represent a safe supportive material for regenerating spinal cord axons.

Calcification of poly(2-hydroxyethyl methacrylate) hydrogel sponges implanted in the rabbit cornea: a 3-month study

Poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels have been used in the past as ocular implants. In a recent development, PHEMA sponges have shown suitable properties as materials for the peripheral component of an artificial cornea (keratoprosthesis). However, the propensity of PHEMA to calcify could threaten the long-term stability of the implanted devices. In an attempt to improve the understanding of the calcification mechanism, the dynamics, extent, and nature of calcified deposits within PHEMA sponges implanted in the cornea were investigated in this study, and the possible correlation between necrosis of cells and calcification was critically examined. Samples of a PHEMA sponge were implanted in rabbit corneas and explanted at predetermined time points (2, 4, and 12 weeks). The samples were examined by microscopy (light, transmission, scanning) and energy dispersive analysis of X-rays. Histological assessment and semiquantitative analysis of the amount of calcium deposited was performed using image analysis. An in vitro experiment was also performed by incubating sponge samples for 2 weeks in a solution of calcium and phosphate ions at a ratio similar to that in hydroxyapatite, in the absence of cells. Calcification was not seen in the 2- and 4-week explants, however, small deposits were detected in two of the 12-week explants, both within and on the sponge's constituent polymer particles. The deposit volumes represented 0.094% and 0.21%, respectively, of the total sponge volumes. Calcium deposits were present in large amounts both within the constituent polymer particles and on the surface of the sponges incubated in the abiotic calcifying solution. Cooperative mechanisms are suggested for the calcification of PHEMA sponges in vivo. The initial event may occur at a molecular level, when plasma proteins are adsorbed onto the polymer surface and bound through chelation to the calcium ions present in the medium. After their natural degradation, these structures may act as nucleation sites for calcium phosphate crystallization. Concurrently, the calcium ions can diffuse into the hydrogel particles and then the spontaneous precipitation of calcium phosphate may be caused by supersaturation due to the lower content of water in polymer, an effect which is likely predominant in vitro. The second event is the recruitment of phagocytic cells to clear calcium debris. Degeneration of these cells may then form nucleation sites for secondary calcification.

Preparation of macroporous poly(2-hydroxyethyl methacrylate) hydrogels by enhanced phase separation

Macroporous poly(2-hydroxyethyl methacrylate) (p(HEMA)) hydrogels were prepared in the presence of a 0.3-0.7 M NaCl solution. The pore morphology of the p(HEMA) hydrogels was dependent on the concentration of NaCl for a constant monomer solution to aqueous solution ratio. Swelling studies showed an increase in equilibrium water content and hydrogel porosity as the NaCl concentration in the polymerization medium increased from 0 to 0.7 M. The equilibrium water content, however, decreased as the NaCl concentration in the swelling medium increased. The frozen water content increased and non-frozen water decreased with an increase in the NaCl concentration in the polymerization medium. Mechanical testing indicated that the elastic modulus of the hydrogels was not affected by the increased porosity until the pores became interconnected. These data suggest that the addition of NaCl to the polymerization medium results in a multi-phase separation during fabrication that produces macroporous hydrogels of controlled morphology.

Synthesis, physical characterization, and biological performance of sequential homointerpenetrating polymer network sponges based on poly(2-hydroxyethyl methacrylate)

A limitation in the use of hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) sponges as implantable devices is their inherently poor mechanical strength. This precludes proper surgical manipulation, especially in the eye where the size of the implant is usually small. In this study a new method was developed to produce mechanically stronger PHEMA sponges. Sequential homointerpenetrating polymer network (homo-IPN) sponges were made by using HEMA as the precursor for generating both the first network and the successive interpenetrated networks. Following the formation of network I, the sponge was squeezed to remove the interstitial water, soaked in the second monomer (also HEMA), and squeezed again to remove the excess monomer from the pores before being subjected to the second polymerization leading to the formation of network II. Two two-component IPN sponges (K2 and K4) with increasing HEMA content in the network II and a three-component IPN sponge (K3) were produced, and their properties were compared to those of a homopolymer PHEMA sponge (control). Apart from elongation, the tensile properties were all significantly enhanced in the IPN sponges; the water content was the same as in the control sponge, except for sponge K4, which was lower. Light microscopy revealed similar pore morphologies of the control and IPN sponges K2 and K3, and the majority of the pores were around 25 microm. Sponge K4 displayed smaller pores of around 10 microm. Cellular invasion into the sponges was examined in vitro (incubation with 3T3 fibroblasts) and in vivo (implantation in rabbit corneas). Although the in vitro assay detected a change in the cell behavior in the early stage of invasion, which was probably due to the formation of IPNs, such changes were not reflected in the longer term in vivo experiment. There was a proper integration of sponges K2 and K3 with the corneal stroma, but much less cellular invasion and no neovascularization in sponge K4. We concluded that IPN formation is a valid method to enhance the strength of PHEMA sponges, provided that the content of HEMA in the successive networks is not too high.

The modulation of corneal keratocyte and epithelial cell responses to poly(2-hydroxyethyl methacrylate) hydrogel surfaces: phosphorylation decreases collagenase production in vitro

We examined the regulation of collagenase production by rabbit keratocyte, epithelial and mixed keratocyte/epithelial cell cultures which were exposed to poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel surfaces with different chemistries and morphologies (sponge and homogeneous gels). Tissue culture modified polystyrene (TCP), used as a control surface, induced the maximum collagenase response with all cell culture types. Copolymer homogeneous gels containing 2-ethoxyethyl methacrylate (EEMA) or methyl methacrylate (MMA) induced a high response in keratocyte cultures, whilst PHEMA hydrogels induced a moderate response and the phosphorylated PHEMA (phos-PHEMA) hydrogel induced no response. Epithelial cells cultured on PHEMA, copolymer and phos-PHEMA hydrogels produced less collagenase activity than the keratocyte cells. The profile of collagenases produced by epithelial cells in response to phos-PHEMA was different to that for the other hydrogels. Co-cultured cells produced higher levels of collagenase (relative to the TCP) in response to hydrogels than did either the keratocytes or epithelial cells alone, but the response of phos-PHEMA was still the lowest. The overall enzyme response to the sponge hydrogels was lower than that to the homogeneous hydrogels, although this effect was less prominent in the keratocyte cultures. The markedly reduced and alternative collagenase responses to phosphorylated surfaces was not a consequence of cell death, and may be a phenomenon related to changes in cell surface charge and morphology.

The modulation of cellular responses to poly(2-hydroxyethyl methacrylate) hydrogel surfaces: phosphorylation decreases macrophage collagenase production in vitro

We examined the regulation of collagenase production by the monocyte/macrophage THP-1 cell line when these cells were exposed to poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel surfaces with different chemistries and morphologies. Tissue culture modified polystyrene (TCP), used as a control surface, induced the maximum collagenase response. Copolymer hydrogels containing 2-ethoxyethyl methacrylate (EMA) or methyl methacrylate (MMA) also induced a high response, while PHEMA hydrogels induced a low level response and the phosphorylated hydrogel induced no response. This pattern was altered when the morphology of the hydrogels was changed to that of a sponge. The overall enzyme response to the sponge hydrogels was lower than that to the homogeneous hydrogels. Sponges containing EMA and MMA produced low level response relative to the TCP control. PHEMA and phosphorylated sponges produced little and no response respectively. The dramatically reduced enzyme response to phosphorylated surfaces was not a consequence of cell death, and may be a phenomenon related to changes in cell surface charge.

Keratoprostheses: advancing toward a true artificial cornea

Keratoprosthesis surgery is carried out in very few centers. Elaborate surgical techniques and high complication rates limit the application of currently available keratoprostheses (KPros). However, the clinical need for an alternative to donor tissue has sparked considerable research interest in the development of new KPros. This paper charts the evolution of KPros from the earliest devices to those currently used, describes their drawbacks and discusses the specifications of an ideal device. Recent research focuses upon the use of porous polymers as the skirt component of core-and-skirt KPros in order to obtain improved biological integration of the prosthetic material. Developments in biomaterials technology make a KPro analogous to a donor corneal button an increasingly realistic goal. However, two particular problems still need to be addressed. First, it must be demonstrated that secure long-term fixation that is able to withstand trauma is achievable in a full-thickness artificial cornea. Second, an ideal artificial cornea for a wet eye requires an epithelialized surface, and this has yet to be achieved.

Keratoprosthesis: preliminary results of an artificial corneal button as a full-thickness implant in the rabbit model

PURPOSE

To develop a prototype artificial cornea and evaluate it in the rabbit model.

METHODS

Hydrogel core-and-skirt keratoprostheses were made and were inserted as full-thickness implants covered with conjunctival flaps in the right eyes of eight rabbits.

RESULTS

Peroperative complications related to inadequate mechanical strength led to failure in the early postoperative period in three animals, one was euthanased for an unrelated reason and the remaining four have been successful for up to 16 weeks' follow-up.

CONCLUSIONS

Full-thickness implantation of an artificial cornea, analogous to penetrating keratoplasty, has been achieved in the rabbit model. Histological findings confirm that integration of the prosthesis with host tissue occurs. The main complications encountered in this preliminary series were related to inadequate strength of the sponge skirt of this prototype device. Work in our laboratories is now concentrated upon improving the mechanical qualities of the hydrogel skirt and on the enhancement of biointegration.

Preliminary Evaluation of a Hydrogel Core-and-Skirt Keratoprosthesis in the Rabbit Cornea

BACKGROUND

We developed a core-and-skirt keratoprosthesis, with both components made from poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels. The identical chemical nature of both spongy skirt and transparent core assures a permanent union between them. We have previously shown that PHEMA sponges, within a certain range of pore size, can support cellular invasion and neovascularization when implanted into the rabbit cornea. The present study is the first to evaluate the behavior of the whole prosthesis after implantation into the rabbit cornea.

METHODS

Hydrogel keratoprostheses were inserted intrastromally into the corneas of seven rabbits and histologically examined by light microscopy in five eyes enucleated at 8, 12, and 14 weeks.

RESULTS

None of the implants extruded over this period. Both clinical and histopathologic examination showed that the keratoprostheses were well tolerated by the host tissue. The porous skirt was fully integrated into the stroma by fibrovascular invasion, and no capsule formed around the implants. Stromal melting anterior to the implant occurred in two cases, but this did not affect the fixation of the keratoprostheses.

CONCLUSIONS

This study indicates that our keratoprosthesis can prevent extrusion in the short term when inserted into an intrastromal pocket of the rabbit eye.

Production of neocollagen by cells invading hydrogel sponges implanted in the rabbit cornea

BACKGROUND

Poly(2-hydroxyethyl methacrylate) sponges are artificial tissue-equivalent matrices with potential value as materials for the peripheral zone of artificial corneas. A keratoprosthetic device was developed incorporating a poly(HEMA) spongy skirt which allowed cellular invasion. The present in vivo study investigated the biosynthetic activity of stromal fibroblasts growing within a poly(HEMA) sponge implanted into the rabbit cornea.

METHODS

A porous poly(HEMA) hydrogel was synthesized by polymerization in a large excess of water. Specimens with a pore size larger than 10 microns were impregnated with collagen type I and then implanted into the limbal region of cornea in four rabbits. The animals were followed clinically for 28 days, when they were anaesthetized and new sponge specimens were implanted in their second eye. After 2 h, both eyes were enucleated. The 28-day and 2-h explants were subjected to autoradiographic analysis following labelling with tritiated proline and to an immunostaining technique using antibodies to collagen types I-VI.

RESULTS

The autoradiographic analysis showed that the fibroblasts within the 28-day explants continued to be synthetically active and deposited proteins. Using the immunostaining technique, the deposition was most clearly demonstrated by the localization of collagen type III in the tissue invading the sponge. Both techniques failed to indicate any cellular activity in the short-time implants.

CONCLUSIONS

The presence of collagen type III is consistent with a normal healing response of the stromal fibroblasts and indicates that poly(-HEMA) sponges are able to function as tissue-equivalent matrices.

Axonal growth within poly (2-hydroxyethyl methacrylate) sponges infiltrated with Schwann cells and implanted into the lesioned rat optic tract

Porous hydrophilic sponges made from 2-hydroxyethyl methacrylate (HEMA) have a number of possible biomedical applications. We have investigated whether these poly(HEMA) hydrogels, when coated with collagen and infiltrated in vitro with cultured Schwann cells, can be implanted into the lesioned optic tract and act as prosthetic bridges to promote axonal regeneration. Nineteen rats (20-21 days old) were given hydrogel/Schwann cell implants. No obvious toxic effects were seen, either to the transplanted glia or in the adjacent host tissue. Schwann cells survived the implantation technique and were immunopositive for the low affinity nerve growth factor receptor, S100 and laminin. Immunohistochemical studies showed that host non-neuronal cells (astrocytes, oligodendroglia and macrophages) migrated into the implanted hydrogels. Astrocytes were the most frequently observed host cell in the polymer bridges. RT97-positive axons were seen in about two thirds of the implants. The axons were closely associated with transplanted Schwann cells and, in some cases, host glia (astrocytes). Individual axons regrowing within the implanted hydrogels could be traced for up to 900 microns, showing that there was continuity in the network of channels within the polymer scaffold. Axons did not appear to be myelinated by either Schwann cells or by migrated host oligodendroglia. In three rats, anterograde tracing with WGA/HRP failed to demonstrate the presence of retinal axons within the hydrogels. The data indicate that poly(HEMA) hydrogels containing Schwann cells have the potential to provide a stable three-dimensional scaffold which is capable of supporting axonal regeneration in the damaged CNS.

Interpenetrating polymer network (IPN) as a permanent joint between the elements of a new type of artificial cornea

The combination at the interface between two chemically identical polymers was investigated by light and electron (scanning, transmission) microscopy. The polymers constitute elements of a new type of artificial cornea in which the peripheral skirt is made from spongy poly(2-hydroxyethyl methacrylate) (PHEMA) and the central optical zone from homogeneous, transparent PHEMA. Their two-phase combination along the boundary fulfill formally the requirements for an interpenetrating polymer network (IPN). The procedure for the manufacture of prosthesis was described in detail. Thin and ultrathin sections excised from the interface region were investigated using microscopic techniques. Light microscopy allowed the measurement of the diffusion path length of transparent PHEMA into sponge, which was approximately 0.5 mm. Transmission electron microscopy revealed a cellular-like morphology as well as larger segregated zones, which indicated network interpenetration on a molecular level and also a relatively poor miscibility of the two polymers despite their identical chemical structure. The latter was interpreted as a result of the submicroscopic restraints imposed by polymer I (sponge) upon polymer II. This study provides evidence that the interface combination of the prosthetic elements should be regarded as a gradient homo-IPN. This system offers a union between elements much stronger than those previously reported in artificial corneas.