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

Advances in Hybrid Fabrication toward Hierarchical Tissue Constructs

The diversity of manufacturing processes used to fabricate 3D implants, scaffolds, and tissue constructs is continuously increasing. This growing number of different applicable fabrication technologies include electrospinning, melt electrowriting, volumetric-, extrusion-, and laser-based bioprinting, the Kenzan method, and magnetic and acoustic levitational bioassembly, to name a few. Each of these fabrication technologies feature specific advantages and limitations, so that a combination of different approaches opens new and otherwise unreachable opportunities for the fabrication of hierarchical cell-material constructs. Ongoing challenges such as vascularization, limited volume, and repeatability of tissue constructs at the resolution required to mimic natural tissue is most likely greater than what one manufacturing technology can overcome. Therefore, the combination of at least two different manufacturing technologies is seen as a clear and necessary emerging trend, especially within biofabrication. This hybrid approach allows more complex mechanics and discrete biomimetic structures to address mechanotransduction and chemotactic/haptotactic cues. Pioneering milestone papers in hybrid fabrication for biomedical purposes are presented and recent trends toward future manufacturing platforms are analyzed.

Host reaction to poly(2-hydroxyethyl methacrylate) scaffolds in a small spinal cord injury model

Tissue engineered scaffolds and matrices have been investigated over the past decade for their potential in spinal cord repair. They provide a 3-D substrate that can be permissive for nerve regeneration yet have other roles including neuroprotection, altering the inflammatory cascade and mechanically stabilizing spinal cord tissue after injury. In this study we investigated very small lesions (approx. 0.25 μL in volume) of the dorsal column into which a phase-separated poly(2-hydroxyethyl methacrylate) hydrogel scaffold is implanted. Using fluorescent immunohistochemistry to quantify glial scarring, the poly(2-hydroxyethyl methacrylate) scaffold group showed reduced intensity compared to lesion controls for GFAP and the chondroitin sulfate proteoglycan neurocan after 6 days. However, the scaffold and tissue was also pushed dorsally after 6 days while the scaffold was not integrated into the spinal cord after 28 days. Overall, this small-lesion spinal cord injury model provided information on the host tissue reaction of a TE scaffold while reducing animal discomfort and care.

The formation of a tissue-engineered cornea using plastically compressed collagen scaffolds and limbal stem cells

Collagen has excellent biocompatibility, is biodegradable, and possesses low immunogenicity. Therefore, this protein is a very suitable substrate for the formation of a corneal scaffold for therapeutic use. The highly hydrated nature of conventional collagen gels, however, results in a gel that is structurally weak and difficult to manipulate. In this chapter, we describe a novel method to cultivate limbal epithelial cells (LEC) on a compressed collagen scaffold. The compressed collagen scaffold can be rapidly constructed using a cell-independent process, which produces dense and mechanically strong collagen constructs with controllable microscale features.We embedded corneal keratocytes in a collagen gel, which we subsequently compressed and coated with laminin. The resulting construct supported the physiological morphology and stratification of LEC. The expression of a specific marker for differentiated LEC, cytokeratin 3 (CK3), and a marker for undifferentiated LEC, cytokeratin 14 (CK14), were similar in LEC expanded on both the compressed collagen construct and the leading conventional scaffold, denuded amniotic membrane (AM). We therefore demonstrate that a laminin-coated, compressed collagen gel containing keratocytes can support LEC expansion, stratification, and differentiation to a degree that is comparable to denuded AM. Our novel compressed collagen/keratocyte construct has potential for use as a tissue-engineered artificial cornea.

Towards the use of hydrogels in the treatment of limbal stem cell deficiency

Corneal blindness caused by limbal stem cell deficiency (LSCD) is a prevailing disorder worldwide. Clinical outcomes for LSCD therapy using amniotic membrane (AM) are unpredictable. Hydrogels can eliminate limitations of standard therapy for LSCD, because they present all the advantages of AM (i.e. biocompatibility, inertness and a biodegradable structure) but unlike AM, they are structurally uniform and can be easily manipulated to alter mechanical and physical properties. Hydrogels can be delivered with minimum trauma to the ocular surface and do not require extensive serological screening before clinical application. The hydrogel structure is also amenable to modifications which direct stem cell fate. In this focussed review we highlight hydrogels as biomaterial substrates which may replace and/or complement AM in the treatment of LSCD.

AlphaCor artificial cornea: clinical outcome

PURPOSE

The purpose of this study was to describe the long-term results of AlphaCor implantations, and to evaluate the main complications and risk factors.

METHODS

Retrospective analysis of preoperative and follow-up data from 15 AlphaCor implantations. Analysis of outcomes, trends, and associations was performed and compared with data from published clinical trials and a literature review.

RESULTS

The survival rate of the device at 1, 2, and 3 years was 87%, 58%, and 42%, respectively. Postoperative visual acuity ranged from hand movement to 0.8. The most significant complications were stromal melt (nine cases), optic deposition (three eyes), and retroprosthetic membrane formation (three eyes). The most common device-unrelated complication was trauma (three patients). All complications were managed without loss of the eye.

CONCLUSION

AlphaCor provides a treatment option for patients with corneal blindness in which a donor tissue graft would not succeed.

Glucose-permeable interpenetrating polymer network hydrogels for corneal implant applications: a pilot study

Epithelialization of a keratoprosthesis requires that the implant material be sufficiently permeable to glucose. We have developed a poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network (IPN) hydrogel that can provide adequate passage of glucose from the aqueous humor to the epithelium in vivo. A series of PEG/PAA IPNs with varying PEG macromonomer molecular weights were synthesized and evaluated through swelling studies to determine their water content and diffusion experiments to assess their permeability to glucose. One of the PEG/PAA hydrogels prepared in this study had a glucose diffusion coefficient nearly identical to that of the human cornea (approximately 2.5 x 10(-6) cm(2)/sec). When implanted intrastromally in rabbit corneas, this hydrogel was retained and well-tolerated in 9 out of 10 cases for a period of 14 days. The retained hydrogels stayed optically clear and the epithelium remained intact and multilayered, indicating that the material facilitated glucose transport from the aqueous humor to the anterior part of the eye. The results from these experiments indicate that PEG/PAA hydrogels are promising candidates for corneal implant applications such as keratoprostheses and intracorneal lenses, and that the PEG/PAA IPN system in general is useful for creating permeable substrates for ophthalmic and other biomedical applications.

AlphaCor: Clinical outcomes

PURPOSE

To study the outcomes of AlphaCor implantation.

METHODS

: The AlphaCor artificial cornea is indicated for corneal blindness not treatable by donor grafting. Prospective preoperative and follow-up data were collected. Data were evaluated using SPSS for statistical analysis of outcomes, trends, and associations.

RESULTS

This report includes data returned through February 28, 2006, for all 322 devices implanted, with mean follow-up in situ of 15.5 months and a maximum of 7.4 years. The probability of AlphaCor retention at 6 months and 1 and 2 years for protocol cases was 92%, 80%, and 62%, respectively, and off-label cases were at higher risk (P = 0.010), as were cases not prescribed medroxyprogesterone (MPG; P = 0.001). Currently, the most common complications were stromal melting, fibrous reclosure of the posterior lamellar opening, and white intraoptic deposits, with incidences in 2005 of 11.4%, 5.1%, and 2.6%, respectively. MPG seems to protect against melts, and eyes with a history of herpetic keratitis were not at increased risk. A history of glaucoma or the presence of tubes did not affect device retention. Complications culminated in loss of an eye in 1.3%. Mean preoperative visual acuity (VA) was hand movements. The VA achieved postoperatively (light perception to 20/20) was affected by previous pathology and postoperative course, with a mean improvement of 2 lines.

CONCLUSION

AlphaCor provides a treatment option where a donor tissue graft would not succeed in severe corneal conditions, while being reversible to a donor graft in the event of complications for anatomic integrity. Surgical technique and adjunctive therapies are evolving with experience. Continued data collection is important for a fuller understanding of AlphaCor's role.

Histology of AlphaCor skirts: evaluation of biointegration

PURPOSE

To report histologic findings in 14 AlphaCor artificial corneas implanted during clinical trials and subsequently explanted from human subjects following complications, so as to evaluate biointegration within the device skirt.

METHODS

Explants were fixed and sectioned in paraffin. Histologic findings related to the device skirt were compared with earlier histologic results from animal studies and correlated with clinical histories.

RESULTS

Two devices had been removed due to complications related to the optic alone, 11 following stromal melting overlying the biointegratable sponge skirt and 1 due to a retroprosthetic membrane. All devices demonstrated normal skirt porosity. Biointegration was similar to that found in animal studies but qualitatively appeared reduced in the affected areas in patients with overlying stromal melting prior to explantation. Patients with a longer history of melting prior to explantation demonstrated presence of inflammatory cells around the device.

CONCLUSIONS

Histologic findings of the AlphaCor skirt in humans are consistent with earlier animal studies. This study confirms that biointegration by host fibroblastic cells, with collagen deposition occurs after AlphaCor implantation in humans. In cases in which stromal melting had occurred, biointegration is seen to be reduced. On correlating preoperative clinical factors with biointegration observed histologically, preoperative vascularization appears not to be required for AlphaCor biointegration.

A photopolymerized sealant for corneal lacerations

PURPOSE

To determine whether a novel photocrosslinkable polymer synthesized from hyaluronic acid would seal experimental full-thickness corneal lacerations in a rabbit model.

METHODS

A solution of hyaluronic acid was modified with methacrylate groups (HA-MA), precipitated, dried, reconstituted in an aqueous solution, and sterilized before use. The viscous polymer solution was applied to 38 of 43 experimental corneal lacerations in rabbits and subsequently irradiated with a low-intensity argon laser beam to produce a clear flexible polysaccharide hydrogel patch. The ability of this sealant to repair corneal lacerations was evaluated in four types of full-thickness, 3-mm corneal wounds (linear, linear + epithelium removed, stellate, and stellate + epithelium removed). Slit-lamp examinations, measurements of intraocular pressure, Seidel tests, and histologic studies were performed at selected intervals to evaluate the wound and determine the rate of healing.

RESULTS

Corneal perforations were completely sealed and the anterior chambers had reformed by 6 hours in HA-MA-treated eyes. There was no evidence of leakage at this or later times in 37 of the 38 eyes. Intraocular pressure had risen to near-normal levels by day 7 in all four groups, and the sealant was still present in most eyes at day 7. In contrast, the anterior chambers did not re-form in control eyes (five) with untreated perforations because of aqueous leakage through the wounds. Minimal inflammation was observed clinically or in histologic sections of treated corneas. There was extensive proliferation of stromal cells and formation of new extracellular matrix at the wound edges, which became tightly adherent between days 4 and 7.

CONCLUSION

Our novel photocrosslinkable methacrylated hyaluronan polymer sealed 97% (37/38) of the experimental corneal lacerations. HA-MA may prove useful for sealing corneal lacerations in patients and for other sutureless ophthalmic surgical procedures.

Tailoring of new polymeric biomaterials for the repair of medium-sized corneal perforations

The aim of this study was to investigate whether polymeric biomaterials can be designed such that they become suitable for surgical closure of medium-sized perforations in the cornea, the transparent tissue in the front of the eye. Such a biomaterial must meet stringent requirements in terms of hydrophilicity, strength, transparency, flexibility, and biocompatibility. Four different copolymers of n-butyl methacrylate (BMA) and hexa(ethylene glycol) methacrylate (HEGMA) were prepared and characterized. Poly(BMA) was made as a reference material. Physicochemical properties were measured (contact angles, glass-transition temperatures, thermal degradation, water uptake and swelling), and cytotoxicity in vitro was assessed with a MTT test. Moreover, the interaction between the materials and cultured human corneal epithelial cells was studied. The copolymers may be useful for temporary closure of corneal perforations.

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.

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.

Development and clinical assessment of an artificial cornea

Keratoprosthesis research has been a gradual, rather fragmentary process with advances being made by isolated groups of researchers. This has arisen partly because of poor funding in the area; research groups which have achieved commercial support have often had constraints upon the full disclosure of their findings. Despite these difficulties there has been real progress over the last decade by several independent groups. This article concentrates upon our own development of a hydrogel core-and-skirt keratoprosthesis, the Chirila KPro, in order to illustrate the scientific and clinical problems common to keratoprosthesis research. Pilot data from a clinical trial is presented and the priorities for future research are discussed.

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.

Cell interactions with perfluoropolyether-based network copolymers

We have investigated the potential of several polymers based on perfluoropolyether (PFPE) macromonomers for use in biomaterial applications. Polymer networks were synthesised from the PFPE macromonomers of increasing chain length and the adhesion and proliferation of corneal, vascular and bone cells was evaluated on these polymers. The polymer surfaces were quite hydrophobic, having sessile air-water contact angles ranging between 96 and 125 degrees. However, these polymers supported the attachment and growth of bovine corneal epithelial and endothelial cells and fibroblasts at 60-100% of the rate of cell growth on the culture substratum, TCPS. Furthermore, the PFPE polymers supported the attachment and growth of vascular endothelial cells (from human umbilical artery) and human bone-derived cells over a 7 day period at an equal level to TCPS. The relationship between the macromonomer chain length (n = 1 to 4) and the ability of the resulting PFPE homopolymer to support the overgrowth of corneal epithelial tissue was also evaluated. The PFPE-containing polymers supported corneal epithelial tissue overgrowth, with the most effective having a performance equivalent to that of TCPS. In addition to these homopolymers, copolymers comprising of PFPE and N,N-dimethylaminoethyl methacrylate (DMAEMA) were also synthesised. Surprisingly, the addition of DMAEMA to the PFPE polymer network lead to a reduction in the growth and attachment of corneal epithelial cells and fibroblasts. These results indicate that PFPE-based materials show a potential for use in the development of biomaterials in the ocular, vascular and orthopaedic areas.

Implantation of a synthetic cornea: design, development and biological response

Our goal was to evaluate 3 different designs of synthetic corneas in vivo. All devices had a transparent hydrogel center molded to a porous peripheral skirt. Over 30 devices were implanted into rabbits and followed for up to 6 months. The devices were preseeded with rabbit stromal fibroblasts, which enhanced the rate of fibroplasia. The anterior surface of the hydrogel was modified using argon rf plasma treatments. Clinical examinations were performed, and histological analyses were conducted on tissue throughout the time course. Our optimal model ranged from 4.5 to 6 mm and had an extended porous skirt increasing the surface area for fibroplasia and ultimate anchorage of the device. Fibroplasia occurred in this model, and collagen was detected by 28 days. The anterior chamber was normal with no detectable leakage of aqueous humor. Glycosaminoglycans were detected and followed the time course outlined previously when porous material itself was inserted into the stroma. We present the first demonstration that rabbit limbal epithelial cells can migrate onto the synthetic cornea in vivo.

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.