Engineering artificial corneas for restoring vision
27 January 2015
Author: Mark Ahearne, senior research fellow, Trinity Centre for Bioengineering, Trinity Biomedical Sciences Institute
Blindness is a debilitating condition that affects many people throughout Ireland and across the world. According to the National Council for the Blind of Ireland, some 6,500 people are registered blind in this country.
Every five seconds, somebody in world will go blind while in Europe, approximately one in 30 people will experience sight loss at some point in their lives. The average unemployment rate among blind and partially-sighted people of working age is estimated to be over 75%. This has created significant social and economic challenges in many countries that have yet to be overcome. These challenges could be partially alleviated by improving or developing new treatments for restoring vision.
Corneal blindness is among the most common causes of blindness worldwide. The cornea is the transparent front outer layer of the eye whose primary function is to focus light as it enters the eye. A loss of transparency results in corneal blindness, which can be caused by a variety of medical conditions including keratoconus, a condition that results in corneal thinning and distortion, Fuch’s dystrophy, a condition that results in corneal swelling and opacity or Stephen-Johnson syndrome, a condition that can cause erosion and blistering of the cornea. In addition, corneal blindness may result from physical injury or exposure of toxic chemicals to the surface of the eye.
In some cases, non-surgical interventions can be used to restore vision. However, in more severe cases, a corneal transplant is required. Currently, the main limitation with corneal transplants is the significant shortage of suitable donor cornea available for transplant. In Ireland, all corneas used for transplantation have to be imported from abroad due to the theoretical risk of CJD transmission from Irish donors.
In addition, many donated corneas are unsuitable for transplantation for a variety of reasons including contamination and poor endothelial cell density on the inner corneal surface. All of these problems have led researchers to investigate new approaches to restoring vision such as potentially developing bioengineered corneal implants that can replace the function of the damaged cornea.
Bioengineering artificial corneas
The approach taken by Dr Mark Ahearne and his research group at the Trinity Centre for Bioengineering (TCBE) in Trinity College Dublin is to combine stem-cell technology, biomaterials synthesis and tissue-engineering techniques to develop new approaches to bioengineer artificial corneas and regenerate corneal tissue.
Among the more promising approaches being investigated is the use of adipose-derived stem cells for corneal regeneration. These stem cells can be easily isolated from excess body fat, like that normally discarded during liposuction. When cultured under the right conditions, these stem cells can be manipulated to behave like corneal stromal cells or keratocytes, thus providing a valuable source of cells for bioengineering corneal tissue.
The challenge, however, is to find the optimal combination of biochemical and environmental factors to control how the cells behave. Recently, the group has shown that by making adjustments to the composition of the solution in which the stem cells are initially grown, the ability of the stem cells to behave like keratocytes can be affected (Ahearne et al, 2014).
In addition to examining cell behaviour, researchers in Dr Ahearne’s group at TCBE have been investigating different approaches to manufacturing three-dimensional scaffolds that could support corneal regeneration and potentially be used as a corneal implant. In tissue engineering, scaffolds are used to provide a three-dimensional structure that acts as a temporary artificial template. When cells are added to the scaffold, they replace the scaffold materials with proteins and other organic components that they generate themselves, thus leading to the formation of a tissue.
Among the types of scaffold under investigation at TCBE for corneal regeneration are hydrogel based scaffolds that have been manufactured from natural biopolymers. Hydrogels are formed from water-swollen polymer networks and depending on the type of polymer material, they can have excellent biocompatibility and transparency while still retaining the ability to be remodelled by cells.
An example of another type of scaffold being fabricated at TCBE is the decellularised porcine corneal scaffold, currently being developed by Amy Lynch, a PhD student at Trinity College Dublin. Lynch’s research has recently led to the publication of a paper analysing different approaches for generating decellularised corneal scaffolds (Lynch et al, 2013).
With the decellularisation approach, the cornea is removed from an enucleated pig eye and all the cells and cellular components within the cornea are destroyed using a specialised cocktail of chemicals. These cells need to be removed to prevent them from eliciting an immune response after implantation. The result is a cell-free or acellular scaffold that retains the composition and structure of the native cornea.
The transparency and mechanical strength of the scaffolds after decellularisation are also similar to those found in a healthy cornea. Human cells can then be incorporated into the scaffold, thus providing a tissue engineered corneal construct. Recently, work has been carried out in collaboration with the Academic Ophthalmology Unit at the University of Nottingham to find the optimal techniques for seeding human corneal cells into these scaffolds.
An alternative approach to generating a corneal scaffold that is also under investigation at TCBE involves the application of electrospinning to manufacture nanofibres. The native cornea consists of parallel bundles of aligned collagen fibrils that form lamella. These lamella are arranged perpendicular to each other, giving the cornea a unique structure that allows it to remain both transparent and mechanically resilient. In electrospinning, lamella sheets of aligned fibers can be produced, similar to those found in the native cornea.
Electrospinning works by extruding a biopolymer through a needle whilst connected to a high-voltage power supply. The charge on the polymer causes it to be stretch rapidly upon leaving the needle, leading to the formation of thin fibres. The aligned fibre sheets can be assembled perpendicularly to each other to generate a three-dimensional scaffold that mimics the structure of the native cornea. Different materials for generating the scaffolds are currently under investigation to determine the most suitable for use in manufacturing a corneal scaffold. Once assembled, cells can be incorporated into the scaffolds with the aim of generating a functional engineered tissue.
The overall aim of the research being undertaken is to generate new therapies that can be used to treat patients suffering from corneal blindness. To date, this research has been supported by Science Foundation Ireland and the Marie-Curie COFUND (11/SIRG/B2104).
The recent award of a European Research Council (ERC) Starting Grant, entitled ‘Engineering a scaffold-based therapy for corneal regeneration (EyeRegen)’, will enable this research to prosper and hopefully help develop new products to restore vision to those affected by corneal blindness.
More information on the research being undertaken by the Dr Mark Ahearne and his research group at Trinity College Dublin is available from their website.
Ahearne M, Lysaght J, Lynch AP. ‘Combined influence of basal media and fibroblast growth factor on the expansion and subsequent differentiation capabilities of adipose-derived stem cells.’ Cell Regeneration 2014; 3: 13.
Lynch AP, Ahearne M. ‘Strategies for developing decellularized corneal scaffolds.’ Exp Eye Res 2013; 108: 42-47.