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| Biocompatible Materials Developments For New Medical Implants |
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By D. Hodgins and J.M Wasikiewicz
Recent work on modifying silicone rubber to improve water permeability and biocompatibility is described. In addition, modifications to the interface between an active implanted device and the body are reported, which have led to reduced power consumption and improved device performance.
Long term requirements
The Healthy Aims project, funded under the European Framework 6 Programme, has developed a range of micro-nano-bio technologies beyond the current state of the art and integrated these into future medical implants. Functions such as hearing, sight and bladder control lost through illness or disability will be restored and the quality of life will be improved for millions of Europeans Union citizens suffering from those desabilities. This article describes the biomaterials that form an essential part of these new implants.
The implants developed under the healthy Aims projects are a cochlear implant, a retina implant, an intracranial pressure sensor implant and funtional electrical stimulation systems for limb, bladder and bowel control. The system and product specifications for those devices included the user requirements in terms of functionality, location and physical constraints. They all require biocompatible coating material(s) that must remain stable for the lifetime of the implant, which in many cases can be the lifetime of a person.
Ultrathin coating materials
A group at Queen Mary University of London (QMUL) (London, UK) has been working with the Healthy Aims product partners to develop biocompatible coatings that are thin, flexible and rugged enough for handling during surgery. An important requirement to allow an implantable electronic device to perform acceptably within the electrolyte medium of the inside of the body is that it is hermetically packed using biocompatible barrier materials. There is a wide range of new, generally polymeric materials proposed as encapsulants. However, the introduction of new biomaterials involves extended testing and detailed study over lengthy time periods for regulatory approval. Thus, there are advantages to adopting and modifying existing materials that already have regulatory approval. Because Healthy Aims wished to move new products into clinical trials within four years the decision was taken to modify existing materials
Silicone rubber
This material has been the most frequently investigated and is already medically approved as a bioinert sealing material. Unfortunately, its packaging effectiveness is far from perfect, not least with regard to the uptake and transmission of water. Moreover, although silicone rubber has good biocompatibility, this could be improved. Hence, the group proposed a silicone modification through physical entrapment of synthetic hydrophylic lipid (HL) to try to reduce its water permeability/uptake and to improve biocompatibility. This modification is a purely physical process and therefore does not involve the creation of any potentially toxic side products, which occur during chemical synthesis. Because silicone and the lipid are clinically approved materials, introduction to medical market could be significantly accelerated.
The figure shows the results of liquid water transmission through lipid modified silicone. Small but significantly improved water resistance can be observed with increasing HL concentration up to 1% weight/volume (w/v); any further increase of HL concentration has no effect on water transmission through the membranes.
Another advantage of lipid modified silicone is improved biocompatibility. The figure shows the results of the AlamarBleue cytotoxicity test for these materials. The increase in lipid concentration led to increased cell growth on the membranes, which suggests biocompatibility. It shows also the retina dummy structure coated with 1% w/v of HL modified silicone.
Diamond like carbon (DLC)
This is another material that has been considered by the group.This ultrathin (< 1 micron) hydrogen carbon alloy is plasticiser free and is obtained via plasma assisted chemical vapour deposition directly onto a substrate material/device. DLC provides hard but flexible layers that are resistent to wear and shear forces. Its chemical inertness guarantees a corrosion barrier and its dense amorphous atomic structure helps create a secure barrier to the diffusion of molecules, even small diffusants such as noble gases. DLC adheres strongly to a range of materials used for bioengineering and surgical purposes, including metals and polymers. Thus it can provide chemically protective, impermeable coatings to these materials in a biological environment. In vitro tests have been previously reported with mouse macrophage cells and fibroblast cells. Measured enzyme activity indicated that there was no inflammatory response or loss of cell integrity on contact with DLC. Morphological examination has confirmed the biochemical results and that no cellular damage apparently occurs.
The QMUL group optimised DLC disposition parameters for various silicone compositions on implantable microelectronic devices and showed that it is possible to obtain uniform adherent ultra-thin layers.
Improved implant interface
A group at INEX (Newcastle upon Tyne, UK) has been working on ways of optimising the electrode-nerve interface by discouraging growth of scar tissue on electrodes and/or encouraging the growth of neuronal cells on the electrodes that encourages electron flow at the interface. Understanding the interface between any implanted device and the body is critical to ensure appropriate interactions. Increasingly modifying this interface has brought about additional device funtionality such as reducing power consumption and improving performance. The group has investigated physical and chemical techniques for their ability to define the biologogical response to active implanted devices.
Topographical surface modification
Standard photolithography techniques have been used to produce three dimentional physical features integrated into the planar surface of an active electrode. Successful optimisation of the parameters has demonstrated the alignment of more than 90% of extending neurites to the underlying features. The modiolus electrode, which is designed to recover lost hearing function, directly contacts neural tissue. Integrating these topographical features into devices such as the modiolus electrode will, it is hoped, increase performance by reducing the distance between the biologically active cells and the electrically active surface of the device. In an alternative approach, subcellular scale topographies have been demonstrated to reduce the adhesion and proliferation of a glial cell line. The ability to prevent biofouling of an electrode surface with nanoelectrically active cells is expected to reduce electrode impedance and thus reduce the power consumption of these types of device.
Chemical functionalisation
Changes in surface chemistry have been shown to affect the properties of cell adhesion and behaviour.The group has investigated the adhesion of a model neuronal cell line, PC12 cells, to a range of diferent silane modified substrate surfaces and studied their effect on cell differentiation. The investigations have shown a differential response to diferent functional groups. This work is being optimised and will be applied to active electrode surface in the final stages of this project.
Biomolecular techniques
The subsequent funtionalisation of the chemically modified surfaces with active biomolecule has been extensively studied. Micro contact printing was used to pattern extracellular matrix proteins such as collagen to bring about neurite orientation in a similar way to the topographic features. The immobilisation of active growth factors to promote neurite diferentiation has also been demonstrated within the project. This alternative technique of defining neuronal alignment using active biomolecules may be targeted at specific neuronal populations depending on the function to be replaced in future implantable devices. Biomolecules associated with the prevention of cell adhesion such as chondroitin sulphate and polyethylene glycol (PEG) have also been immobilised to active surfaces where they are shown to reduce cell adhesion and proliferation and; as described above, contibute to improve device performance.
End game
In the final stages of the project the Healthy Aims product partners are now evaluating the biocompatible barrier materials that heve already been successfully demonstrated in the labotory by QMUL with the view to introducing them into their products. INEX has also successfully demonstrated that surface funtionalisation can be used to influence and direct cell behaviour and in the final stages of the project these modifications are being integrated into product demonstrators to investigate their effect on performance.
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