Medical Device Link.

MATERIALS

Long term requirements

Figure 1: Liquid water transport through lipid modified silicones. Error bar represents standard deviation of three simultaneous measurements. (click image to enlarge)

The Healthy Aims project,¹ 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 European Union citizens suffering from those disabilities. This article describes the biomaterials that form an essential part of these new implants.

The implants developed under the Healthy Aims Project are a cochlear implant, a retina implant, an intra-cranial pressure sensor implant and functional electrical stimulation systems for limb, bladder and bowel control. The system and product specifications for these 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

Figure 2 : Relative, Swiss albino murine fibroblast cell growth at 96 h (almarBlue cytotoxicity test) on lipid modified silicones. Mean ± 95% CI (n=12 for controls and 6 for test materials). (click image to enlarge)

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.² 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 a synthetic hydrophilic 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.

Figure 3: Retina dummy structure coated with 1 % w/v HL modified silicone.

Figure 1 shows the results of liquid water transmission through lipid modified silicones. 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 silicones is improved biocompatibility. Figure 2 shows the results of the AlamarBlue cytotoxicity test for these materials. The increase in lipid concentration led to increased cell growth on the membranes, which suggests improved biocompatibility. Figure 3 shows the retina dummy structure coated with 1% w/v of HL modified silicone.

Figure 4 : Optical microscopy image of DLC layer deposited on 1% w/v HL modified silicone membrane.

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 resistant 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.³ 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.⁴

The QMUL group optimised DLC deposition parameters for various silicone compositions on implantable microelectronic devices and showed that it is possible to obtain uniform adherent ultra-thin layers. Figure 4 shows one example of a uniform DLC coating and Figure 5 shows the intracranial pressure sensor implant coated with optimised DLC layer.

Improved implant interface

Figure 5: Retina dummy structure coated with 1 % w/v HL modified silicone.

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.

Figure 6 : Alignment of PC12 neurites with an underlying grooved topography (INEX, UK).

Increasingly modifying this interface has brought about additional device functionality such as reducing power consumption and improving performance. The group has investigated physical and chemical techniques for their ability to define the biological response to active implanted devices.

Topographical surface modification. Standard photolithography techniques have been used to produce three dimensional 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 (Figure 6). 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 nonelectrically active cells is expected to reduce electrode impedance and thus reduce the power consumption of these types of device.

Figure 7: Different silane chemistries can been seen to affect cell adhesion both in the presence and absence of collagen (INEX, UK). (click image to enlarge)

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 different silane modified substrate surfaces and studied their effect on cell differentiation. The investigations have shown a differential response to different functional groups (Figure 7). This work is being optimised and will be applied to active electrode surfaces in the final stages of this project.

Figure 8 : Functional modifications with active biomolecules; chondroitin sulphate (centre) and PEG (right) can be shown to reduce cell adhesion compared with decorin (left), which showed no difference to the control (INEX, UK). (click image to enlarge)

Biomolecular techniques. The subsequent functionalisation of the chemically modified surfaces with active biomolecules 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 differentiation 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 (Figure 8) and, as described above, contribute to improved device performance.

End game

In the final stages of the project the Healthy Aims product partners are now evaluating the biocompatible barrier materials that have already been successfully demonstrated in the laboratory by QMUL with the view to introducing them into their products. INEX has also successfully demonstrated that surface functionalisation 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.

Dr Diana Hodgins* MBE DSc (Honorary) is Project Co-ordinator of Healthy Aims and Managing Director of European Technology for Business Ltd, Codicote Innovation Centre, St. Albans Road, Codicote, SG4 8WH, UK tel. +44 1438 822 822 e-mail: diana.hodgins@etb.co.uk, www.etb.co.uk

Dr J.M. Wasikiewicz is Research Assistant, e-mail: j.m.wasikiewicz@qmul.ac.uk, www.materials.qmul.ac.uk/irc/

Dr M.F. Grahn Senior Lecturer, e-mail: m.f.grahn@qmul.ac.uk, www.icms.qmul.ac.uk/centres/surgery

Dr D. Paul is Research Assistant, e-mail: d.paul@qmul.ac.uk

N. Roohpour is a Ph.D. student, e-mail: n.roohpour@qmul.ac.uk

Professor P. Vadgama is Head of IRC in Biomedical Materials, e-mail: p.vadgama@qmul.ac.uk, www.materials.qmul.ac.uk/irc/

Dr Angela Silmon is Project Leader, e-mail: a.m.silmon@ncl.ac.uk, www.inex.org.uk

Dr Bernard Verdon is postdoctoral research associate, e-mail: Bernard.verdon@ncl.ac.uk, www.inex.org.uk

Dr Brian Cousins post doctoral research associate, e-mail: brian.cousins@manchester.ac.uk, www.materials.manchester.ac.uk/

* To whom all correspondence should be directed.