Implantable thick conducting polymer devices for tissue regeneration

Targeted, low-frequency electrical stimulation of body tissues has therapeutic applications in aiding healing and tissue regeneration. Low-frequency electric fields can be applied by wearable electrodes on the surface of the body for accelerating the healing of wounds on the skin, or by internally implantable electrodes to accelerate and aid nerve regeneration following injury, for example spinal cord damage.

In implantable devices in particular, matching the stiffness of the materials to the surrounding body tissues is important to avoid damaging tissues and triggering rejection of the device by the immune system, which would limit the lifespan of the device. The requirement for relatively soft materials that can conduct electricity with low impedance can be met using electrically conducting polymer films.

Such films can be bonded on top of a thin metal electrode to lower the electrode's impedance and increase capacitance while maintaining flexibility.

Low-frequency stimulation requires electrodes with high charge injection capacity. Charge injection capacity can be increased by increasing electrode volume, but implanted devices with larger footprint are more likely to cause damage and more vulnerable to rejection by the body so have shorter implantation lifespan. It is therefore desirable to design an electrode with a thick layer of the conducting polymer.

Poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) is a popular conducting polymer film in implantable bioelectronics, with established fabrication techniques such as spin-coating that produce high quality films of controllable thicknesses. However, PEDOT fabrication techniques are only established for film thicknesses up to 2 micrometres.

The film thickness required to facilitate the low-frequency stimulation to accelerate wound healing and nerve regeneration is upwards of 100 micrometres. The aim is to establish a reliable protocol for thick PEDOT:PSS film fabrication using single drop-casting combined with laser patterning, through iteration on the fabrication process and characterisation of the resulting films.

In addition to conducting polymer film fabrication, the other components of the device must be designed, as well as their assembly and bonding together into the final device. This includes the metal foil electrode, wire, and the insulating and adhesive layers. The metal foil layer must also be optimised for low stiffness to avoid tissue damage, high yield strength to avoid permanent deformation and damage to the device, and high surface area to minimise impedance.

This metal layer will be fabricated by lasers according to a pattern drawn in computer-aided design software. This pattern will be designed and adapted according to the results of tests on the properties of candidate patterns.

The objective of this project is the design and reliable fabrication protocol of a flexible, biocompatible device capable of low-frequency electrical stimulation of body tissues and suitable for wearable or implantable use, based on a thick conducting polymer film. Such a device has the potential to improve quality of life and healthcare outcomes for patients in a wide range of situations.

This project falls within the EPSRC healthcare technologies research area.