Thursday 8 May 2014

Biohybird retinal implant

Recently I found very interesting scientific article "Biohybrid Visual Prosthesis for Restoring Blindness" of Tohru Yagi.

Dr. Tohru Yagi's group has been conducting basic research and system design/integration on a biohybird retinal implant, which consists of cultured neurons on MEMS (Microelectromechanical Systems). Accordingly, “bio-hybrid" visual prosthesis combines the characteristics of regenerative medicine and visual prostheses. The first prototype consists of an external and an internal device. In operation, visual information is captured by a video camera in the external device. After encoding, this information is then sent to the internal device through an infrared (IR) communication unit. After the internal device receives the IR data, it generates appropriate electric pulses for stimulating the cultured neurons. Then cultured neurons send signals to the brain and the user can recognize visual information. In a biohybrid implant, it is the most prominent feature that the axons of transplanted neurons are used as living electric cables to form functional connections between neurons on the electrode array and the CNS.

The biohybrid implants require the implantation of not only the MEMS, but also the transplantation of nerve cells. Recently, it has been shown that when nerve cells and Schwann cells are together, irrespective of their origin, the visual cortex or periphery, the lengthening of nerve fibers is promoted by factors produced by Schwann cells, and myelin sheath formation occurs. Аn artificial optic nerve is prepared from Schwann cells (a semipermeable membrane tube filled with cultured Schwann cells, extracellular matrix, and neurotrophic factors), the axons of these nerve cells are guided to the higher visual cortex, connecting the MEMS with the visual cortex.

So, the nerve cells are used as a ‘living electrical cable". Once the connection is complete, it is considered that nerve cells transmit signals to the visual cortex in response to electrical pulses provided by the electrode array. Because nerve cells are transplanted as part of the process of fitting this visual prosthesis, a biohybrid implant is appropriate for blind patientswhose optic nerves and/or retinal ganglion cells are NOT intact such as glaucoma and diabetic retinopathy patients. Although biohybrid implants have advantages, there are many challenges related to nerve cell transplantation. Even if the axons of nerve cells can be guided to the visual cortex, unless a connection is formed between the neurons of the visual cortex and synapses, and a functional connection achieved via neurotransmitters, the signals cannot be communicated. 

The fundamental challenge for this prosthesis is the reliable reconstruction of signaltransmission function between an artificial device and transplanted nerve cells, and between transplanted nerve cells and the visual cortex.In addition, the long-term use of metallic electrodes induces connective tissues covering metal parts, and causes glioma aggregation and/or scar formation. Dr. Yagi's group regards it may be possible to develop a conductive polymer electrode that has a high affinity to biological tissues. This electrode may be bound to neural tissues at the molecular level so that a neuron will be stimulated intracellularly or quasi-intracellularly to decrease the threshold current significantly, and the functionality; biocompatibility of electrodes will be improved. For that purpose, they have been developing the technique of micro/nanofabrication of conductive polymers.

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