Acta Neurochir Suppl (2006) 99: 141–144 # Springer-Verlag 2006 Printed in Austria Neural prosthesis in the wake of nanotechnology: controlled growth of neurons using surface nanostructures J. K. Lee 1 , H. Baac 1 , S.-H. Song 2 , E. Jang 3 , S.-D. Lee 3 , D. Park 2 , and S. J. Kim 1;3 1 Nano-Bioelectronics and Systems Research Center, Seoul National University, Seoul, Korea 2 School of Biological Sciences, College of Natural Sciences, Seoul National University, Seoul, Korea 3 School of Electrical Engineering, College of Engineering, Seoul National University, Seoul, Korea Summary Neural prosthesis has been successfully applied to patients with mo- tional or sensory disabilities for clinical purpose. To enhance the per- formance of the neural prosthetic device, the electrodes for the biosignal recording or electrical stimulation should be located in closer proximity to target neurons than they are now. Instead of revising the prior implant- ing surgery to improve the electrical contact of neurons, we propose a technique that can bring the neurons closer to the electrode sites. A new method is investigated that can control the direction of neural cell growth using surface nanostructures. We successfully guide the neurons to the position of the microelectrodes by providing a surface topograph- ical cue presented by the surface nanostructure on a photoresponsive polymer material. Because the surface structure formed by laser holo- graphy is reversible and repeatable, the geometrical positioning of the neurons to microelectrodes can be adjusted by applying laser treatment during the surgery for the purpose of improving the performance of neural prosthetic device. Keywords: Photoresponsive polymer; surface structure; neural cell guide; growth control; neural prosthesis. Introduction Neural prosthesis is a technology which rehabilitates a patient with motional or sensory disability by electrical recording or stimulation of the neurons [2]. The neural prosthesis has become an important therapeutic option for these disabilities because transplantation and stem cell approaches for the neural cell are promising, but still distant. Significant progress has been made in several areas of neural prosthesis. The cochlear implant system restores the auditory sense in the deaf [4]. The artificial retina system stimulates the remained retinal neural cells in patients with degenerated photoreceptors to restore vision [8]. The cortical prosthetic device is proposed to monitor and stimulate the motor cortex for quadriplegic patients due to cervical injury and amyotrophic lateral sclerosis [7]. Functional electrical stimulation is also promising for patients with neurogenic bladder, paraple- gia due to lumbar injury and so on [10]. Microelectrodes in these devices play a crucial role in stimulating and recording neural signals for monitoring and controlling the activities of the nervous system. To record the bio- signals and stimulate the neurons effectively with high resolution, the electrodes have to maintain contact with the targeted neuronal cells closely enough to focus the electrical current onto the target cells. The electrode sur- face needs to be within 100 nm of the nerve cell in order to obtain a reasonable signal to noise ratio [5]. However, in almost all prosthetic devices, the relative position of electrodes with respect to target neurons is determined at the moment of the operation. Correct positioning of the electrode with micro and nanoscale precision is not possible with conventional surgery. Moreover, once the device is implanted into the body of the patient, the position cannot be adjusted without revision of the pre- vious implantation surgery. In an approach of enhancing the interfacing electrical activity [1], a semiconductor fabrication technique with silicon, silicon nitride, and gold was used to provide a substrate to align neurons by reaction to the topography. This approach succeeded in guiding the neurons by providing micro or nano struc- ture on the surface of the devices. Nevertheless, the limitation of such approach is that the topography in the substrate is fixed after fabrication, so modifying the physical structure to adjust the position of the neurons after surgery is not possible. A material with dynamically changeable properties would open a