The great mismatch in physical and chemical properties between the hard electrodes and the soft brain tissues is still a barrier to achieve ideal neural prosthetic devices with excellent long-term performance. The current solutions focus on tailoring the properties of the electrode surface. This review summarizes the impediments that hinder the development of the neural electrodes including poor biocompatibility, low sensitivity, high impedance and poor stability, and then presents the progress in the surface modification and functionalization of neural electrodes. Conducting polymers and carbon nanotubes have been applied to modify the electrode surface due to their good electrical conductivity. Finally, promising strategies and methods for the development in the field are prospected, which include controlling surface morphology to optimize the mechanical properties, doping of biospecies and improving the interfacial adhesion between the modifying coatings and the electrode substrates.

Contents
1 Introduction
2 The development of neural electrodes
3 The challenges in the development of neural electrodes
3.1 Biocompatibility
3.2 Signal sensitivity
3.3 Mechanical match
3.4 Long-term stability
4 Surface modification with conducting polymers
4.1 Properties of conducting polymers
4.2 The methods of surface modification with conducting polymers
4.3 The challenges of surface modification with conducting polymers
5 Surface modification with carbon nanotubes
5.1 Properties of carbon nanotubes
5.2 The methods of surface modification with carbon nanotubes
5.3 The composite coating of conducting polymers and carbon nanotubes
6 Conclusions and prospects