Neuroadhesive L1 coating attenuates acute microglial attachment to neural electrodes as revealed by live two-photon microscopy.

Implantable neural electrode technologies for chronic neural recordings can restore functional control to paralysis and limb loss victims through brain-machine interfaces. These probes, however, have high failure rates partly due to the biological responses to the probe which generate an inflammatory scar and subsequent neuronal cell death. L1 is a neuronal specific cell adhesion molecule and has been shown to minimize glial scar formation and promote electrode-neuron integration when covalently attached to the surface of neural probes. In this work, the acute microglial response to L1-coated neural probes was evaluated in vivo by implanting coated devices into the cortex of mice with fluorescently labeled microglia, and tracking microglial dynamics with multi-photon microscopy for the ensuing 6 h in order to understand L1's cellular mechanisms of action. Microglia became activated immediately after implantation, extending processes towards both L1-coated and uncoated control probes at similar velocities. After the processes made contact with the probes, microglial processes expanded to cover 47.7% of the control probes' surfaces. For L1-coated probes, however, there was a statistically significant 83% reduction in microglial surface coverage. This effect was sustained through the experiment. At 6 h post-implant, the radius of microglia activation was reduced for the L1 probes by 20%, shifting from 130.0 to 103.5 µm with the coating. Microglia as far as 270 µm from the implant site displayed significantly lower morphological characteristics of activation for the L1 group. These results suggest that the L1 surface treatment works in an acute setting by microglial mediated mechanisms.

Tuning peptide affinity for biofunctionalized surfaces.

The control of the interaction between biological systems and surfaces plays a major role in the development of bioactive implants. Random absorbance of different compounds of the body liquids attach at the implant site after surgery. This protein layer triggers the activation of immune cells and is a breeding ground for pathogens, which may induce inflammation processes. Many efforts have been made to block these fouling processes such as PEGylation and unspecific coatings. These systems lead to bioinert implant surfaces that lower the inflammation potential of implanted materials. In contrast, the biomimetic approach attempts the functionalization of implant surfaces with compounds such as peptides, proteins, or sugars that form an artificial layer on the implant that corresponds to the naturally occurring extracellular matrix. This enables the controlled recruitment of cells that improve the healing processes or enhance the osseointegration into the implanted material. An improved connection of implants with cells that enhances the healing processes or tightens the connection of implants with the surrounding tissue is obtained by this approach. However, for bioactive functionalization of implant materials, efficient and robust immobilization techniques are required. Peptides owing to their low-toxicity and their multifunctionality are interesting agents that can act as molecular glue to surfaces. Here, an overview is provided of the development of surface binding peptides, the molecular mechanisms of peptide-surface interactions, and the application of surface binding peptides in the development of multifunctional biomaterials that facilitate beneficial characteristics in vitro and in vivo.