Retinitis pigmentosa and macular degeneration are two common retinal degenerative diseases that affect 1.5 and 30 million people globally and lead to end-stage blindness.

In a normal eye, once light enters the eye, it is converted into a signal that is sent to the bipolar cells and the ganglion cells, both of which are photoreceptor cells. The light signal eventually travels via the optic nerve to the brain where it is integrated and perceived as an image. However, patients with retinitis pigmentosa or macular degeneration experience a loss of the light-sensing cells – the photoreceptor cells – which are the rods for night vision and cones for day and color vision. As the photoreceptors degenerate, the eyes become insensitive to light and eventually they are unable to capture light and convert it into a signal that can be sent to the brain.

A company called LambdaVision, Inc. has developed a protein-based retinal implant. The implant itself makes use of the light-activated protein called bacteriorhodopsin, which has a light-activated proton-pump that pumps a proton unidirectionally from the intracellular surface of the protein to the extracellular surface thereby initiating the visual signaling cascade.

A layer-by-layer manufacturing process is used to manufacture these artificial retinas. As many as 100 layers are deposited, alternating between an ion-permeable scaffold and a polycation bacteriorhodopsin protein layer to form the final retinal implant. The increased number of layers directly correlates with a higher quality signal. Additionally, consistency in the thin-film orientation is important in order to generate a uni-directional ion gradient. This implant is surgically inserted as a replacement for the damaged photoreceptors.

While such a protein-based retinal implant provides significant improvement compared to other leading therapies on the market, manufacturing has been a challenge. On Earth where the effects of gravity are ubiquitous, convection currents, sedimentation, evaporation, surface tension are key physical phenomena at play, and contribute to inefficient and irregular protein deposition, reduced implant homogeneity, and ultimately a lower performing artificial retina However, when manufactured in the microgravity environment of Low Earth Orbit, the implants showed significant improvements in quality. Lack of sedimentation and convection currents resulted in increased homogeneity of layers with fewer to no defects.

Thus, the microgravity environment confers a unique advantage for fabricating high-quality thin films, via layer-by-layer deposition, which have a variety of applications beyond the artificial retina.