Tear film is a multilayered biological barrier that protects, lubricates, and provides a smooth optical surface for proper vision. The tear film lipid layer (TFLL) is the outermost layer, which plays a crucial role in stabilizing the film by reducing surface tension and constraining water evaporation. Perturbations of the TFLL lead to dry eye disease (DED), a ubiquitous condition that affects 10–30% of the world population. The biophysical mechanism by which the TFLL prevents tear film evaporation remains uncertain. To gain mechanistic insight, we developed a novel experimental method called constrained drop surfactometry (CDS) to generate high-fidelity biophysical simulations of the TFLL under physiologically relevant conditions.
The cover art for the February 1 issue of Biophysical Journal is an artistic rendition of the CDS that is exploited in our TELL studies and illustrates its bio-anatomical components. The image features a painting of a human face, courtesy of Charita Goyal. The left side of the face is redrawn as a seemingly robotic caricature, accentuated by an oversized water droplet, courtesy of Saksham Tandon, in place of the eye. The water droplet is “constrained” on a pedestal with sharp edges. The air-water surface of the droplet is encircled by an artificial TFLL, composed of nonpolar lipids (yellow), phospholipids (red headgroups and green headgroups), and fatty acid esters of hydroxy fatty acids featuring two hydrophilic headgroups (purple). The droplet contains well-defined electrolytes, peptides, and proteins in tear film, including lipocalin, lysozyme, and lactoferrin, which are depicted as inclusions in the droplet.
Our study suggests that the major biophysical function of phospholipids in the TFLL is to reduce surface tension, while the primary function of fatty acid esters of hydroxy fatty acids is to optimize the rheological properties of the TFLL. We anticipate that these findings will help facilitate the development of targeted ophthalmic TFLL replacement and therapeutic agents in the treatment of DED as well as other disorders of the mammalian tear film.
For more information about tear film and CDS, please visit our website at www2.hawaii.edu/~yzuo.
- Yi Y. Zuo