Micro fabrication technologies for tear fluid sensors for functional contact lens applications

Functional multi-purpose contact lenses have recently attracted attention as suitable means to exploit the characteristics of the eye to diagnose diseases and for drug delivery. Such enrichment of the normal contact lenses are mainly achieved by the integration of different components like micro sensors and microelectronics onto the contact lens substrate. The surface of our eye is an interesting chemical interface, which acts like a window into the human body. For example, intraocular pressure is a surrogate marker of cardiovascular health. Just like the blood, the eye tear contains many health parameters which could be used to diagnose various illness. Therefore, a contact lens with integrated biosensors shall provide health professionals with a new tool for research studies and for diagnosing diseases in a novel way without traditional lab works. Motivated by this fact, this thesis deals with the integration of biosensors on contact lens to sense the eye tear film solution. The contact lens is made of polydimethylsiloxane (PDMS) and the sensor is bare gold electrodes patterned using micro fabrication technology. However there are several challenges that need to be addressed before reaching the final functional contact lens with incorporated micro devices and micro sensors for biomedical applications. One such challenge is related to the metallization on polymer substrates. The results are presented in the direction of realizing robust metal patterns on polymer substrates. Comparison study of various metallization techniques on different polymer substrates like PDMS, PET(Polyethylene terephthalate) to name a few and different metals patterned like Ag, Au to name a few are presented and thus emphasizing the reasons for the selection of polymer and micro fabrication technique used for the final device. Surface characterization of the patterned metal is performed using various techniques and the analyzed data are presented. Specific steps and solutions to issues like crack formation and adhesion of the metal on the polymer substrates are also discussed. An electrochemical sensor for the diagnosis of Keratoconjuctivitis sicca or dry eye syndrome (DES) is presented. Aqueous layer in the tear film plays an important role in maintaining the moisture inside the eye. This layer consists of proteins, carbohydrates, oxygen and some inorganic salts. It has been shown that in dry eye patients, the salt content in the tear film is higher compared to that of a healthy person. So to diagnose dry eye syndrome, we developed an impedimetric sensor to measure the resistance of the eye tear solution and we conduct associated experiments to analyze the performance. An inverse relationship between solution resistance and saline concentration in a log10vslog10 plot was confirmed. The fabricated sensors were tested for reproducibility of the results and the DES severity scale falls well inside the detection limit of the sensor. The work is wrapped up by the proof of concept experiments performed for the wireless transmission of the data from the sensor using resonance RLC circuit.