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Hongjie Jiang1 Vaibhav Jain1 Manuel Ochoa1 Babak Ziaie1

1, Purdue University, West Lafayette, Indiana, United States

In the US, 15% of diabetics develop chronic wounds, of which 12-25% result in amputations due to their non-healing status. Among the many methodologies for treating such wounds, oxygen plays a very important role through all wound healing stages from inflammation for prevention of infection to tissue remodeling for promoting collagen synthesis. As a result, pharmaceutical and medical device companies have recently placed a strong emphasis on the development of oxygen based therapies with various delivery modalities, including hyperbaric oxygen (HBO), topical oxygen (TO) and continuous diffusion of oxygen (CDO). HBO usually requires expensive, sophisticated, and bulky equipment and is time-consuming, thus limiting its suitability to only high-end clinics. TO can avoid the side effect of excessive oxygen associated HBO but still requires an external oxygen resource and thus immobilizes the patients, which also limits their utility in large population. CDO, providing a slow flow rate of oxygen for continuous delivery, has more practical approach for its portability but is still not cost-effective. Therefore, as a low-cost alternative CDO, we have developed a personalizable, foot pressure-triggered, oxygen-releasing insole that delivers oxygen specifically to foot regions with an ulcer. The insole consists of an oxygen-filled polydimethylsiloxane (PDMS) chamber composed of two layers, one to provide the interface to the foot with the selective laser-machined region targeting the ulcer position, and the other to store the oxygen providing structural support via an array of 1cm diameter pillars distributed with the spatial ratio of 12.4% mm3/mm3. Both layers are bonded together using oxygen plasma and a strong adhesive film (3M 300LSE) and filled with pure oxygen. The entire fabrication can be realized through a scalable and layer-by-layer process. The high permeability of PDMS to oxygen along with its strong but flexible mechanical properties make it an ideal material for use in foot insole applications with oxygen transport properties. The thickness of the upper layer of the insole is patterned by laser-machining to alter the surface-to-volume ratio of the chamber and tune the oxygen permeability of the material, thus enabling higher permeability in ulcer regions but lower permeability elsewhere. When a person applies pressure during normal walking or standing, the insole releases oxygen to the ulcer regions. Mechanical characterization of the insole revealed an average bond strength (peel test) of 6.85N. The O2 release of the insole was investigated by placing it over a 12x8x9 mm3 0.35% agarose gel (mimicking the wound bed) while the O2 in the insole was pressurized to match the pressure exerted by a normal weight adult (between 50 to 60 kg). The oxygen was measured 4mm into the gel for one day. The average O2 delivery rate was measured to be 0.05 ppb/min/mm2 at 0.9mm thick PDMS region, 2.1 times of the non-laser-machined region.

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