A 3D photonic sensor integrated tissue model for strain sensing

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University of Delaware
The study of wound healing and wound healing therapies is motivated by the need to prevent the formation of thick scar tissue in pathologically healing wounds and tissues that rely on their elasticity and modulus to perform their function, such as cardiac and vocal fold tissues. The development of in vitro platforms that can detect cell-induced strain in mimics of healing wounds has expanded our understanding of the mechanical, chemical, and physical cues that drive wound healing. However, these platforms are limited in their resolution, dimensionality, and ability to gather information about changes in strain throughout thick, opaque tissue models. In this work we describe the development of flexible, deterministically buckled 3D photonic device arrays that are designed and fabricated to meet the specific spatial, temporal, and strain resolution requirements needed for the detection of cell-induced strains in a millimeters-thick tissue model. ☐ A polymer or silicone-clad Ge23Sb7S70 chalcogenide glass resonant cavity array is selected for this application, as high-quality chalcogenide glasses devices can be deposited at low temperatures onto flexible and cytocompatible substrates. However, the reliability of these and other highly sensitive chalcogenide glass devices is affected by their aging-induced structural relaxation. The refractive index shifts resulting from this relaxation are on the same order of magnitude as the index shifts used to small-scale strain with our device arrays. In order to overcome this limitation, we develop and demonstrate a high-precision refractometry technique that tracks small changes in the refractive index of Ge23Sb7S70 chalcogenide glass, down to 10-5 RIU. This technique allows us to both identify the aging mechanism in this glass with high accuracy and compare different index stabilization methods to optimize our device processing. ☐ The expected performance of these arrays was tested both through finite element modeling and a proof-of-concept in vitro experiment. In the modeling experiments, PDMS buckled geometries were deformed in cardiac graft tissue-like environments. From these experiments we showed that devices embedded in these materials could easily detect small, localized changes in stiffness theoretically caused by limited perfusion of growth factor throughout this model. In vitro, an SU-8 clad, symmetrically buckled device was exposed to a contracting collagen gel, and the device response as a result of this deformation was analyzed. ☐ These deterministically buckled arrays of polymer or silicone-clad chalcogenide glass resonant cavities demonstrate sensitivity to relevant strains in 3D cell culture platforms, excellent ease of use, and the potential for a wide range of applications. This technique can be used as a standalone, low cost, plug-and-play local strain gauge for use in soft material systems. Thus, this technique’s flexibility both in terms of its deformability and range of applications easily surpasses other methods of in vitro force or strain detection.