Robert Chang and Jeeseong Hwang


NIST, Optical Technology Division, 100 Bureau Dr, Gaithersburg, MD 20899-8311


In vivo biological tissues assume complex well-organized layered three-dimensional architectures. In situ cells are surrounded by other cells, where many extracellular ligands and other subtle biochemical complexities act in concert not only to facilitate attachments and signaling between cells and the matrix milieu but also grant access to oxygen, hormones, and nutrients, along with waste removal. Therefore, a primary aim in the engineering of an optimal tissue optical phantom is to fabricate an optimal analog of the in vivo scenario. This challenge can be addressed by applying emerging layered biofabrication approaches in which the precise configuration and composition of cells and other biological constituents can simulate the well-defined three-dimensional microenvironments that precisely represent the natural context of tissues with cell-cell and cell-matrix interactions. Furthermore, the advent of and refinements in microfabricated systems presents physical and chemical cues to cells in a predictably and reproducibly with microfluidic laminar flows, resulting in high-fidelity, high-throughput in vitro models capable of simulating both normal and diseased processes in vivo, as well as a serving as a conduit for deploying fluorescent reporters to tissue . The physical tissue phantom model developed in this research involves the combinatorial setup of an automated syringe-based, layered direct cell writing process with micropatterning techniques to fabricate a microscale in vitro device with resident three-dimensional hydrogel-based tissue microspheres in defined design shape and dimensions that biomimic the cellís natural microenvironment for enhanced performance and functionality. One primary application is wound healing in which oxygen-carrying hemoglobin proteins are encapsulated within a hydrogel matrix and fabricated as integrated microspheres. Reflectance and transmittance measurements are then carried out to predict the bulk scattering properties of the microspheres based on scattering coefficients as predicted by Mie theory. The repeatability in absorption and scattering properties of these reproducibly fabricated microspheres effectively eliminates the systematic variation in the preparation of phantoms. In the proposed work, this three-dimensional tissue-based model subject to physiological perfusion flow will enable broad application as an in vitro testbed amenable to the measurement of standard optical properties to probe and stratify various biological and disease phenomena that emerge from complex cellular processes.