Microfabricated tissue gauges to measure and manipulate forces from 3D microtissues

Physical forces generated by cells drive morphologic changes during development and can feedback to regulate cellular phenotypes. Because these phenomena typically occur within a 3-dimensional (3D) matrix in vivo, we used microelectromechanical systems (MEMS) technology to generate arrays of microtissues consisting of cells encapsulated within 3D micropatterned matrices. Microcantilevers were used to simultaneously constrain the remodeling of a collagen gel and to report forces generated during this process. By concurrently measuring forces and observing matrix remodeling at cellular length scales, we report an initial correlation and later decoupling between cellular contractile forces and changes in tissue morphology. Independently varying the mechanical stiffness of the cantilevers and collagen matrix revealed that cellular forces increased with boundary or matrix rigidity whereas levels of cytoskeletal and extracellular matrix (ECM) proteins correlated with levels of mechanical stress. By mapping these relationships between cellular and matrix mechanics, cellular forces, and protein expression onto a bio-chemo-mechanical model of microtissue contractility, we demonstrate how intratissue gradients of mechanical stress can emerge from collective cellular contractility and finally, how such gradients can be used to engineer protein composition and organization within a 3D tissue. Together, these findings highlight a complex and dynamic relationship between cellular forces, ECM remodeling, and cellular phenotype and describe a system to study and apply this relationship within engineered 3D microtissues.

A theory of slow feature analysis for transformation-based input signals with an application to complex cells

We develop a group-theoretical analysis of slow feature analysis for the case where the input data are generated by applying a set of continuous transformations to static templates. As an application of the theory, we analytically derive nonlinear visual receptive fields and show that their optimal stimuli, as well as the orientation and frequency tuning, are in good agreement with previous simulations of complex cells in primary visual cortex (Berkes and Wiskott, 2005). The theory suggests that side and end stopping can be interpreted as a weak breaking of translation invariance. Direction selectivity is also discussed. © 2011 Massachusetts Institute of Technology.

The effect of dislocations on the efficiency of InGaN/GaN solar cells

Two solar cells based on an InGaN/GaN p-i-n hetero-junction, but having different dislocation densities, were fabricated and characterized. The structures were grown on c-plane (0001) GaN-on-sapphire templates with different threading dislocation (TD) densities of 5×108 and 5×109 cm-2. Structural characterization revealed the presence of V-defects in the InGaN epilayer. Since each V-defect was associated with a TD, the structural as well as the optical properties worsened with a higher TD density in the GaN/sapphire template. It was also found that additional dislocations were generated in the p-GaN layer over the V-defects in the InGaN layer. Because of its superior structural quality, the peak external quantum efficiency (EQE) of the low TD density sample was three times higher than that of the high TD density sample. © 2013 Elsevier B.V.