Hydrogels are considered as excellent candidates for tissue substitutes by virtue of their high water content and biphasic nature. However, the fact that they are soft, wet and floppy renders them difficult to process and use as custom-designed scaffolds. To address this problem alginate hydrogels were modeled and characterized by measuring stress-strain and creep behavior as well as viscosity as a function of sodium alginate concentration, cross-linking time and calcium ion concentration. The gels were then microfabricated into scaffolds using the pressure-assisted microsyringe. The mechanical and viscous characteristics were used to generate a processing window in the form of a phase diagram which describes the fidelity of the scaffolds as a function of the material and machine parameters. The approach can be applied to a variety of microfabrication methods and biomaterials in order to design well-controlled custom scaffolds.
We analyze the robustness properties of a recently introduced strategy for stabilization with limited information. We show that if the nominal closed-loop system can be made Input-to-State Stable (ISS) with respect to measurement errors, parameter uncertainty and exogenous disturbances then this robustness is preserved when only limited information is available on the plant. More precisely, if a sufficiently bandwidth is available on the communication network, then the resulting closed-loop is semiglobally practically ISS.
Variable Stiffness Actuation (VSA) devices are being used to jointly address the issues of safety and performance in physical human-robot interaction. With reference to the VSA-II prototype, we present a feedback linearization approach that allows the simultaneous decoupling and accurate tracking of motion and stiffness reference profiles. The operative condition that avoids control singularities is characterized. Moreover, a momentum-based collision detection scheme is introduced, which does not require joint torque sensing nor information on the time-varying stiffness of the device. Based on the residual signal, a collision reaction strategy is proposed that takes advantage of the proposed nonlinear control to rapidly let the arm bounce away after detecting the impact, while limiting contact forces through a sudden reduction of the stiffness. Simulations results are presented to illustrate the performance of the overall approach. Extensions to the multi-dof case of robot manipulators equipped with VSA-II devices are also considered.