Published papers

This paper is concerned with robust instability analysis of linear feedback systems subject to a dynamic uncertainty. The work is motivated by, and provides a basic foundation for, a more challenging problem of analyzing persistence of oscillations in nonlinear dynamical systems. We first formalize the problem for SISO LTI systems by introducing a notion of the robust instability radius (RIR). We provide a method for calculating the RIR exactly for a certain class of systems and show that it works well for a class of second order systems. This result is applied to the FitzHugh-Nagumo model for neuronal dynamics, and the effectiveness is confirmed by numerical simulations, where we properly care for the change of the equilibrium point.

© 2020 IEEE. This paper describes a locally bendable stimuli-responsive hydrogel actuator with axially patterned functional materials fabricated by a valve-controlled microfluidic device. By application of an external stimuli to the complete actuator, it can be locally bended owing to the axial pattern of the responsive and non-responsive material. These materials are axially patterned on a microfiber through the Y-shaped channel coupled with the open/close motion of the valves in the microfluidic device. The microfluidic device can regulate the on/off of the flow by thin membrane pneumatic valves controlled by a computer. The patterning frequency of the two materials in the microfiber could be adjusted by modifying the switching intervals of the solutions. Also, we successfully demonstrated the curvature-change through heating and cooling. We believe that this locally bendable stimuli-responsive hydrogel could contribute to microfiber-shaped soft actuators, biomedical sensors, and self-assembly of complex 3D microstructures.

© 2019 IEEE. This paper proposes a computationally tractable algebraic condition for Turing instability, a type of local instability inducing self-organized spatial pattern formation, in molecular communication networks. The molecular communication networks consist of spatially distributed homogeneous compartments, or biological cells, that interact with neighbor compartments using a small number of diffusible chemical species. Thus, the underlying spatio-temporal dynamics of the system can be modeled by reaction-diffusion equations whose diffusion terms are zero for some chemical species. We show that the molecular communication networks are not Turing unstable if and only if certain polynomials are non-negative. This leads to sum-of-squares optimizations for Turing instability analysis. The proposed approach is capable of predicting the formation of spatial patterns in molecular communication networks based on the mathematically rigorous analysis of Turing instability.

© 2017 IEEE. Cells are the minimum unit of function for a living body. Cells are in a dynamic equilibrium state with mutually interacting with the microenvironment surrounding the cells. We focused on whether cellular responses also could be regulated in the interaction of cells with the other cells and/or the microenvironment of them. Here, we try to uncover the mechanism for the macrophages to acquire a stable response at the high cell density through simulations of the way for cell-cell communication via IFN-β. As a result, the extracellular concentration of IFN-β increased rapidly, and uniformly reached the effective concentration in any place in high-density culture condition. On the other hand, the local concentration of IFN-β in the low-density culture condition could rise transiently but easily decreased by diffusion, indicating that it was hard to reach the effective concentration in the most area. The cell density-dependent differences in IFN-β field formation were also shown to have a decisive effect on the stability of cellular responses from the population of cells. Collectively, we successfully demonstrated that the cells themselves dynamically form the surrounding microenvironment to regulate their own activity.