Theoretical Studies of Mechanics in Active Matter

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical research, outreach and education on mechanically-excitable active solids that lie at the interface of physics and biology. In the beating heart, cells called cardiomyocytes contract in a coordinated fashion to generate a contractile wavefront that crosses from one end of the heart to the other, pushing blood through the heart with each beat. Cardiomyocytes in the adult heart excite other cells to contract using electrical signals involving ions. In the early embryonic heart, however, the PI and collaborators have suggested that the mechanical stress resulting from the contraction of a cell signals other cells to contract. The embryonic heart is therefore a mechanically-excitable solid in which active components called cardiomyocytes exert stresses on surrounding tissue that excite contraction of other cardiomyocytes (and therefore the generation of more stress). Like their simpler cousins, chemically-excitable systems, mechanically-excitable active solids can show rich behavior, such as wavefront propagation as in the contractile wavefront of the heart, the formation of patterns such as stripes or spots, or chaotic behavior. In this project, the PI will investigate the physics of mechanically-excitable active solids such as the embryonic heart. Mechanically-excitable active solids are a class of active matter--systems that contain many small components, for example cardiomyocytes, that interact strongly with each other and that supply energy to the system. The physics of active matter can be fundamentally different from the physics of traditional passive matter, in which individual microscopic components cannot supply energy. This award supports training physics graduate students who, in the course of their research, will bring together ideas and techniques from many subfields of condensed matter physics and interact closely with biologists. Their work will not only broaden the study of active matter within physics, but will bring a new perspective to the workings of the heart and may lead to the design of new synthetic materials as active solids. TECHNICAL SUMMARY This award supports theoretical research and education on active matter at the interface with biology. Active matter is a form of matter maintained out of equilibrium by energy injected at the microscopic scale. A canonical example is an active fluid of motile particles. This project focuses on excitable active solids, in which constituents inject energy by generating stress. An example is the beating heart, in which cardiomyocyte cells inject energy into the tissue by contracting in a coordinated fashion to create a contractile wavefront that traverses the heart with each beat to pump blood. In the adult heart, the contractile wavefront is understood as wavefront propagation in a chemo-electrical excitability problem. Ions from one cell trigger calcium release in the next cell to initiate its contraction. In the embryonic heart, however, the principal investigator and collaborators proposed that the contractile wavefront is a mechanical excitability phenomenon: stress is generated when a cell contracts, effectively diffuses through elastoviscous tissue, and triggers ion release in the next cell to cause its contraction. The heart exhibits the reverse energy cascade characteristic of active matter: energy injected at the cellular scale by cardiomyocyte contraction is transduced, via the nonlinear dynamics of wavefront propagation, up to the macroscopic organ scale, where it leads to a collective function - the pumping action of the heart. This theoretical project has three main goals: (1) to develop a theoretical description for active solids in which energy is injected at the microscopic level via stress generation; (2) to construct a coherent theoretical framework for mechano-electrical reaction-diffusion in the heart that is consistent with experimental observations, and to understand its implications for development and evolution of the heart; (3) to understand mechanical reaction-diffusion systems more generally, using theoretical techniques developed in the nonlinear dynamics community for chemical reaction-diffusion systems. Here the aim is to calculate phase diagrams for steady-state behavior and transient phenomena. Such a phase diagram might have regions denoted, wavefront propagation, pattern formation, temporal oscillations, or quiescent behavior. This award will support physics graduate students to work at the interface of soft matter physics, mechanobiology and physiology. In the course of their research, these students will bring together ideas and techniques from many subfields of condensed matter physics and interact closely with biologists. Their work will not only broaden the study of active matter within soft matter physics, but will bring a new perspective to early developing and early evolving hearts and may lead to the design of new synthetic materials as active solids.