Emergent biohybrid active materials

Conventional robots are made from hard materials. Bio hybrid robots use living cells and extracellular matrix (ECM), in addition to engineered scaffolds. These living materials become active through a self-assembly process. Initially, they are dispensed as a liquid mixture of cells and ECM. After the ECM cures, the cells interact with one another as well as with the extra cellular matrix. These interactions result in a collective behavior leading to the emergence of the active material, i.e., the tissue. We will show that this emergence relies on a phase transition process as a function of cell-cell distance. If the cell-cell distance is high then the cells cannot interact with each other, and the collective behavior is suppressed. With increasing cell density, cell-cell distance decreases to a critical value when the cells begin to interact. Cells then approach each other, and the collective behavior proceeds in a feed forward way, eventually forming an active tissue with ordered structure, i.e., cells undergo a disorder to order transition. For example, muscle tissues for biohybrid robots can be formed only when the initial cell density is above a critical value. A simple mathematical model will be presented that captures this phase transition process.