Materials Zeitgeist: Biological Inspiration

Traditional approaches to materials synthesis have largely relied on thermodynamic phases leading to static “condensed matter” structures, e.g., monolithic single crystals. Over the past several decades, these approaches have led to the development of a wide range of technologically useful materials including semiconducting, ferroelectric, non-linear optical, superconducting, and piezoelectric materials. Important as they are, the functions of these materials will ultimately be constrained by their dependence on time-invariant single-length scales, e.g., the crystal unit cell dimensions.

By contrast, most biological functional systems are neither monolithic nor equilibrated.  They consist of heterogeneous mixtures of widely disparate building blocks, organized temporally  and spatially over vastly different length- and time scales. They exhibit dynamic responsiveness by existing far outside thermal equilibrium, harnessing energy from their environment, and accessing kinetic states in their free energy landscapes. This spatio-temporal mode of organization in living systems is directly responsible for many of the complex, emergent, and adaptive functions and behaviors – unmatched by synthetic systems.  

From the vantage of materials science and materials fabrication, these notions of spatio-temporal and energy dissipative self-organization represent a major shift in emphasis from thermodynamic to kinetic regimes. Here, equilibrium structures (global minima) are replaced by higher-order organizational states (e.g., transitions between various local metastable minima of different structures). Examples of synthetic materials deliberately fabricated using a dynamic self-assembly approach are rare, reflecting the current lack of fundamental understanding of the process. In contrast, such examples abound in biology. For instance, a typical biological membrane, a 3-5 nm thick bimolecular layer, is a multi-component nanostructured fluid composed of a phospholipid matrix phase with a number of functional components including glycolipids, membrane proteins, and ion channels. Complex dynamics reflecting coupling between chemical composition, molecular structure, and physical interactions lead to an impressive set of time-dependent functions for the membrane, including active and passive ion transport, molecular recognition, and energy transduction. To facilitate such functions, membrane architectures often undergo dynamic reorganization to permit the temporal emergence of functional sub-structures. 

We assert that the creation of new materials capable of performing advanced functions, e.g., complex and co-operative processes for memory, self-replication, and self-repair, will ultimately rely upon understanding and controlling dynamic self-assembly in both natural and synthetic systems. Fundamental challenges to stably design such out-of-equilibrium structures are many: (1) how does one design a material system whose energy functional contains multiple minima? (2) how does one ensure that the accessible minima are roughly equivalent in energy allowing for dynamics through transitions between minima with weak interactions? (3) how can one fashion dissipative processes so as to achieve maximum regulation and structure evolution with minimal energy consumption?  These challenges and questions constitute the dominant intellectual and scientific framework of our efforts.