Session Q20: Physics of Bio-Inspired Materials

Manipulating light and color with soft and structured matter

Investigations of nature’s most fascinating light manipulation strategies can inspire design concepts for synthetic, hierarchically structured, functional optical materials and devices. While soft and fluid matter frequently enables tunable and stimuli-responsive optical characteristics in biological photonic systems, soft and fluid components still represent an underutilized class of materials in the optical engineers’ toolbox. My group tries to understand how organisms grow and form light-manipulating material architectures to translate useful biological design concepts into bioinspired dynamic optical materials. In this presentation, I will briefly discuss our efforts on visualizing butterfly scale structure formation and will then focus on the manufacture of bio-inspired dynamic photonic materials with nano-scale feature control and macroscale area throughput.   

Combining Experimental and Simulation Approaches to Model Color Generation in Colloidal Assemblies Under Spherical Confinement

Structural colors are produced by constructive interference of specific wavelengths of light as it moves through a nanostructured material. Inspired by the mechanism of structural color production in avian species, researchers have fabricated colloidal glasses using either non-absorbing particles (like silica) or highly absorbing particles (like melanin). Modeling color generation from such systems have posed challenges in terms of elucidating the structural characteristics of the colloidal assembly, handling multiple scattering, and accounting for large refractive index contrast and high broadband absorption. In this work, we present a robust method of performing optical modeling using experimental and simulation tools that includes a) extraction of structural information from colloidal assemblies using scattering techniques, b) reconstruction of the assemblies using the previously developed CREASE method, and c) optical simulation of the reconstructed structures using finite-difference time-domain calculations to produce reflectance profiles that match the experimental reflectance spectra. This full-cycle approach presents opportunities to model and predict color generation from complex hierarchical structures that can open exciting avenues to tune structural colors.   

Liquid Collection on Welwitschia-Inspired Wavy Surfaces

The separation of liquids from miscible species (especially those in vapor phase) is of important consequence in myriad processes, notably dehumidification, fog collection, and chemical mist elimination. Most current research incorporates some type of omniphobicity to promote liquid collection, but there exists a lack of universality for surface modifications across a gamut of different working fluids and conditions, including surface tension, supersaturation, and particle size. A difficulty to many existing designs is the struggle to create stable phobicity for low-surface tension liquids and large vapor supersaturations. Here we propose an omniphilic wavy surface design, inspired by Welwitschia mirabilis, that possesses the requisite characteristics for efficient liquid collection: a low energy barrier for nucleation and fast liquid transport.  Utilizing a curvature gradient, liquids on these surfaces experience Laplace-Pressure-driven flow when in the filmwise regime. For both phase-change and isothermal processes, this surface spontaneously refreshes where liquid preferentially deposits by quickly transporting the liquid away to designated locations. We envision that this new surface design can remove the need for stable phobicity, and enable far greater applicability.   

Mechanically self-adaptable materials, a novel bio-inspired pathway for overcoming fatigue failure.

Classical approach of developing new engineering materials rely on enhancing their properties. Unexpected crack growth initiating at stress concentrations under fatigue loading leads to premature failures. Interestingly, many materials in nature can dynamically change their properties to conform with different loading conditions. Bone is an excellent example for this capability. Building upon our previous work demonstrating proportional mineral deposition as a function of a stress using piezoelectric scaffold [1], we report a bioinspired pathway for next generation materials with improved fatigue resistance. We found that pre-notched piezoelectric samples submerged in a simulated body fluid showed their ability to adapt in response to applied fatigue loading without external intervention. As piezoelectric materials generate charges proportional to the applied stress, stress-concentration leads to higher deposition rates resulting in lower overall stress at crack tip. Accordingly, significant enhancement was observed in the fatigue life. The proposed pathway can help increase the components’ reliability and reduce material overdesign.

[1] Orrego, Santiago, et al. "Bioinspired Materials with Self‐Adaptable Mechanical Properties." Advanced Materials 32.21 (2020): 1906970.   

Effect of Wettability on Fog Collection on a Cylindrical Wire

Fog collection is a promising and sustainable solution to the global water scarcity problem, and is of great importance to industries including recapturing water in cooling towers of thermal power plants. In the past few decades, great progress on understanding the coupled effects of various fog collection system parameters has been made. However, how surface wettability of a single cylindrical wire affects detailed steps of fog collection and its performance remains unclear. Here, we conduct experiments using individual wires – the primarily components of conventional fog-collection systems. Experimental results show that the projected surface area of wires in the air flow direction during the fog collection process is affected by surface characteristics, namely wire diameter and wettability, and wind speed, which then significantly influence the fog collection rate of wires. The interesting observed droplet growth and transport phenomena on the wires are modeled by computer simulation. As such, our results and analyses provide an explanation for how surface characteristics influence fog collection, through experiments and simulations, and suggest how they can be utilized to create advanced fog-collection techniques, thus benefitting the fight against the global water crisis.   

Mechanical metamaterial based on cellular structures

We introduce an amorphous mechanical metamaterial inspired by how cells pack in biological tissues. The spatial heterogeneity in the local stiffness of these materials has been recently shown to impact the mechanics of confluent biological tissues and cancer tumors. Here we use this bio-inspired model as a design template and show that this heterogeneity can give rise to amorphous cellular solids with large, tunable phononic bandgaps. Unlike in phononic crystals, the band gaps here are directionally isotropic due to their complete lack of positional order. The size of the bandgap can be tuned by a combination of local stiffness heterogeneity and the local elasticity modulus. To further demonstrate the existence of bandgaps, we dynamically perturb the system with an external sinusoidal wave in the perpendicular and horizontal directions. The transmission coefficients are calculated and show valleys that coincide with the bandgaps. Experimentally this design should lead to the engineering of self-assembled rigid phononic structures with full bandgaps that can be controlled via mechanical tuning and promote applications in a broad area from vibration isolations to mechanical waveguides.   

Bioinspired dopamine-functionalized polymers for enhanced adhesion in construction applications

The design of bio-inspired polymers has long been an area of intense study, however applications to the design of concrete admixtures for improved materials performance have been relatively unexplored. In this work, we functionalized poly(acrylic acid) (PAA), a simple analogue to polycarboxylate ether admixtures in concrete, with dopamine to form a catechol-bearing polymer (PAA-g-DA). A synthetic route utilizing hydroxybenzotriazole (HOBt) as an activating agent was examined for its ability in grafting dopamine to the PAA backbone. The primary goal of this work was to focus our efforts on incorporating mussel-inspired adhesives for building and construction materials, with an emphasis on concrete.  As a wet setting system, DOPA-functionalized polymers may help to strengthen the bond between paste and aggregate, known as the interfacial transition zone (ITZ). The ITZ is important from a structural materials perspective as it represents the weakest point in concrete. We have performed various experiments ranging from modified tensile to peel testing on aggregate substrates and on fresh cement pastes. Furthermore, we have conducted studies on the effect of commercially available additives such as polyethylene glycol towards enhancement of the bio-inspired polymers adhesion strength.   

Bioinspired liquid-infused materials for self-healing and self-stiffening

Natural materials such as coral reefs and bones can repair damages and adapt to changes of mechanical environment. Some of their common features are use of porous structures and environmental liquids such as seawater and blood. Inspired by these examples, we report a bioinspired pathway for synthetic self-healing and self-stiffening materials based on porous liquid-infused piezoelectric scaffolds. Building on our previous work demonstrating mineral deposition from surrounding mineral solution in response to mechanical loading[1], the new material system is for non-liquid environment. We investigated the synthesis of open porous piezoelectric composites by studying effects of solution concentration, curing temperature and particle size. From x-ray computed tomography, we confirmed the average pore size is ~89 um, which is well below capillary length of water. We will present our characterization of piezoelectric properties, liquid-infusion capability as well as self-healing and self-stiffening capability. We envision our approach would be beneficial for diverse environments overcoming some challenges of current synthetic materials for load-bearing application.

1. Orrego et al. Bioinspired Materials with Self-Adaptable Mechanical Properties. Advanced Materials 32 (2020): 1906970.   

Collective behavior triggers chemomechanical oscillations in active hydrogels

Quorum sensing refers to the ability of yeast and other unicellular organisms to switch behavior in response to increasing numbers or density, inducing responses such as oscillating production of chemicals and biofilm formation. There has been much interest in obtaining a quorum sensing behavior in synthetic analogues, employing, for example the enzyme urease or the Belousov Zhabotinsky (BZ) reaction, for novel bioinspired smart material. In this work, we demonstrate that the quorum sensing response of an assembly of soft small BZ hydrogels (smaller than their critical size to oscillate on their own) can trigger their chemomechanical oscillations. We first report that a single spherical BZ gel smaller than a critical size of 350 micrometers in diameter does not chemically oscillate when left alone. In contrast, a 2D layer made of the same spherical BZ gels present chemical oscillations that last for days. To understand the physicochemical cause of this critical size, we experimentally engineer the coupling of single BZ gels to their environment by imposing a flow of reactants around them. By comparing the oscillation patterns obtained in a convective flow set up with those obtained when only diffusion takes place, we establish experimentally that the passive transport of the inhibitor outside the bead produced by the BZ reaction solely controls the pattern of oscillations. We finally discuss why this discovery paves the way to the design of optimized BZ chemomechanical machines.    

Thermodynamic law for control of biopolymer reorganization through energy consumption

Energy dissipation has been previously shown to alter structure and dynamics of non-equilbrium liquids through the renormalization of interparticle interaction strength [Tociu et al. PRX 2019]. Cytoskeletal networks are active systems that rely on complex component interactions to undergo structural changes in response to external stimuli. In this work, we address the interplay between structure and energy dissipation in a model system which transitions between aster-like and bundle-like configurations of actin filaments, which interact through the action of myosin molecular motors. Our phenomenological model and numerical simulations indicate that dynamic biasing of energy dissipation by the system induces a structural transformation similar to the one observed when a property of myosin motors, their rigidity, is modulated. These findings serve as a basis for a novel non-equilibrium thermodynamic control principle for altering the properties of complex materials through modulation of the system's energy budget.   

Spatiotemporal control of Mechanical phase transition in 3D active networks

Activity-driven self-assemblies of crosslinked networks of cytoskeletal filaments provide a unique platform to build reconfigurable active materials. However, it remains unclear how cytoskeletal stresses regulate the emergent mechanical properties of living matter. Here, we demonstrate that active networks composed of molecular motors and biopolymers exhibit an activity-controlled phase transition from an active liquid that spontaneously bends in-plane to an active solid that spontaneously buckles out-of-plane. We are mapping the phase space of active and elastic stresses to better understand these two active instabilities. Finally, we establish an optogenetic control over this non-equilibrium phase transition by optically modulating the activity of light-activable motors. Our results elucidate how to spatiotemporal control active and elastic stresses in biomimetic active materials, a fundamental step towards understanding how cytoskeletal stresses control the self-organization of living systems.