SEASON 1 , June - September, 2020

Abstract:

Soft biological tissues typically consist of large amounts of water molecules (30-80 wt%) and networks of flexible polymers. The water molecules provide a liquid-like medium in which other molecules migrate and react. The polymer networks enable a solid-like structure of properties such as modulus, toughness, and fatigue resistance. These tissues often have complex shapes and heterogeneous structures. The forms and functions of biological tissues are always mimicked to enable diverse applications, such as tissue engineering, regenerative medicine, and soft robots. 3D printing technologies have opened engaging possibilities to manufacture complex shaped soft structures for the purpose of bionics. Myriads of approaches have been developed, such as extrusion printing and stereolithography. However, the 3D printed soft materials still face several fundamental challenges: fatigue and adhesion between heterogeneous materials. The advance in this field will greatly expand the function and application of 3D printed soft materials. We will show their applications in shape morphing, ionotronics and biomedical engineering.

Abstract:

A diversity of chemical motors based on Marangoni propulsive forces has been developed in recent years. However, most motors are non-functional due to poor performance, a lack of control, and the use of toxic materials. To overcome these limitations, we have developed multifunctional and biodegradable self-propelled motors from structural proteins and an anesthetic metabolite. The protein motors surpass previous reports in performance output and efficiency by several orders of magnitude, and they offer control of their propulsion modes, speed, mobility lifetime, and directionality by regulating the protein nanostructure via local and external stimuli, resulting in programmable and complex locomotion. We demonstrate diverse functionalities of these motors in environmental remediation, microrobot powering, and cargo delivery applications. These versatile and degradable protein motors enable design, control, and actuation strategies in microrobotics as modular propulsion sources for autonomous operation.

Abstract:

Adhering two surfaces together to form an integrated structure is one of the most fundamental ways of processing materials. Not surprisingly, various methods and strategies of adhesion have been workhorses that drive countless innovations and technological breakthroughs in modern engineering. For example, our daily objects and structures such as cell phones, laptops, houses, and even cars are full of various types of adhesives and glues, without which we may not be able to enjoy slim and seamless designs and integrity of these modern technological marvels. However, these magical adhesives mostly fail, often exhibiting strikingly poor adhesion, when they meet wet surfaces like biological tissues and hydrogels. This notable incompatibility of existing adhesives to wet surfaces originates from the fact that most existing adhesive technologies are designed for “dry” conventional engineering materials such as ceramics, metals, glasses, and rubbers. Despite the lack of good existing solutions, the importance of robust adhesion for wet materials is ever increasing with high unmet demand in a broad range of fields.

In this talk, I will introduce a series of wet adhesion technologies to address this lingering challenge with a focus on the mechanism, design principle, and translation of each technology to real-world applications. In specific, I will cover three key aspects of translational wet adhesion technologies including 1) fundamental mechanisms for rapid and robust wet adhesion, 2) novel methods for diverse materials such as engineering solids, hydrogels, conducting polymers, and biological tissues, and 3) real-world applications of wet adhesion technologies. At the end of the talk, future perspectives for translational wet adhesion technologies will be briefly discussed including the remaining challenges and opportunities.

Abstract:

4 billion people in the world are facing severe water scarcity[1]. To address this problem, mainstream research has been focused on desalination of brine[2], water harvesting from air[3], etc. However, the effort to reduce water consumption in daily life has not received enough attention. For example, more than 141 million m3 of fresh water is flushed away in toilets every day globally[4], nearly 6 times as much as the daily water consumption of the entire Africa population[5]. To reduce the flushing water, toilet surfaces need to repel sticky viscoelastic solid (e.g. human fecal waste). Here, we create a design criterion for viscoelastic solid repellent coatings, and develop a facile, scalable coating method for materials in toilets, which significantly reduces the adhesion of the human fecal waste and saves considerable amount of flushing water. Specifically, we demonstrate that our designed liquid-entrenched smooth surfaces (LESS) are only 10% of the adhesion of untreated smooth surfaces to synthetic human feces, and saves up to 90% of water used for cleaning untreated surfaces. In addition, LESS show great anti-bacteria performance, even better comparing to slippery liquid-infused porous surfaces (SLIPS)[6] with underlying surface roughness. With all the function of LESS, we provide a lubricant replenishment strategy to address the concern of coating longevity, and we demonstrate its effectiveness on maintaining the coating’s functionality from both physics modeling and real testing. We believe that the slippery coating in this study provides new possibilities in solving small problems in daily life; while contributing globally to the mitigation of the water scarcity, as well as improving quality of life in developing regions.

Reference

[1] M. M. Mekonnen and A. Y. Hoekstra, "Four billion people facing severe water scarcity," Science Advances, vol. 2, 2016.

[2] M. Elimelech and W. A. Phillip, "The Future of Seawater Desalination: Energy, Technology, and the Environment," Science, vol. 333, pp. 712-717, Aug 2011.

[3] H. Kim, S. Yang, S. R. Rao, S. Narayanan, E. A. Kapustin, H. Furukawa, et al., "Water harvesting from air with metal-organic frameworks powered by natural sunlight," Science, 2017.

[4] W. H. Organization and UNICEF, Progress on sanitation and drinking water–2015 update and MDG assessment: World Health Organization, 2015.

[5] U. N. D. Programme, Human Development Report: UNDP, 2006.

[6] T. S. Wong, S. H. Kang, S. K. Y. Tang, E. J. Smythe, B. D. Hatton, A. Grinthal, et al., "Bioinspired Self-repairing Slippery Surfaces with Pressure-stable Omniphobicity," Nature, vol. 477, p. 443, 2011.

Abstract:

Polymer interfaces play a critical role in a variety of applications, including consumer products, structural components, biomedical devices, and electronics. In many cases, polymers need to be bonded with adhesives to create structural joints or multi-layer structures. For adhesives to efficiently wet and bond to a substrate, the surface free energy of the substrate must be equal to or higher than the surface free energy of the adhesive. However, the surface energy of most polymers is low, which makes adhesion difficult. Thus, there often is a need to increase the surface energy of a polymer without changing the bulk mechanical and chemical properties.

In this work, we demonstrate that atomic layer deposition (ALD) can be applied on poly(methyl methacrylate) (PMMA) and fluorinated ethylene propylene (FEP) to increase their surface energies and, hence, to increase the interfacial fracture toughness when bonded to an epoxy adhesive.

ALD alumina films were deposited on each type of polymer to modify the surfaces towards high energy surfaces of metal oxide. Transmission-electron microscopy (TEM) and atomic-force microscopy (AFM) were used to study the film morphology on the polymers. These indicated that the ALD treatment increased the surface roughness and changed the sub-surface chemistry by vapor-phase infiltration (VPI). The increase in surface energy after ALD was measured by the sessile-drop test with water, ethylene glycol and glycerol.

The interfacial fracture toughness of each polymer-epoxy interface was measured using a customized motor-controlled wedge tester. After ALD film growth, the interfacial fracture toughness of the PMMA-epoxy and FEP-epoxy interfaces increased by factors of up to 7 and 60, respectively. The two ALD samples and two control samples were tested at the same level of humidity. Furthermore, we observed stress-corrosion cracking of the ALD-polymer interfaces. By conducting wedge tests in different levels of humidity, we found that although ALD increased equilibrium interfacial fracture toughness at all humidity, the effect decreased as humidity increased. Scanning-electron microscopy (SEM) of samples after testing provided additional evidence for stress-corrosion cracking of the ALD-polymer interface. These results suggest a new application of ALD for engineering the mechanical properties of chemically inert surfaces.

Abstract:

Grippers are widely used for the gripping, manipulation, and assembly of objects with a wide range of scales, shapes, and quantities in research, industry, and our daily lives. A simple yet universal solution is very challenging. Here, we manage to address this challenge utilizing a simple shape memory polymer (SMP) block. The embedding of objects into the SMP enables the gripping while the shape recovery upon stimulation facilitates the releasing. Systematic studies show that friction, suction, and interlocking effects dominate the grip force individually or collectively. This universal SMP gripper design provides a versatile solution to grip and manipulate multi-scaled (from centimeter scale down to 10-um scale) 3D objects with arbitrary shapes, in individual, deterministic, or massive, selective ways. These extraordinary capabilities are demonstrated by the gripping and manipulation of macro-scaled objects, meso-scaled steel sphere arrays and microparticles, and the selective and patterned transfer printing of micro light-emitting diodes.

Abstract:

Owning to the shape-morphing capacities of Kirigami, this paper cutting ancient art have inspired a variety of engineering advancements, especially in soft robotics and flexible electronics. Distinct from shape-shifting of an origami sheet which is mainly governed by folding pattern geometries, shape-morphing of a kirigami patterned sheet is governed by both pattern geometry and applied loading conditions through stretching, bending, and twisting. Through experiments, theoretical formulation, and finite element simulation, I will demonstrate how to exploit the simplest form of kirigami, linear parallel cut pattern, for tunnable architected materials and soft robotic gripper.

Abstract:

Kirigami, has recently enabled the design of stretchable mechanical metamaterials that can be easily realized by embedding arrays of periodic cuts into an elastic sheet. While in flat kirigami sheets, the ligaments buckle simultaneously as Euler columns, leading to a continuous phase transition; in this talk, I will demonstrate that kirigami shells can also support discontinuous phase transitions. Specifically, in cylindrical kirigami shells, the snapping-induced curvature inversion of the initially bent ligaments results in a pop-up process that first localizes near an imperfection and then, as the deformation is increased, progressively spreads through the structure. We find that the width of the transition zone as well as the stress at which propagation of the instability is triggered can be controlled by carefully selecting the geometry of the cuts and the curvature of the shell.

Moreover, we also exploit kirigami principles to design inflatables that can mimic target shapes upon pressurization. Our system comprises a kirigami sheet embedded into an unstructured elastomeric membrane. We show that the inflated shape can be controlled by tuning the geometric parameters of the kirigami pattern. Then, by applying a simple optimization algorithm, we identify the best parameters that enable the kirigami inflatables to transform into a family of target shapes at a given pressure. Furthermore, thanks to the tessellated nature of the kirigami, we show that we can selectively manipulate the parameters of the single units to allow the reproduction of features at different scales and ultimately enable a more accurate mimicking of the target.

Abstract:

The integration of soft materials—biological tissues, hydrogels, ionogels, and elastomers—is a rapidly developing fundamental technology of our time. Whereas hard materials have been adhered using adhesives of hard polymers since antiquity, these hard polymers are in general unsuited to adhere soft materials, because the hard polymers constrain the deformation of the soft materials. This paper describes a design principle to use hard polymers to adhere soft materials, such that adhesion remains tough after the adhered soft materials are subject to many cycles of large stretch in the plane of their interface. The two soft materials have stretchable polymer networks, but need not have functional groups for adhesion. We adhere the two soft materials by forming, in situ at their interface, islands of a hard polymer. The adhesion is tough if the islands themselves are strong, and the polymers of the islands are in topological entanglement with the polymer networks of the two soft materials. The adhesion is stretchable if the islands are smaller than the flaw sensitivity length. We demonstrate several methods of forming the hard polymer islands, and study the mechanics and chemistry of adhesion. The design principle will enable many hard polymers to form tough and stretchable adhesion between soft materials.

Abstract:

Wrinkling patterns in soft materials have been extensively studied due to their important roles in determining surface morphologies in biological structures and developing multifunctional devices. Most existing work focuses on relatively simple geometries, such as flat structures and curved structures with constant curvature such as the cylinder and 2-sphere. In this talk I discuss wrinkling patterns on a torus, the Gaussian and mean curvatures of which vary along the poloidal direction. We observe eight different wrinkling patterns from large-scale finite element simulations and construct a phase diagram for these patterns. We further show that the non-uniform curvature and anisotropic deformation play critical roles in determining the formation and evolution of these wrinkling patterns. The anisotropic deformation along the toroidal and poloidal directions controls pattern transitions from stripes to hexagons and the non-uniform curvatures determine the nucleation sites of the wrinkling patterns. Our results show that global deformations of a torus lead to strong coupling between elasticity and curvature which may enlarge the design space as well as the dynamically control of wrinkling patterns. If time permitted, I will discuss our ongoing work on the wrinkles on a cone.

Abstract:

Recent experiments on hydrogels subjected to large elongations have shown elastic instabilities resulting in the formation of geometrically intricate fringe and fingering deformation patterns on the specimens boundaries. I will present a robust computational framework addressing the challenges that emerge in the simulation of this complex material response from the onset of instability to the post-bifurcation behavior. The numerical difficulties stem from the non-convexity of the strain energy density in the near-incompressible, large-deformation regime, which is responsible for the coexistence of multiple equilibrium paths with vastly-different, sinuous deformation patterns immediately after bifurcation. The ingredients to overcome these challenges include: a high order of interpolation in the finite element approximation, an arc-length-based nonlinear solution procedure that follows the entire equilibrium path of the system, and an implementation enabling parallel, large-scale simulations. The resulting computational approach provides the ability to conduct highly-resolved, truly quasi-static simulations of complex elastic instabilities. I will show numerical results illustrating the ability of the path-following approach to describe the full evolution of the fringe and fingering instabilities observed experimentally.

Abstract:

Recently, ferromagnetic soft continuum robots – a type of slender, thread-like robots that can be steered magnetically – have demonstrated the capability to navigate through the brain's narrow and winding vasculature, offering a range of captivating applications such as robotic endovascular neurosurgery. Composed of soft polymers with embedded hard-magnetic particles as distributed actuation sources, ferromagnetic soft continuum robots produce large-scale elastic deflections through magnetic torques and/or forces generated from the intrinsic magnetic dipoles under the influence of external magnetic fields. This unique actuation mechanism based on distributed intrinsic dipoles yields better steering and navigational capabilities at much smaller scales, which differentiate them from previously developed continuum robots. To account for the presence of intrinsic magnetic polarities, this emerging class of magnetic continuum robots provides a new type of active structure – hard-magnetic elastica – which means a thin, elastic strip or rod with hard-magnetic properties. In this work, we present a nonlinear theory for hard-magnetic elastica, which allows accurate prediction of large deflections induced by the magnetic body torque and force in the presence of an external magnetic field. From our model, explicit analytical solutions can be readily obtained when the applied magnetic field is spatially uniform. Our model is validated by comparing the obtained solutions with both experimental results and finite element simulations. The validated model is then used to calculate required magnetic fields for the robot’s end tip to reach a target point in space, which essentially is an inverse problem challenging to solve with a linear theory or finite-element simulation. Providing facile routes to analyze nonlinear behavior of hard-magnetic elastica, the presented theory can be used to guide the design and control of the emerging class of magnetically steerable soft continuum robots.

Abstract:

Glass is highly demanded in electronics, photovoltaic system and building structures for its transparency, hardness and chemical stability. However, its applications are limited by its inherent brittleness and low deformability. Lamination and tempering can improve impact responses of glass but do not change its brittle behaviors. Through millions of years of evolution, nature provides us with solutions to solve the problem of brittleness. Many hard biological materials, such as mollusk shells and tooth enamel, are made of brittle minerals but have toughness thousands of times higher than these minerals due to their intricate microstructures. Inspired by the structures of mollusk shells and tooth enamel, we developed tough and deformable transparent architectured glasses. The materials have highly controlled three-dimensional meso-architectures introduced by laser engraving, and interfaces made of ductile polymers. Although made of more than 90 wt% of brittle glass, the bio-inspired glasses obtain deformability and toughness comparable to many tough polymers, and impact resistance two to three times higher than laminated glass and tempered glass.

Abstract:

Understanding high-velocity microparticle impact is essential for many fields, from space exploration to additive manufacturing and needle-less drug delivery. While impact dynamics of macroscale projectiles has been studied in real time using high-speed imaging, investigations of microscale impact have been hitherto limited to post-mortem analysis of impacted specimens. In a laser-induced particle impact test, we observe single-microparticle impact events using an ultra-high-speed camera with nanosecond time resolution. The method is used to the unit process of metal additive manufacturing by cold spray, i.e, single metallic particle impact on metallic substrates, and the transition from metallic bonding to erosion at higher impact velocities.

Abstract:

A long-term challenge in the design and manufacture of materials and structures is to create porous materials that are simultaneously lightweight, and strong. Here we advanced the 3D microfabrication methodology in combination of two-photon lithography and high-temperature pyrolysis, to fabricate nanoarchitected pyrolytic carbon of two prototype unit-cell geometries, octet- and iso-truss, and achieved combined excellent properties with low density, high specific strength, imperfection insensitivity as well as good resilience.

Abstract:

Skin serves as a physical and hygroscopic barrier to protect the inner body, and also contains sensory receptors to perceive environmental and mechanical stimuli. To recapitulate these salient features, hydrogel-based artificial skins have been developed. However, existing designs are constrained by limited functionality, low stability, and requirement of external power. Herein, a novel artificial ionic skin (AIskin) – an analog of the diode based on controlled ion mobility – is demonstrated with high toughness, stretchability, ambient stability and transparency. The AIskin consists of a bilayer of oppositely-charged, double-network hydrogel, and converts mechanical stimuli and humidity into signals of resistance, capacitance, open-circuit voltage (OCV), and short-circuit current (SCC), among which the OCV- and SCC-based sensing signals are self-generated. Its multimodal sensation is maintained in a wide range of relative humidities (13–85%). It is demonstrated for wearable strain-humidity sensing, human–machine interaction and walking energy harvesting. This work will open new avenues toward next-generation, skin-inspired wearable electronics.

Abstract:

Mechanical metamaterials are artificial materials with tailored structural cells that exploits motion, deformation and shape morphing. These materials can transform into different shapes in response to mechanical stimuli, leading to superior properties such as tunable stiffness, reconfigurable topology and advantage in energy absorption. In recent years, origami and kirigami provide many inspirations in the geometry structure of the metamaterials. This talk will start from an introduction to the Deployable Structure Group at Oxford, and explain analytical approaches to study origami through kinematic models of linkage mechanisms. Then, a series of origami/kirigami inspired kinematic-induced metamaterials will be presented. The reconfigurability of the origami structures provides the material with a continuously changing shape, which can be used in electromagnetic, photonic and acoustic fields for tunable metamaterials. Some applications of the proposed metamaterial will be included, and discussion on future work will be given in the end.

Abstract:

Rigid electromagnetic actuators serve our society in a myriad of ways for more than two hundred years. Yet, their bulky nature restricts close collaboration with humans. Here we introduce soft electromagnetic actuators (SEMAs) by replacing solid metal coils with liquid metal channels embedded in elastomeric shells. We demonstrate human-friendly, simple, stretchable, fast, durable, and programmable centimeter-scale SEMAs that drive a soft shark, interact with everyday objects, or rapidly mix a dye with water. A multi-coil flower SEMA with individually controlled petals blooms or closes within tens of milliseconds and a cubic SEMA performs programmed, arbitrary motion sequences. We develop a numerical model supporting design and opening potential routes toward miniaturization, reduction of power consumption and increase of mechanical efficiency. SEMAs are electrically controlled shape morphing systems that are potentially empowering future applications from soft grippers to minimally invasive medicine.

Abstract:

Salivary acquired pellicle (SAP) is a layer of saliva protein and glycoprotein and can stay on tooth surface for a long time. Statherin is the main compound of salivary acquire pellicle which is a 43 amino acid phosphopeptide consisting of proline and tyrosine. Statherin has a unique composition with a high degree of structural and charge asymmetry, present in human parotid and submandibular salivas. The Nterminated of the six amino acid DDDEEK of statherin have a strong adsorption to the hydroxyapatite surface. The purpose of this work was to design a series of short peptides modified PAMAM, metal-polyphenol net, CsgA, DOPA. Manipulation of the surface properties of biominerals is very important for their biomedical applications. However, the straightforward preparation of a multifunctional and stable coating on biominerals remains a challenge. Herein we report a rapid and universal method for the preparation of multifunctional coatings on various biominerals using a salivary acquired pellicle (SAP) inspired dendrimer. The dendrimer has a highly branched structure and an external surface modified with DDDEEKC peptide. It mimics the adsorption function of statherin, which is one of the main components of SAP, to endow the coating with a universal capability for adhesion on various biominerals such as hydroxyapatite, tertiary calcium phosphate, calcium carbonate, pearls, enamel, dentin, and bone.

Abstract:

We present experimental and theoretical results of the adhesion and entry of magnetite nanoparticles (MNP) and MNP/cancer drug (paclitaxel and prodigiosin) complexes into MDA-MB-231 breast cancer cells. The adhesion between luteinizing hormone releasing hormone (LHRH) and breast cancer cells is studied using atomic force microscopy (AFM) technique. The results present that the adhesion force between LHRH coated AFM tips and MDA-MB-231 breast cancer cells is about twice as much as that between bare AFM tips and breast caner cells; the adhesion force between LHRH-MNP coated AFM tips is also approximately twice as much as that between MNP coated AFM tips and breast cancer cells. The increased adhesion from LHRH coated tips elucidate that LHRH can be used for breast cancer targeting. Similarly, the adhesion between the constituents of MNP/cancer drug nanoparticle complexes is elucidated via atomic force microscopy. Finally, the receptor-mediated entry of nanoparticles into breast cancer cells is investigated using thermodynamics and kinetics models. The predictions are shown to be in good agreement with both in-vitro and in-vivo experimental observations of nanoparticle entry into cells.

Abstract:

Diagnosis of many mental diseases heavily relies on behavioral assessment. To understand the principles of neural circuitry mechanisms and develop effective treatments, it is essential to discover the causative link between behavioral output and cellular activity. Engineering approaches can provide methodological assistance to establish tool platform. Ideally, the tool platform will have the features including minimal invasiveness to biological tissue; functional longevity; widespread coverage of neural circuits; the capability to manipulate neurons at multiple scales, ranging from individual synapse to broad neural circuits; and the specificity to identify targeted neural populations. I have been developing effective engineering tools in neural-material interfaces to investigate the dynamics of neural circuits. This talk will cover my two main research directions: precise interventional tools for remotely controlled neuromodulation and real-time recording techniques to monitor neural dynamics. In my talk, I will first mainly present a magnetic toolkit for remote neuromodulation, which allows remotely controlled release of pharmacological compounds to modulate targeted neural circuits. This chemomagnetic technique combines magnetic tools and behavioral neuroscience to enable temporally precise modulation of specific neural circuits underlying motivation and social interactions. In the second part, I will briefly introduce an optical recording system to monitor neural dynamics from multiple sites across the central nervous system in freely behaving mice with simultaneous behavioral output.

Abstract:

Complex 3D functional architectures are of widespread interest due to their potential applications in biomedical devices, metamaterials, energy storage and conversion platforms, and electronics systems. In this talk, I will discuss exploiting structural buckling to create flexible, multifunctional 3D microstructures and electronics from thin-film materials including polymers, metals, semiconductor silicon, and a heterogeneous combination of those materials, which are inaccessible with existing technologies. Both the fundamental buckling mechanics and a wide range of assembled 3D functional thin structures, including shape-programmable 3D structures and 3D electronic scaffolds with sensing and stimulation capabilities, will be presented. I will conclude my talk by briefly discussing new opportunities in the development of multifunctional materials and structures for many applications.

Abstract:

Shape memory polymers (SMPs) are a class of smart materials that can remember temporary shapes and recover to permanent shapes responding with external stimuli. SMPs have a wide range of applications in a variety of fields, including aerospace, biomedicine, flexible electronics, and other fields. Due to the limited species and applications of SMP materials, the development of the SMP has been in a state of stagnation for a long time. This research mainly studies the new properties brought by the introduction of reversible bonds to SMPs and expands the species and applications of SMPs.

Reversible bonds are a class of bonds that can be broken and reconstructed in particular states. In this work, temperature sensitive reversible bonds are introduced into SMP systems. Thus, this novel SMP has another reversible bond activation temperature range besides the phase transition temperature of conventional SMPs. Within this reversible bond activation temperature range, the mechanical properties of the material are greatly affected by temperature. By introducing suitable reversible bonds and adjusting the network structure, the novel SMP exhibits the elastic behavior of conventional SMPs below the reversible bond activation temperature and exhibits the plastic behavior in the reversible bond activation temperature range.

Abstract:

The double network concept has been revolutionary in its ability to turn soft, brittle hydrogels into tough, robust materials with mechanical properties that match the best synthetic elastomers. Double network hydrogels consist of two interpenetrating networks, where each network has a specific mechanical response: the “first network” acts as a sacrificial network, consisting of a rigid, extended network, and the “second network” is a globally percolated, stretchable network. When a double network hydrogel is stretched, covalent bonds of the first network break, dissipating energy; this process continues with increasing strain, until the sacrificial network is completely broken and the second network ruptures. The goal of this research is to demonstrate that the “sacrificial bond concept” is applicable at length-scales beyond the molecular scale. We aim to incorporate this design concept universally for application in structural and medical devices.

Like double network hydrogels, our system consists of a rigid “first network,” 3d printed polyurethane/polyacrylate grids, embedded in a soft and stretchable “second network,” silicone rubber. We found that when the strength of the matrix exceeds the strength of the grid, local fracture occurs in the grid, and stretching is isolated to the rubber in the fractured region. As stretching increases, the force increases, and when the local force exceeds the global strength of the grid, fracture will occur elsewhere in the composite. This process continues sequentially throughout the sample until all grid fracture sites are exhausted, and the matrix ruptures. By tuning the stiffness of the grid, we can independently control the yield strength and fracture strain of the composite, until a point where the grid strength exceeds the matrix strength, and the multiple fracture process no longer occurs.

We also systematically studied the interfacial interactions between the matrix and the reinforcing grid. Both interfacial adhesion as well as topological interlocking are important towards developing a robust composite. By adhesive interactions alone, only minimal fracture of the reinforcing phase occurs; topological interlocking is required to maximize fracture. Based on this result, we systematically change the grid size to modify the number of fracture events. In the optimized form, an increase in work of extension of ~50% over the neat matrix was achieved, representing a ~70% toughening efficiency versus the calculated maximum toughness. These results demonstrate that macroscale double networks can dramatically increase the toughness of soft materials.

Abstract:

Vitrimer possesses covalently crosslinked network that is capable of the topological rearrangement with network integrity preserved through thermally activated bond exchange reactions, thus showing a variety of unique properties such as stress relaxation, self-healing and recycling. Mechanics study on these behaviors can be useful to its practical applications and will be the focus of this talk. First, we study the bulk thermo-mechanical behaviors of vitrimers through finite element methods. The effects of the non-uniform and evolving temperature field is explored, with a focus on the complex coupling between mechanical deformation, heat conduction and bond exchange reactions. We show that in a thermoforming process, the non-uniform heating causes the material to creep in the high-temperature region, leading to redistribution of the deformation field and thus a final shape deviating from the prescribed shape. Such deviation can be controlled by adjusting the heating region, time and temperature to achieve desired final shapes of the vitrimer in a thermoforming process. Second, to gain insights into the powder-based recycling of vitrimers, we study the consolidation and fusion of vitrimer particles under heat press. A computational model is presented for this process, where multiple mechanisms are captured: random particle packing, large deformation, inter-particle contact, and thermally activated bulk stress relaxation and interface healing. Using the model, we evaluate the fusion extent by the evolution of porosity during consolidation and the effective modulus and strength of fused vitrimer, and study how the fusion process depends on the particle size and processing conditions.

Abstract:

Granular material in a swirled container exhibits a curious transition as the number of particles is increased: at low densities the particle cluster rotates in the same direction as the swirling motion of the container, while at high densities it rotates in the opposite direction. We investigate this phenomenon experimentally and numerically using a co-rotating reference frame in which the system reaches a statistical steady-state. In this steady-state the particles form a cluster whose translational degrees of freedom are stationary, while the individual particles constantly circulate around the cluster's center of mass, similar to a ball rolling along the wall within a rotating drum. We show that the transition to counterrotation is friction-dependent. At high particle densities, frictional effects result in geometric frustration which prevents particles from cooperatively rolling and spinning. Consequently, the particle cluster rolls like a rigid body with no-slip conditions on the container wall, which necessarily counterrotates around its own axis. Numerical simulations verify that both wall-disc friction and disc-disc friction are critical for inducing counterrotation.

Abstract:

With the commercialization of haptic devices, understanding behavior under various environmental conditions is crucial for product optimization and cost reduction. Specifically, for surface haptic devices, the dependence of the friction force and the electroadhesion effect on the environmental relative humidity and the finger hydration level can directly impact their design and performance. This talk presents finger-surface friction force with and without electroadhesion with various relative humidity. The random multi-capillary model is introduced to describe the capillary formation in the interface. Mechanisms including changes to Young's modulus of skin, contact angle change and capillary force will be discussed separately with experimental and numerical methods.

Abstract:

The manipulation of elastic waves has been of special interest for a long period of time, owing to their wide applications in structural health monitoring, aseismic design of structures, and medical ultrasonography, etc. Elastic metasurfaces have been recently developed to offer a preferable possibility to manipulate elastic waves, showing the compact and simple-to-implement features. Herein, by artificially tuning the phase and amplitude of certain interfaces, we propose and experimentally demonstrate some examples to manipulate guided elastic waves using metasurfaces, such as source illusions, asymmetric wave propagation, metacages and metaguides. This work may underlie the design of compact devices with complete control over guided elastic waves, and may be potentially useful for applications in elastodynamics like nondestructive testing, high-resolution ultrasonography and analog signal processing.

Abstract:

Thermo-mechanical properties of polymers are dependent on the material chemistry of the base materials. Various methods have been developed to perform atomistic-scale simulations for the cross-linking of polymers. Most of these methods involve connecting the reactive sites of the monomers, but these typically do not capture the entire reaction process from the reactants to final products through transition states. Experimental time scales for cross-linking reactions in polymers range from minutes to hours, which are time scales that are inaccessible to atomistic scale simulations. Because simulating reactions on realistic time scales is computationally expensive, in this investigation, an accelerated simulation method was developed within the ReaxFF reactive force field framework. This new method allows simulation of cross-linking at realistic, low temperatures, which helps to mimic chemical reactions and avoids unwanted high-temperature side reactions and still allows us to reject high-barrier events. This new method - Accelerated ReaxFF - can be used to correctly simulate the molecular structures of thermosetting polymers and polymer derived ceramics, and shows good agreement with experimental results.

Abstract:

Neurite outgrowth inhibition by NogoA via receptors has been a hurdle for therapeutic intervention in neurodegenerative diseases and spinal cord injury. Although NogoA knockout exhibits enhanced neuron regeneration in spinal cord injured mice, inhibition of Nogo receptors does not recover to the extent observed in NogoA knockout mice, suggesting the presence of other pathways. Here, we identified a novel pathway by NogoA, which inhibits neurite outgrowth via decreasing a glycolytic enzyme, Pgk1. Pgk1 stimulates neurite outgrowth independent from its glycolytic role, while depletion of secreted Pgk1 inhibits neurite outgrowth in cell line and zebrafish without activating known NogoA-dependent pathway. We revealed that extracellular Pgk1 inhibits neurite outgrowth by triggering a reduction of p-Cofilin-S3, a growth cone collapse marker, through decreasing a novel Rac1-GTP/p-Pak1-T423/p-P38-T180/p-MK2-T334/p-Limk1-S323/p-Cofilin-S3 molecular pathway. Not only did supplementary Pgk1 enhance neurite outgrowth in a cell line, but injection of Pgk1 also rescued denervation in muscle-specific NogoA-overexpression of zebrafish and an Amyotrophic Lateral Sclerosis mouse model. Collectively, our results provided insight into NogoA-mediated neurite outgrowth inhibition and shed new light on potential therapeutic in NogoA-mediated neurodegenerative diseases and spinal cord injury.

Abstract:

Nanocalorimetry is a technique to do calorimetry on extremely small samples using arrays of micromachined calorimetry sensors. It is widely useful for studying phase transformations in thin film materials. I will talk about how we used differential nanocalorimetry to study the martensite-austenite transformation in CuZr-based shape memory alloys. Additionally, I will talk about resistance sensor arrays and their usage in screening for novel shape memory alloys.

Abstract:

Due to advances in micro/nanofabrication, condensation heat transfer has seen a renaissance in recent years. Compared to conventional heat transfer surfaces, micro/nanostructured surfaces impregnated with lubricated oil have been demonstrated to improve condensation heat transfer by facilitating droplet nucleation, growth, and departure. The presence of micro/nanoscale roughness improves performance longevity by immobilizing the lubrication film. The enhancement, however, is short-lived due to the eventual loss of lubrication oil by departing droplets. Using high speed fluorescence imaging and thermogravimetric analysis, this work shows that (a) departing droplets leave behind satellite droplets that help to retain the oil in the wetting ridge, (b) the composition of the satellite droplets left behind is water, (c) the major source of oil depletion is the wetting ridge; not the wrapping layer, and (d) nearly half of the oil used for lubrication remains on the surface after 10 h condensation. We attribute the slow rate of oil depletion to the nanostructures on our tubes. The insights gained from this work can guide the rational design of long lasting lubricated surfaces for phase-change condensation.

Abstract:

Salivary acquired pellicle (SAP) is a layer of saliva protein and glycoprotein and can stay on tooth surface for a long time. Statherin is the main compound of salivary acquire pellicle which is a 43 amino acid phosphopeptide consisting of proline and tyrosine. Statherin has a unique composition with a high degree of structural and charge asymmetry, present in human parotid and submandibular salivas. The N-terminated of the six amino acid DDDEEK of statherin have a strong adsorption to the hydroxyapatite surface. The purpose of this work was to design a series of short peptides modified PAMAM, metal-polyphenol net, CsgA, DOPA. After modified by DDDEEK, the macromolecule can adhesive to the materials surface endow the material surface with biominelization, antibacterial, antifouling and cell adhesion property. It has a wide application in tooth / bone repair materials, tooth / bone implant materials and tooth / bone tissue engineering field.

Abstract:

Biodegradable and biocompatible elastic materials for soft robotics, tissue engineering or stretchable electronics with good mechanical properties, tunability, modifiability or healing properties drive technological advance, and yet they are not durable under ambient conditions and do not combine all the attributes in a single platform. We have developed a versatile gelatin-based biogel, which is highly resilient with outstanding elastic characteristics, yet degrades fully when disposed. It self-adheres, is rapidly healable and derived entirely from natural and food-safe constituents. We merge all the favourable attributes in one material that is easy to reproduce and scalable, and has a low-cost production under ambient conditions. This biogel is a step towards durable, life-like soft robotic and electronic systems that are sustainable and closely mimic their natural antetypes.

Abstract:

Due to their continuous and natural motion, soft robots have shown potential in a range of robotic applications including bio-inspired robots, wearable devices, and industry. Despite these advantages and the rapid developments in recent years, robots using soft actuators still have several challenging issues to be addressed. In this talk, I'll present some of our recent work in the area of soft robotics, covering the topics of actuation, sensation and control of soft robots.

Abstract:

The fundamental topology of cellular structures―the location, number, and connectivity of nodes and compartments―can profoundly impact their acoustic, electrical, chemical, mechanical, and optical properties, as well as heat, fluid and particle transport. Approaches harnessing swelling, electromagnetic actuation as well as mechanical instabilities in cellular materials have enabled a variety of interesting wall deformations and compartment shape alterations, but the resulting structures generally preserve the defining connectivity features of the initial topology, as overcoming the particularly high elastic resistance for repacking at each node presents a unique challenge. Here we introduce a two-tiered dynamic strategy to achieve systematic reversible transformations of the fundamental topology of cellular microstructures that can be applied to a wide range of material compositions and geometries. Our approach only requires exposing the structure to a liquid whose composition is selected to have the ability to first infiltrate and soften the material at the molecular scale, and then, upon evaporation, to form a network of localized capillary forces at the architectural scale that zip the edges of the softened lattice into a new topological structure, which subsequently re-stiffens and remains kinetically trapped. Reversibility is induced by applying a mixture of liquids that offer modular temporal control over the sequence and extent of the softening-evaporation-stiffening actions, restoring the original lattice or providing access to a repertoire of intermediate reconfiguration modes. Guided by a generalized theoretical model connecting cellular geometries, material stiffness and capillary forces, we first demonstrate programmed reversible topological transformations of triangular lattices into hexagonal ones, and then expand this strategy to a variety of complex closed-cell geometries. We then harness dynamic topologies for developing active surfaces with information encryption, selective particle trapping, and tunable mechanical, chemical and acoustic properties, as well as multi-stimuli actuation.