Towards enduring autonomous robots via embodied energy.

Autonomous robots comprise actuation, energy, sensory and control systems built from materials and structures that are not necessarily designed and integrated for multifunctionality. Yet, animals and other organisms that robots strive to emulate contain highly sophisticated and interconnected systems at all organizational levels, which allow multiple functions to be performed simultaneously. Herein, we examine how system integration and multifunctionality in nature inspires a new paradigm for autonomous robots that we call Embodied Energy. Whereas most untethered robots use batteries to store energy and power their operation, recent advancements in energy-storage techniques enable chemical or electrical energy sources to be embodied directly within the structures and materials used to create robots, rather than requiring separate battery packs. This perspective highlights emerging examples of Embodied Energy in the context of developing autonomous robots.

Electrolytic vascular systems for energy-dense robots.

Modern robots lack the multifunctional interconnected systems found in living organisms and are consequently unable to reproduce their efficiency and autonomy. Energy-storage systems are among the most crucial limitations to robot autonomy, but their size, weight, material and design constraints can be re-examined in the context of multifunctional, bio-inspired applications. Here we present a synthetic energy-dense circulatory system embedded in an untethered, aquatic soft robot. Modelled after redox flow batteries, this synthetic vascular system combines the functions of hydraulic force transmission, actuation and energy storage into a single integrated design that geometrically increases the energy density of the robot to enable operation for long durations (up to 36 hours). The fabrication techniques and flexible materials used in its construction enable the vascular system to be created with complex form factors that continuously deform with the robot's movement. This use of electrochemical energy storage in hydraulic fluids could facilitate increased energy density, autonomy, efficiency and multifunctionality in future robot designs.

Magnetohydrodynamic levitation for high-performance flexible pumps.

We use magnetohydrodynamic levitation as a means to create a soft, elastomeric, solenoid-driven pump (ESP). We present a theoretical framework and fabrication of a pump designed to address the unique challenges of soft robotics, maintaining pumping performance under deformation. Using a permanent magnet as a piston and ferrofluid as a liquid seal, we model and construct a deformable displacement pump. The magnet is driven back and forth along the length of a flexible core tube by a series of solenoids made of thin conductive wire. The magnet piston is kept concentric within the tube by Maxwell stresses within the ferrofluid and magnetohydrodynamic levitation, as viscous lift pressure is created due to its forward velocity. The centering of the magnet reduces shear stresses during pumping and improves efficiency. We provide a predictive model and capture the transient nonlinear dynamics of the magnet during operation, leading to a parametric performance curve characterizing the ESP, enabling goal-driven design. In our experimental validation, we report a shut-off pressure of 2 to 8 kPa and run-out flow rate of 50 to 320 mL⋅min-1, while subject to deformation of its own length scale, drawing a total of 0.17 W. This performance leads to the highest reported duty point (i.e., pressure and flow rate provided under load) for a pump that operates under deformation of its own length scale. We then integrate the pump into an elastomeric chassis and squeeze it through a tortuous pathway while providing continuous fluid pressure and flow rate; the vehicle then emerges at the other end and propels itself swimming.

Simple synthesis of soft, tough, and cytocompatible biohybrid composites.

Collagen is the most abundant component of mammalian extracellular matrices. As such, the development of materials that mimic the biological and mechanical properties of collagenous tissues is an enduring goal of the biomaterials community. Despite the development of molded and 3D printed collagen hydrogel platforms, their use as biomaterials and tissue engineering scaffolds is hindered by either low stiffness and toughness or processing complexity. Here, we demonstrate the development of stiff and tough biohybrid composites by combining collagen with a zwitterionic hydrogel through simple mixing. This combination led to the self-assembly of a nanostructured fibrillar network of collagen that was ionically linked to the surrounding zwitterionic hydrogel matrix, leading to a composite microstructure reminiscent of soft biological tissues. The addition of 5-15 mg mL-1 collagen and the formation of nanostructured fibrils increased the elastic modulus of the composite system by 40% compared to the base zwitterionic matrix. Most notably, the addition of collagen increased the fracture energy nearly 11-fold ([Formula: see text] 180 J m-2) and clearly delayed crack initiation and propagation. These composites exhibit elastic modulus ([Formula: see text] 0.180 MJ) and toughness ([Formula: see text]0.617 MJ m-3) approaching that of biological tissues such as articular cartilage. Maintenance of the fibrillar structure of collagen also greatly enhanced cytocompatibility, improving cell adhesion more than 100-fold with >90% cell viability.

Valveless microliter combustion for densely packed arrays of powerful soft actuators.

Existing tactile stimulation technologies powered by small actuators offer low-resolution stimuli compared to the enormous mechanoreceptor density of human skin. Arrays of soft pneumatic actuators initially show promise as small-resolution (1- to 3-mm diameter), highly conformable tactile display strategies yet ultimately fail because of their need for valves bulkier than the actuators themselves. In this paper, we demonstrate an array of individually addressable, soft fluidic actuators that operate without electromechanical valves. We achieve this by using microscale combustion and localized thermal flame quenching. Precisely, liquid metal electrodes produce sparks to ignite fuel lean methane-oxygen mixtures in a 5-mm diameter, 2-mm tall silicone cylinder. The exothermic reaction quickly pressurizes the cylinder, displacing a silicone membrane up to 6 mm in under 1 ms. This device has an estimated free-inflation instantaneous stroke power of 3 W. The maximum reported operational frequency of these cylinders is 1.2 kHz with average displacements of ∼100 µm. We demonstrate that, at these small scales, the wall-quenching flame behavior also allows operation of a 3 × 3 array of 3-mm diameter cylinders with 4-mm pitch. Though we primarily present our device as a tactile display technology, it is a platform microactuator technology with application beyond this one.

Underactuated fluidic control of a continuous multistable membrane.

This work addresses the challenge of underactuated pattern generation in continuous multistable structures. The examined configuration is a slender membrane which can concurrently sustain two different equilibria states, separated by transition regions, and is actuated by a viscous fluid. We first demonstrate the formation and motion of a single transition region and then sequencing of several such moving transition regions to achieve arbitrary patterns by controlling the inlet pressure of the actuating fluid. Finally, we show that nonuniform membrane properties, along with transient dynamics of the fluid, can be leveraged to directly snap through any segment of the membrane.

Fluid flow exposure promotes epithelial-to-mesenchymal transition and adhesion of breast cancer cells to endothelial cells.

BACKGROUND: Mechanical interactions between tumor cells and microenvironments are frequent phenomena during breast cancer progression, however, it is not well understood how these interactions affect Epithelial-to-Mesenchymal Transition (EMT). EMT is associated with the progression of most carcinomas through induction of new transcriptional programs within affected epithelial cells, resulting in cells becoming more motile and adhesive to endothelial cells. METHODS: MDA-MB-231, SK-BR-3, BT-474, and MCF-7 cells and normal Human Mammary Epithelial Cells (HMECs) were exposed to fluid flow in a parallel-plate bioreactor system. Changes in expression were quantified using microarrays, qPCR, immunocytochemistry, and western blots. Gene-gene interactions were elucidated using network analysis, and key modified genes were examined in clinical datasets. Potential involvement of Smads was investigated using siRNA knockdown studies. Finally, the ability of flow-stimulated and unstimulated cancer cells to adhere to an endothelial monolayer, migrate and invade membrane pores was evaluated in flow and static adhesion experiments. RESULTS: Fluid flow stimulation resulted in upregulation of EMT inducers and downregulation of repressors. Specifically, Vimentin and Snail were upregulated both at the gene and protein expression levels in flow stimulated HMECs and MDA-MB-231 cells, suggesting progression towards an EMT phenotype. Flow-stimulated SNAI2 was abrogated with Smad3 siRNA. Flow-induced overexpression of a panel of cell adhesion genes was also observed. Network analysis revealed genes involved in cell flow responses including FN1, PLAU, and ALCAM. When evaluated in clinical datasets, overexpression of FN1, PLAU, and ALCAM was observed in patients with different subtypes of breast cancer. We also observed increased adhesion, migration and invasion of flow-stimulated breast cancer cells compared to unstimulated controls. CONCLUSIONS: This study shows that fluid forces on the order of 1 Pa promote EMT and adhesion of breast cancer cells to an endothelial monolayer and identified biomarkers were distinctly expressed in patient populations. A better understanding of how biophysical forces such as shear stress affect cellular processes involved in metastatic progression of breast cancer is important for identifying new molecular markers for disease progression, and for predicting metastatic risk.

A transparent, self-healing and high-κ dielectric for low-field-emission stretchable optoelectronics.

Stretchable optoelectronic materials are essential for applications in wearable electronics, human-machine interfaces and soft robots. However, intrinsically stretchable optoelectronic devices such as light-emitting capacitors usually require high driving alternating voltages and excitation frequencies to achieve sufficient luminance in ambient lighting conditions. Here, we present a healable, low-field illuminating optoelectronic stretchable (HELIOS) device by introducing a transparent, high permittivity polymeric dielectric material. The HELIOS device turns on at an alternating voltage of 23 V and a frequency below 1 kHz, safe operating conditions for human-machine interactions. We achieved a brightness of 1,460 cd m-2 at 2.5 V µm-1 with stable illumination demonstrated up to a maximum of 800% strain. The materials also self-healed mechanically and electronically from punctures or when severed. We further demonstrate various HELIOS light-emitting capacitor devices in environment sensing using optical feedback. Moreover, our devices can be powered wirelessly, potentially enabling applications for untethered damage-resilient soft robots.

Simple Synthesis of Elastomeric Photomechanical Switches That Self-Heal.

This article introduces a simple two-stage method to synthesize and program a photomechanical elastomer (PME) for light-driven artificial muscle-like actuations in soft robotics. First, photochromic azobenzene molecules are covalently attached to a polyurethane backbone via a two-part step-growth polymerization. Next, mechanical alignment is applied to induce anisotropic deformations in the PME-actuating films. Cross-linked through dynamic hydrogen bonds, the PMEs also possess autonomic self-healing properties without external energy input. This self-healing allows for a single alignment step of the PME film and subsequent "cut and paste" assembly for multi-axis actuation of a self-folded soft-robotic gripper from a single degree of freedom optical input.

Bio-inspired Design and Additive Manufacturing of Soft Materials, Machines, Robots, and Haptic Interfaces.

Soft materials possess several distinctive characteristics, such as controllable deformation, infinite degrees of freedom, and self-assembly, which make them promising candidates for building soft machines, robots, and haptic interfaces. In this Review, we give an overview of recent advances in these areas, with an emphasis on two specific topics: bio-inspired design and additive manufacturing. Biology is an abundant source of inspiration for functional materials and systems that mimic the function or mechanism of biological tissues, agents, and behaviors. Additive manufacturing has enabled the fabrication of materials and structures prevalent in biology, thereby leading to more-capable soft robots and machines. We believe that bio-inspired design and additive manufacturing have been, and will continue to be, important tools for the design of soft robots.

Development of a Magnetic Attachment Method for Bionic Eye Applications.

Successful visual prostheses require stable, long-term attachment. Epiretinal prostheses, in particular, require attachment methods to fix the prosthesis onto the retina. The most common method is fixation with a retinal tack; however, tacks cause retinal trauma, and surgical proficiency is important to ensure optimal placement of the prosthesis near the macula. Accordingly, alternate attachment methods are required. In this study, we detail a novel method of magnetic attachment for an epiretinal prosthesis using two prostheses components positioned on opposing sides of the retina. The magnetic attachment technique was piloted in a feline animal model (chronic, nonrecovery implantation). We also detail a new method to reliably control the magnet coupling force using heat. It was found that the force exerted upon the tissue that separates the two components could be minimized as the measured force is proportionately smaller at the working distance. We thus detail, for the first time, a surgical method using customized magnets to position and affix an epiretinal prosthesis on the retina. The position of the epiretinal prosthesis is reliable, and its location on the retina is accurately controlled by the placement of a secondary magnet in the suprachoroidal location. The electrode position above the retina is less than 50 microns at the center of the device, although there were pressure points seen at the two edges due to curvature misalignment. The degree of retinal compression found in this study was unacceptably high; nevertheless, the normal structure of the retina remained intact under the electrodes.

Safety and efficacy of explanting or replacing suprachoroidal electrode arrays in a feline model.

BACKGROUND: A key requirement for retinal prostheses is the ability for safe removal or replacement. We examined whether suprachoroidal electrode arrays can be removed or replaced after implantation. METHODS: Suprachoroidal electrode arrays were unilaterally implanted into 13 adult felines. After 1 month, arrays were surgically explanted (n = 6), replaced (n = 5) or undisturbed (n = 2). The retina was assessed periodically using fundus photography and optical coherence tomography. Three months after the initial implantation, the function of replaced or undisturbed arrays was assessed by measuring the responses of the visual cortex to retinal electrical stimulation. The histopathology of tissues surrounding the implant was examined. RESULTS: Array explantation or replacement was successful in all cases. Fundus photography showed localized disruption to the tapetum lucidum near the implant's tip in seven subjects following implantation. Although optical coherence tomography showed localized retinal changes, there were no widespread statistically significant differences in the thickness of the retinal layers or choroid. The distance between the electrodes and retina increased after device replacement but returned to control values within eight weeks (P < 0.03). Staphylomas developed near the scleral wound in five animals after device explantation. Device replacement did not alter the cortical evoked potential threshold. Histopathology showed localized outer nuclear layer thinning, tapetal disruption and pseudo-rosette formation, but the overall retinal morphology was preserved. CONCLUSIONS: It is feasible to remove or replace conformable medical grade silicone electrode arrays implanted suprachoroidally. The scleral wound requires careful closure to minimize the risk of staphylomas.

Effects of chronic cochlear electrical stimulation after an extended period of profound deafness on primary auditory cortex organization in cats.

Extended periods of deafness have profound effects on central auditory system function and organization. Neonatal deafening results in loss of the normal cochleotopic organization of the primary auditory cortex (AI), but environmentally-derived intracochlear electrical stimulation, via a cochlear implant, initiated shortly after deafening, can prevent this loss. We investigated whether such stimulation initiated after an extended period of deafness can restore cochleotopy. In two groups of neonatally-deafened cats, a multi-channel intracochlear electrode array was implanted at 8 weeks of age. One group received only minimal stimulation, associated with brief recordings at 4-6-week intervals, over the following 6 months to check the efficacy of the implant. In the other group, this 6-month period was followed by 6 months of near-continuous intracochlear electrical stimulation from a modified clinical cochlear implant system. We recorded multi-unit clusters in the auditory cortex and used two different methods to define the region of interest in the putative AI. There was no evidence of cochleotopy in any of the minimally stimulated animals, confirming our earlier finding. In three of six chronically stimulated cats there was clear evidence of AI cochleotopy, and in a fourth cat in which the majority of penetrations were in the anterior auditory field there was clear evidence of cochleotopy in that field. The finding that chronic intracochlear electrical stimulation after an extended period of deafness is able to restore cochleotopy in some (but not all) cases has implications for the performance of patients implanted after an extended period of deafness.

Mesoporous silica supraparticles for sustained inner-ear drug delivery.

Mesoporous silica supraparticles (MS-SPs) are prepared via self-assembly of mesoporous silica nanoparticles under capillary force action in confined droplets. The MS-SPs are effective carriers for sustained drug delivery. Animal studies show that these particles are suitable for chronic intracochlear implantation, and neurotrophins released from the MS-SPs can efficiently rescue primary auditory neurons in an in vivo sensorineural hearing loss model.

Mold-templated inorganic-organic hybrid supraparticles for codelivery of drugs.

This paper reports a facile and robust mold-templated technique for the assembly of mesoporous silica (MS) supraparticles and demonstrates their potential as vehicles for codelivery of brain-derived neurotrophic factor (BDNF) and dexamethasone (DEX). The MS supraparticles are assembled using gelatin as a biodegradable adhesive to bind and cross-link the particles. Microfabricated molds made of polydimethylsiloxane are used to control the size and shape of the supraparticles. The obtained mesoporous silica-gelatin hybrid supraparticles (MSG-SPs) are stable in water as well as in organic solvents, such as dimethyl sulfoxide, and efficiently coencapsulate both BDNF and DEX. The MSG-SPs also exhibit sustained release kinetics in simulated physiological conditions (>30 days), making them potential candidates for long-term delivery of therapeutics to the inner ear.

Multigait soft robot.

This manuscript describes a unique class of locomotive robot: A soft robot, composed exclusively of soft materials (elastomeric polymers), which is inspired by animals (e.g., squid, starfish, worms) that do not have hard internal skeletons. Soft lithography was used to fabricate a pneumatically actuated robot capable of sophisticated locomotion (e.g., fluid movement of limbs and multiple gaits). This robot is quadrupedal; it uses no sensors, only five actuators, and a simple pneumatic valving system that operates at low pressures (< 10 psi). A combination of crawling and undulation gaits allowed this robot to navigate a difficult obstacle. This demonstration illustrates an advantage of soft robotics: They are systems in which simple types of actuation produce complex motion.

Development of a surgical procedure for implantation of a prototype suprachoroidal retinal prosthesis.

BACKGROUND: Current surgical techniques for retinal prosthetic implantation require long and complicated surgery, which can increase the risk of complications and adverse outcomes. METHOD: The suprachoroidal position is known to be an easier location to access surgically, and so this study aimed to develop a surgical procedure for implanting a prototype suprachoroidal retinal prosthesis. The array implantation procedure was developed in 14 enucleated eyes. A full-thickness scleral incision was made parallel to the intermuscular septum and superotemporal to the lateral rectus muscle. A pocket was created in the suprachoroidal space, and the moulded electrode array was inserted. The scleral incision was closed and scleral anchor point sutured. In 9 of the 14 eyes examined, the device insertion was obstructed by the posterior ciliary neurovascular bundle. Subsequently, the position of this neurovascular bundle in 10 eyes was characterized. Implantation and lead routing procedure was then developed in six human cadavers. The array was tunnelled forward from behind the pinna to the orbit. Next, a lateral canthotomy was made. Lead fixation was established by creating an orbitotomy drilled in the frontal process of the zygomatic bone. The lateral rectus muscle was detached, and implantation was carried out. Finally, pinna to lateral canthus measurements were taken on 61 patients in order to determine optimal lead length. RESULTS: These results identified potential anatomical obstructions and informed the anatomical fitting of the suprachoroidal retinal prosthesis. CONCLUSION: As a result of this work, a straightforward surgical approach for accurate anatomical suprachoroidal array and lead placement was developed for clinical application.