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.

Self-Detachable Through-Metal Acoustic Wireless Power Transfer.

Unlike electromagnetic waves, acoustic vibrations waves can be used to transfer power directly through metal structures without being shielded. In this article, a novel design of a self-detachable acoustic wireless power transfer system that can be used to transfer power through the thickness of a steel plate is presented, which does not require the use of any couplant. Electro-permanent-magnets (EPMs) were used to provide magnetic clamping force along the perimeter of the receiver transducer disk to enhance coupling to the steel plate, while the transmitter transducer was bonded to the other side of the plate. The EPM clamping force can be switched ON/ OFF electronically with low power consumption. Unlike past work reliant on additional bonding materials or liquid/gel couplant, this approach enables the receiver to be attached and detached at will, opening up the possibility of a simple charging pad for unmanned aerial vehicles (UAVs) or other consumer devices for harsh environment applications. Power transfer efficiency up to 63% was achieved, and the effect of varying steel plate thickness and clamping force was also investigated. A finite element model was also constructed to understand the vibration mode shape.

Laser Direct Structured 3D Circuits on Silicone.

Silicone rubber is a biocompatible elastomeric polymer, with great potential for mechanical and biologic sensing applications, if electrical circuits can be reliably integrated. Laser direct structuring is a bottom-up circuit fabrication process, whereby copper is chemically grown on laser exposed regions of a modified substrate, promoting adhesion by laser roughening the circuit tracks. In this Research Article, we successfully demonstrate this process using superflexible biocompatible silicone (30 hardness on Shore 00) with copper chromite additive, cast into both 2D planar and 3D contour substrates. A horseshoe pattern circuit, meander and Hilbert fractal inductors, and a 3D hemispherical helix trace are fabricated and tested. The range of laser power and copper chromite concentration are explored. Mechanical testing is performed to determine breakage strain and elastic modulus. Material stiffness and trace peel strength are shown to increase with copper chromite concentration. Peel strength is measured to be very high, from approximately 1 to 5 kN/m, depending on dopant loading. With high adhesion and conductivity, the simple laser-writing process presented here enables high-quality circuit integration into elastomeric silicone.

Self-Folding PCB Kirigami: Rapid Prototyping of 3D Electronics via Laser Cutting and Forming.

This paper demonstrates laser forming, localized heating with a laser to induce plastic deformation, can self-fold 2D printed circuit boards (PCBs) into 3D structures with electronic function. There are many methods for self-folding but few are compatible with electronic materials. We use a low-cost commercial laser writer to both cut and fold a commercial flexible PCB. Laser settings are tuned to select between cutting and folding with higher power resulting in cutting and lower power resulting in localized heating for folding into 3D shapes. Since the thin copper traces used in commercial PCBs are highly reflective and difficult to directly fold, two approaches are explored for enabling folding: plating with a nickel/gold coating or using a single, high-power laser exposure to oxidize the surface and improve laser absorption. We characterized the physical effect of the exposure on the sample as well as the fold angle as a function of laser passes and demonstrate the ability to lift weights comparable with circuit packages and passive components. This technique can form complex, multifold structures with integrated electronics; as a demonstrator, we fold a commercial board with a common timing circuit. Laser forming to add a third dimension to printed circuit boards is an important technology to enable the rapid prototyping of complex 3D electronics.

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.

Printing a Pacinian Corpuscle: Modeling and Performance.

The Pacinian corpuscle is a highly sensitive mammalian sensor cell that exhibits a unique band-pass sensitivity to vibrations. The cell achieves this band-pass response through the use of 20 to 70 elastic layers entrapping layers of viscous fluid. This paper develops and explores a scalable mechanical model of the Pacinian corpuscle and uses the model to predict the response of synthetic corpuscles, which could be the basis for future vibration sensors. The -3dB point of the biological cell is accurately mimicked using the geometries and materials available with off-the-shelf 3D printers. The artificial corpuscles here are constructed using uncured photoresist within structures printed in a commercial stereolithography (SLA) 3D printer, allowing the creation of trapped fluid layers analogous to the biological cell. Multi-layer artificial Pacinian corpuscles are vibration tested over the range of 20-3000 Hz and the response is in good agreement with the model.

Mechanically Cloaked Multiphase Magnetic Elastomer Soft Composites for Wearable Wireless Power Transfer.

Wearable electronics allow for new and immersive experiences between technology and the human body, but conventional devices are made from rigid functional components that lack the necessary compliance to safely interact with human tissue. Recently, liquid inclusions have been incorporated into elastomer composites to produce functional materials with high extensibility and ultrasoft mechanical responses. While these materials have shown high thermal and electrical conductivity, there has been an absence of research into compliant magnetic materials through the incorporation of magnetic fluids. Compliant magnetic materials are important for applications in soft matter engineering including sensing, actuation, and power transfer for soft electronics and robotics. In this work, we establish a new class of highly functional soft materials with advanced magnetic and mechanical properties by dispersing magnetic colloidal suspensions as compliant fluid inclusions into soft elastomers. Significantly, the rigid magnetic particles are encapsulated by the fluid. This mechanically cloaks the solid particles and enables a fluid-like mechanical response while imparting high magnetic permeability to the composite. This microstructure reduces the modulus of the composite below that of the initial elastomer to <40 kPa while increasing the permeability by over 100% to greater than 2. We demonstrate the functionality of these materials through conformable magnetic backplanes, which enables a completely soft, coupled inductor system capable of transferring power up to 100% strain and wearable devices for wireless power transfer.

Vibration sensing the mammalian way: an artificial Pacinian corpuscle.

A vibration sensor is presented mimicking the structure of the Pacinian corpuscle. A multi-step casting process is used to create a 5 mm diameter sensor with a liquid metal core, elastomer dielectric, and graphite counter electrode creating a spherical capacitive sensing element with sensitivities on the order of 10 Δ pF/mm-1. A model for the capacitance change of the spherical capacitor as it is formed is developed and its findings support the sensitivities observed. Various elastomer dielectric compositions with integrated barium titanate nanoparticles are tested to increase the dielectric constant. The biological acoustic filter within the corpuscle is mimicked using alternating cast layers of oligomers and elastomers around the spherical sensor element. Vibration sensing is characterized over the low frequency range of 10-300 Hz and the minimum detectable sensitivity is found to be 1 µm with a low power requirement of 7 mW. The artificial Pacinian corpuscle has potential applications in tactile sensing and seismic monitoring devices.

Ultrasonic Lamb Waves for Wireless Power Transfer.

Ultrasonic guided plate waves (Lamb waves) can be used to transfer power along the length of metal plates, achieving longer distance wireless power transfer (WPT), while not being impeded by electromagnetic shielding from the metal plate. In this article, a fundamental study on the performance of Lamb wave WPT is presented, including modeling, simulations, and experimental verification. By using Macro-Fiber Composite (MFC), d33 -mode, piezoelectric transducers bonded to a [Formula: see text] mm aluminum plate using an epoxy, power transfer of 0.47 W with 56% overall power transfer efficiency was achieved at a 204-mm distance. The measured frequency response of the power transfer efficiency matches well with the simulated results, and the effects of complex load impedance matching and transducer sizing were investigated. It is shown that the location where the efficiency is maximized roughly corresponds to the zero-order symmetrical mode (S0) standing wave patterns due to reflections from the plate edges. For practical implementation, the effect of using different methods to temporarily or permanently bond the MFC transducers to the metal plate was also investigated, as well as the effect of electrically grounding the metal plate.

Phased Array Focusing for Acoustic Wireless Power Transfer.

Wireless power transfer (WPT) through acoustic waves can achieve higher efficiencies than inductive coupling when the distance is above several times the transducer size. This paper demonstrates the use of ultrasonic phased arrays to focus power to receivers at arbitrary locations to increase the power transfer efficiency. Using a phased array consisting of 37 elements at a distance nearly 5 times the receiver transducer diameter, a factor of 2.6 increase in efficiency was achieved when compared to a case equivalent to a single large transducer with the same peak efficiency distance. The array has a total diameter of 7 cm, and transmits through air at 40 kHz to a 1.1-cm diameter receiver, achieving a peak overall efficiency of 4% at a distance of 5 cm. By adjusting the focal distance, the efficiency can also be maintained relatively constant at distances up to 9 cm. Numerical models were developed and shown to closely match the experimental energy transfer behavior; modeling results indicate that the efficiency can be further doubled by increasing the number of elements. For comparison, an inductive WPT system was also built with the diameters of the transmitting and receiving coils equivalent to the dimensions of the transmitting ultrasonic phased array and receiver transducer, and the acoustic WPT system achieved higher efficiencies than the inductive WPT system when the transmit-to-receive distance is above 5 cm. In addition, beam angle steering was demonstrated by using a simplified seven-element 1-D array, achieving power transfer less dependent on receiver placement.

Ultrafine Pitch Stencil Printing of Liquid Metal Alloys.

With high conductivity and stretchable for large cross-sections, liquid metals such as galinstan are promising for creating stretchable devices and interconnects. Creating high resolution features in parallel is challenging, with most techniques limited to a hundred micrometers or more. In this work, multilevel electroplated stencils are investigated for printing liquid metals, with galinstan features as small as ten micrometers printed on soft elastomers, a factor of 10 reduction over past liquid metal stencil printing. Capacitors and resistive strain sensors are also demonstrated, showing the potential for creating stretchable conductors and devices.

Magnetic elastomers for stretchable inductors.

In this work, silicone loaded with magnetic particles is investigated for creating a composite with higher permeability while still maintaining stretchability. Magnetic and mechanical properties are first characterized for composites based on both spherical and platelet particle geometries. The first magnetic-core stretchable inductors are then demonstrated using the resulting ferroelastomer. Solenoid inductors based on liquid metal galinstan are then demonstrated around a ferroelastomeric core and shown to survive uniaxial strains up to 100%. Soft elastomers loaded with magnetic particles were found to increase the core permeability and inductance density of stretchable inductors by nearly 200%.

Robust gold nanoparticles stabilized by trithiol for application in chemiresistive sensors.

The use of gold nanoparticles coated with an organic monolayer of thiol for application in chemiresistive sensors was initiated in the late 1990s; since then, such types of sensors have been widely pursued due to their high sensitivities and reversible responses to volatile organic compounds (VOCs). However, a major issue for chemical sensors based on thiol-capped gold nanoparticles is their poor long-term stability as a result of slow degradation of the monothiol-to-gold bonds. We have devised a strategy to overcome this limitation by synthesizing a more robust system using Au nanoparticles capped by trithiol ligands. Compared to its monothiol counterpart, the new system is significantly more stable and also shows improved sensitivity towards different types of polar or non-polar VOCs. Thus, the trithiol-Au nanosensor shows great promise for use in real world applications.

Fabrication of a dual-tier thin film micropolarization array.

A thin film polarization filter has been patterned and etched using reactive ion etching (RIE) in order to create 8 by 8 microns square periodic structures. The micropolarization filters retain the original extinction ratios of the unpatterned thin film. The measured extinction ratios on the micropolarization filters are approximately 1000 in the blue and green visible spectrum and approximately 100 in the red spectrum. Various gas combinations for RIE have been explored in order to determine the right concentration mix of CF(4) and O(2) that gives optimum etching rate, in terms of speed and under-etching. Theoretical explanation for the optimum etching rate has also been presented. In addition, anisotropic etching with 1 microm under cutting of a 10 microm thick film has been achieved. Experimental results for the patterned structures under polarized light are presented. The array of micropolarizers will be deposited on top of a custom made CMOS imaging sensor in order to compute the first three Stokes parameters in real time.