Showing papers by "George M. Whitesides published in 2022"

Soft Robotics.

Abstract: This description of "soft robotics" is not intended to be a conventional review, in the sense of a comprehensive technical summary of a developing field. Rather, its objective is to describe soft robotics as a new field-one that offers opportunities to chemists and materials scientists who like to make "things" and to work with macroscopic objects that move and exert force. It will give one (personal) view of what soft actuators and robots are, and how this class of soft devices fits into the more highly developed field of conventional "hard" robotics. It will also suggest how and why soft robotics is more than simply a minor technical "tweak" on hard robotics and propose a unique role for chemistry, and materials science, in this field. Soft robotics is, at its core, intellectually and technologically different from hard robotics, both because it has different objectives and uses and because it relies on the properties of materials to assume many of the roles played by sensors, actuators, and controllers in hard robotics.

An outlook on microfluidics: the promise and the challenge.

Abstract: This perspective considers ways in which the field of microfluidics can increase its impact by improving existing technologies and enabling new functionalities. We highlight applications where microfluidics has made or can make important contributions, including diagnostics, food safety, and the production of materials. The success of microfluidics assumes several forms, including fundamental innovations in fluid mechanics that enable the precise manipulation of fluids at small scales and the development of portable microfluidic chips for commercial purposes. We identify outstanding technical challenges whose resolution could increase the accessibility of microfluidics to users with both scientific and non-technical backgrounds. They include the simplification of procedures for sample preparation, the identification of materials for the production of microfluidic devices in both laboratory and commercial settings, and the replacement of auxiliary equipment with automated components for the operation of microfluidic devices.

Nonlinear Phenomena in Microfluidics.

Abstract: This review focuses on experimental work on nonlinear phenomena in microfluidics, which for the most part are phenomena for which the velocity of a fluid flowing through a microfluidic channel does not scale proportionately with the pressure drop. Examples include oscillations, flow-switching behaviors, and bifurcations. These phenomena are qualitatively distinct from laminar, diffusion-limited flows that are often associated with microfluidics. We explore the nonlinear behaviors of bubbles or droplets when they travel alone or in trains through a microfluidic network or when they assemble into either one- or two-dimensional crystals. We consider the nonlinearities that can be induced by the geometry of channels, such as their curvature or the bas-relief patterning of their base. By casting posts, barriers, or membranes─situated inside channels─from stimuli-responsive or flexible materials, the shape, size, or configuration of these elements can be altered by flowing fluids, which may enable autonomous flow control. We also highlight some of the nonlinearities that arise from operating devices at intermediate Reynolds numbers or from using non-Newtonian fluids or liquid metals. We include a brief discussion of relevant practical applications, including flow gating, mixing, and particle separations.

Programmable soft valves for digital and analog control

Abstract: Significance We designed soft pneumatic valves to allow complex sequences of actuation and precise control of forces in both digital and analog formats. These valves leverage the nonlinear mechanics of soft materials. Based on this design, we created systems capable of analog pressure regulation, linear actuation, digital logic, pressure amplification, controlled oscillation, nonvolatile memory storage, and interfacing with human users. Combining digital and analog components enables control circuits with significantly fewer physical valves compared to equivalent, but strictly digital, circuits. The programmability of these valves (i.e., with tunable pressures of actuation and output) simplifies the operation of multipressure systems, helping to untether robots and create intuitive, human-friendly devices.

A buckling-sheet ring oscillator for electronics-free, multimodal locomotion

Abstract: Locomotion of soft robots typically relies on control of multiple inflatable actuators by electronic computers and hard valves. Soft pneumatic oscillators can reduce the demand on controllers by generating complex movements required for locomotion from a single, constant input pressure, but either have been constrained to low rates of flow of air or have required complex fabrication processes. Here, we describe a pneumatic oscillator fabricated from flexible, but inextensible, sheets that provides high rates of airflow for practical locomotion by combining three instabilities: out-of-plane buckling of the sheets, kinking of tubing attached to the sheets, and a system-level instability resulting from connection of an odd number of pneumatic inverters made from these sheets in a loop. This device, which we call a “buckling-sheet ring oscillator” (BRO), directly generates movement from its own interaction with its surroundings and consists only of readily available materials assembled in a simple process—specifically, stacking acetate sheets, nylon film, and double-sided tape, and attaching an elastomeric tube. A device incorporating a BRO is capable of both translational and rotational motion over varied terrain (even without a tether) and can climb upward against gravity and downward against the buoyant force encountered under water. Description The buckling-sheet ring oscillator enables multimodal locomotion from a constant pressure without using electronic controls.

Magnetic fields enhance mass transport during electrocatalytic reduction of CO2

Abstract: The selectivity of electrocatalytic reduction of CO 2 (CDR) is dictated not only by the intrinsic reactivity of the catalyst but also by the transport of reactants to the catalyst (i.e., mass transport). Current methods for increasing mass transport in CDR rely upon either (1) mechanical agitation or (2) use of gas-diffusion electrodes and are unable to eliminate concentration polarization completely. This work demonstrates that magnetic fields orthogonal to the ionic current (i.e., the Lorentz force) can be used to increase mass transport during CDR by generating convective flow in the fluid, thus modifying the observed selectivity of CDR. This increase in mass transport leads to a corresponding increase in current densities (up to 1.3× higher than the analogous system with no B → -field or agitation) and increased selectivity of CDR relative to the hydrogen evolution reaction (up to 2.5× higher than the system with no B → -field or agitation). • During CDR, magnetic fields (B-fields) generate convective currents in the fluid • The convection generated by B-fields decreases pH gradients during electrolysis • The magnitude of the B-field effect is dictated by the structure of the cathode • B-fields decrease the cost of operating a CO 2 electrolyzer by ∼10% At high current densities (>100 mA/cm 2 ), the electrocatalytic reduction of CO 2 (CDR) can be limited by mass transport, resulting in decreased selectivity for CDR relative to the reduction of protons to hydrogen. Common approaches to increase mass transport for CDR rely upon either (1) gas-diffusion electrodes or (2) mechanical agitation. This work demonstrates that magnetic fields acting through the Lorentz force can increase mass transport during CDR. Magnetic fields generate convective currents by the Lorentz force acting on ions moving during electrolysis and decreases the cost of operating a CO 2 electrolyzer (by ∼10% in this work). Magnetic fields parallel to the surface of an electrode can be used to increase mass transport during electrocatalytic reduction of CO 2 .

Controlled Hysteresis of Conductance in Molecular Tunneling Junctions.

Abstract: The problem this paper addresses is the origin of the hysteretic behavior in two-terminal molecular junctions made from an EGaIn electrode and self-assembled monolayers of alkanethiolates terminated in chelates (transition metal dichlorides complexed with 2,2'-bipyridine; BIPY-MCl2). The hysteresis of conductance displayed by these BIPY-MCl2 junctions changes in magnitude depending on the identity of the metal ion (M) and the window of the applied voltage across the junction. The hysteretic behavior of conductance in these junctions appears only in an incoherent (Fowler-Nordheim) tunneling regime. When the complexed metal ion is Mn(II), Fe(II), Co(II), or Ni(II), both incoherent tunneling and hysteresis are observed for a voltage range between +1.0 V and -1.0 V. When the metal ion is Cr(II) or Cu(II), however, only resonant (one-step) tunneling is observed, and the junctions exhibit no hysteresis and do not enter the incoherent tunneling regime. Using this correlation, the conductance characteristics of BIPY-MCl2 junctions can be controlled. This voltage-induced change of conductance demonstrates a simple, fast, and reversible way (i.e., by changing the applied voltage) to modulate conductance in molecular tunneling junctions.

Tube-Balloon Logic for the Exploration of Fluidic Control Elements

Abstract: The control of pneumatically driven soft robots typically requires electronics. Microcontrollers are connected to power electronics that switch valves and pumps on and off. As a recent alternative, fluidic control methods have been introduced, in which soft digital logic gates permit multiple actuation states to be achieved in soft systems. Such systems have demonstrated autonomous behaviors without the use of electronics. However, fluidic controllers have required complex fabrication processes. To democratize the exploration of fluidic controllers, we developed tube-balloon logic circuitry, which consists of logic gates made from straws and balloons. Each tube-balloon logic device takes a novice five minutes to fabricate and costs $0.45. Tube-balloon logic devices can also operate at pressures of up to 200 kPa and oscillate at frequencies of up to 15 Hz. We configure the tube-balloon logic device as NOT-, NAND-, and NOR-gates and assemble them into a three-ring oscillator to demonstrate a vibrating sieve that separates sugar from rice. Because tube-balloon logic devices are low-cost, easy to fabricate, and their operating principle is simple, they are well suited for exploring fundamental concepts of fluidic control schemes while encouraging design inquiry for pneumatically driven soft robots.

An Expanding Foam‐Fabric Orthopedic Cast

Abstract: Traditional orthopedic casting strategies used in the treatment of fractured limbs, such as fiberglass and plaster‐based tapes, suffer from several drawbacks, including technically challenging molding for application, occurrence of skin complications, and the requirement of a potentially hazardous oscillatory saw for removal, which is frightening for pediatric patients. This work presents the design and evaluation of a foam‐fabric cast (FFC) to overcome these drawbacks by integrating strategies from soft materials engineering and functional apparel design. A fabric sleeve is designed to enable the reactive injection molding of a polymer foam and provide a form‐fitting orthopedic cast for the human forearm—with sufficient mechanical reinforcement to stabilize a fractured limb. Through testing with a replica limb and human subjects with a range of forearm volumes, the FFC application process is demonstrated and characterized. The thermal, pressural, chemical, and hygienic safety are comparable to or safer than existing clinical technologies. The FFC weighs only ≈150 g, is water resistant, and represents a robust alternative to traditional casts that can be i) manufactured at a large scale for a low cost; ii) applied to patients simply, rapidly (≈5 min), and reliably; and iii) removed easily with a pair of scissors.

Generating Oscillatory Behavior by Applying a Magnetic Field during Electrocatalytic Oxidation of Glycerol

Abstract: : This work demonstrates how the application of a magnetic field during electrocatalysis can affect the transport of reactants and products at the electrode surface and, under certain conditions, generate complex, oscillatory behavior in amperometric experiments. During the electrocatalytic oxidation of glycerol (EOG), the Lorentz force acts upon hydronium and hydroxide ionic currents to produce fluidic convection, which serves to enhance the mass transport of glycerol and glyceraldehyde. Particle imaging velocimetry shows that the convective fluid flow field depends nonlinearly on the viscosity of the solution (dictated by the concentration of glycerol, which ranged from 2.5% to 40% v/v, to give a viscosity range of 1.3 − 5.3 mPa s). The nonlinear dependence of velocity of the fluid flow on the viscosity of the electrolyte generates time-delayed negative feedback during EOG and results in chemical oscillations. This time-delayed feedback is due to two coupled steps: (i) oxidation of glycerol rapidly decreases the viscosity of the electrolyte near the anode and (ii) at low viscosities magnetic fields increase the rate of mass transport, which subsequently increases the viscosity of the electrolyte near the anode (by increasing the concentration of glycerol). These chemical oscillations can be used to enhance the selectivity of EOG to glyceric acid by a factor of 2.1. This work focuses on the effects of magnetic fields on EOG using a combination of fluid flow analysis, rotating-disk electrode experiments, and electrochemical simulations.

Charge Transport Measured Using the EGaIn Junction through Self-Assembled Monolayers Immersed in Organic Liquids.

Abstract: This paper describes measurements of charge transport by tunneling through molecular junctions comprising a self-assembled monolayer (SAM) supported by a template-stripped metal bottom electrode (MTS), which has been immersed in an organic liquid and contacted by a conical Ga2O3/EGaIn top electrode. These junctions formed in organic liquids are robust; they show stabilities and yields similar to those formed in air. We formed junctions under seven external environments: (I) air, (II) perfluorocarbons, (III) linear hydrocarbons, (IV) cyclic hydrocarbons, (V) aromatic compounds, (VI) large, irregularly shaped hydrocarbons, and (VII) dimethyl siloxanes. Several different lengths of SAMs of n-alkanethiolates, S(CH2)n-1CH3 with n = 4-18, and two different kinds of bottom electrodes (AgTS or AuTS) are employed to assess the mechanism underlying the observed changes in tunneling currents. Measurements of current density through junctions immersed in perfluorocarbons (II) are comparable to junctions measured in air. Junctions immersed in other organic liquids show reductions in the values of current density, compared to the values in air, ranging from 1 (III) to 5 orders of magnitude (IV). We interpret the most plausible mechanism for these reductions in current densities to be an increase in the length of the tunneling pathway, reflecting the formation of thin (0.5-1.5 nm) liquid films at the interface between the SAM and the Ga2O3/EGaIn electrode. Remarkably, the thickness of the liquid film─estimated by the simplified Simmons model, measurements of electrical breakdown of the junction, and simulations of molecular dynamics─is consistent with the existing observations of structured liquid layers that form between two flat interfaces from measurements obtained by the surface force apparatus. These results suggest the use of the EGaIn junction and measurements of charge transport by tunneling as a new form of surface analysis, with the applications in the study of near-surface, weak, molecular interactions and the behavior of liquid films adjacent to non-polar interfaces.

Fabrication of Low-Cost Paper-Based Microfluidic Devices by Embossing or Cut-and-Stack Methods Low-Cost

Abstract: This communication describes the use of embossing, and “cut-and-stack” methods of assembly, to generate microfluidic devices from omniphobic paper, and demonstrates that fluid flowing through these devices behaves similarly to fluid in an open-channel microfluidic device. The porosity of the paper to gasses allows processes not possible in devices made using PDMS or other non-porous materials. Droplet generators and phase separators, for example, could be made by embossing “T”-shaped channels on paper. Vertical stacking of embossed or cut layers of omniphobic paper generated three-dimensional systems of microchannels. The gas permeability of the paper allowed fluid in the microchannel to contact and exchange with environmental or directed gases. An aqueous stream of water containing a pH-indicator, as one demonstration, changed color upon exposure to air containing HCl or NH 3 gases. and M.B.J.A. developed techniques for embossing. J.B., M.M.T. and R.V.M imaged the devices, M.M.T., W.L., J.B., and R.V.M developed the cut-and-stack methodology. R.V.M., M.M.T., and W.L. characterized the performance of the devices. methods. which allows processes possible microfluidic devices made of PDMS or other non-porous polymers.