This review comprises a detailed survey of ongoing methodologies for soft actuators, highlighting approaches suitable for nanometer- to centimeter-scale robotic applications. Soft robots present a special design challenge in that their actuation and sensing mechanisms are often highly integrated with the robot body and overall functionality. When less than a centimeter, they belong to an even more special subcategory of robots or devices, in that they often lack on-board power, sensing, computation, and control. Soft, active materials are particularly well suited for this task, with a wide range of stimulants and a number of impressive examples, demonstrating large deformations, high motion complexities, and varied multifunctionality. Recent research includes both the development of new materials and composites, as well as novel implementations leveraging the unique properties of soft materials.

Soft Actuators for Small-Scale Robotics.

Self-actuating materials capable of transforming between three-dimensional shapes have applications in areas as diverse as biomedicine, robotics, and tunable micro-optics. We introduce a method of photopatterning polymer films that yields temperature-responsive gel sheets that can transform between a flat state and a prescribed three-dimensional shape. Our approach is based on poly(N-isopropylacrylamide) copolymers containing pendent benzophenone units that allow cross-linking to be tuned by irradiation dose. We describe a simple method of halftone gel lithography using only two photomasks, wherein highly cross-linked dots embedded in a lightly cross-linked matrix provide access to nearly continuous, and fully two-dimensional, patterns of swelling. This method is used to fabricate surfaces with constant Gaussian curvature (spherical caps, saddles, and cones) or zero mean curvature (Enneper’s surfaces), as well as more complex and nearly closed shapes.

https://science.sciencemag.org/content/sci/335/6073/1201.full.pdf

Designing responsive buckled surfaces by halftone gel lithography.

Hydrogels in sensing applications

Hydrogel-based actuators: possibilities and limitations

Active motion is intrinsic to all kinds of living organisms from unicellular ones to humans and inspires development of synthetic actively moving materials. Hydrogels, which are three dimensional polymer networks imbibed with aqueous solutions, mimic the swelling/shrinking behavior of plant cells and produce macroscopic actuation upon swelling and shrinking. This Feature Article covers basic principles of design as well as recent advances in the development of hydrogel based actuating systems. It is discussed how simply swelling can be used to generate very complex multistep motion, which can be used to develop new optical devices, sensors, biomaterials, smart surface, etc.

Biomimetic Hydrogel-Based Actuating Systems

Engineered locomotive systems are widespread on the macro scale (e.g., planes, trains, automobiles), yet miniaturized mobile systems are largely limited to the research lab. In particular, integration of specific functions such as sensing or drug delivery into a small robot is a great challenge. The ability to miniaturize parts affecting motion, such as MEMS-based components and electroactive polymers, while widely demonstrated has not led to practical advances in miniature multifunctional mobile robots. Rather complex construction, limits in the use of diverse materials, and cost (in case of the MEMS-based bot) all hinder the production of these robots. The small (micro-/millimeter scale) system within aqueous environments is particularly challenging, yet there are important potential applications for robots in such environments. To date, engineers have explored soft materials for creating components at the micro-/millimeter scales; for example, actuators such as ionic polymer metal composites, conducting polymers, and carbon nanotube actuators, and engineered devices housing hydrogel valves, filters, and lenses, as well as other soft material actuators that mimic natural movement (earthworm, myriapod, etc.). Still, the creation of miniature multifunctional robots that operate in aqueous environments has been elusive. Nature provides numerous examples of small, multifunctional and autonomous ‘‘robots’’ in the insect and marine worlds after which we aimed to model miniature polymeric aquabots. Here we present mini (microto millimeter) soft aquabots that combine multiple functionalities to perform multifunctional operations in aqueous environments, effectively simulating their natural counterparts. Despite extensive research on macroscale robotics and miniature micro-electromechanical systems, relatively little attention has been paid to the creation of miniature soft robots with diverse shapes, actuation mechanisms, and integrated functionalities. As a fabrication method, we describe a technique that takes advantage of the small volume required by amicrofluidic chamber; this will be used for photopolymerizing multifunctional minibots that possess several ‘‘organs,’’ allowing for movement, sensing/signaling, and capture/transport/release. A diverse group of aquabots mimicking three kinds of living organisms – octopus, sperm, and myriapod, each representing a different mode of locomotion – has been developed. These aquabots have the following key features: 1) diverse shape and small size (sub-millimeter feature sizes); 2) a variety of functions (e.g., actuation, sensing) integrated in a single robot; and 3) rapid, scalable fabrication both in dimension and quantity. The aquabots were fabricated out of soft materials, due to the ease of polymerization, as well as the ability to tailor the material composition towards achieving certain functionality (e.g., responsiveness to pH, temperature, or electric/ magnetic field). Sequential in situ photopolymerization within a microfluidic device allowed for precise fabrication of different components (arms, legs, body; see detailed procedure for photopolymerization in Experimental section). The bodies were constructed using a relatively inert polyethylene glycol (PEG) material, whereas ionic electroactive polymers (electroactive hydrogels) were chosen from a large number of available soft materials to serve as the actuators, thus facilitating controlled movement. Depending on the application, some aquabots contained temperature-, pH-, or chemical-sensitive polymers in addition to their electroactive hydrogel and PEG components. The gel network of polymerized actuator material was anionic, causing positively charged surfactant molecules to bind to its surface; this surfactant binding leads to an osmotic pressure difference between the gel interior and the external solution, thus inducing the contraction and curvature of the gel strip. Four parameters control electroactive hydrogelbased actuators: 1)magnitude of the applied potential; 2) angle of the applied potential; and both 3) width and 4) length of the actuator. Figure 1A illustrates the behavior of a rectangular gel actuator under changing parameters (width of post and applied voltage; see Supporting Information Fig. S1 for more complete parameter characterization). In situ photopolymerization facilitates the control of the geometric parameters and is amenable to the use of laser or multiphoton polymerization methods for improved resolution (e.g., further reduce actuator width to achieve faster and larger motion). To date, diverse electroactive hydrogels have been broadly applied to creating polymeric microactuators (see the detailed behavior of electric-sensitive hydrogel in Supporting Information). Here, we demonstrate the use of electroactive hydrogel for fabricating octopus-, sperm-, and myriapod-like aquabots that retain respective characteristic movement (Fig. 2; movies are available for octopus-like aquabot and sperm-like aquabots in Supporting Information – Videos 1 and 2, respectively). The repeated bending test under aquatic conditions was carried out to investigate the durability of the electroactive hydrogel strips (see Supporting Information for a detailed durability test conditions). The operation of the communications

Biomimetic Soft Multifunctional Miniature Aquabots

A sensor for the lethal bacterial enzyme, botulinum neurotoxin type A (BoNT/A), was developed using self-assembled monolayers (SAMs). SAMs consisting of an immobilized synthetic peptide that mimicked the toxin’s in vivo SNAP-25 protein substrate were formed on Au and interfaced with arrayed microfluidic channels. Efforts to optimize SAM composition and assay conditions for greatest reaction efficiency and sensitivity are described in detail. Channel design provided facile fluid manipulation, sample incubation, analyte concentration, and fluorescence detection all within a single microfluidic channel, thus avoiding sample transfer and loss. Peptide SAMs were exposed to varying concentrations of BoNT/A or its catalytic light chain (ALC), resulting in enzymatic cleavage of the peptide substrate from the surface. Fluorescence detection was achieved down to 20 pg/mL ALC and 3 pg/mL BoNT/A in 3 h. Toxin sensing was also accomplished in vegetable soup, demonstrating practicality of the method. The modular design...

Self-assembled peptide monolayers as a toxin sensing mechanism within arrayed microchannels.

Cell concentration via centrifugation is a ubiquitous step in many cell culture procedures. At the macroscale, centrifugation suffers from a number of limitations, particularly when dealing with small numbers of cells (e.g., less than 50 000). On the other hand, typical microscale methods for cell concentration can affect cell physiology and bias readouts of cell behavior and function. In this paper, we present a microfluidic concentrator device that utilizes the effects of gravity to allow cells to gently settle out of a suspension into a collection region without the use of specific adhesion ligands. Dimensional analysis was performed to compare different device designs and was verified with flow modeling to optimize operational parameters. We are able to concentrate low-density cell suspensions in a microfluidic chamber, achieving a cell loss of only 1.1 ± 0.6% (SD, n = 7) with no observed loss during a subsequent cell staining protocol which incorporates ∼36 complete device volume replacements. This m...

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A Microfluidic Cell Concentrator

We have devised a microfluidic platform that incorporates substrate-laden silica beads for sensing the proteolytic activity of botulinum neurotoxin type A (BoNT/A)—one of the most poisonous substances known and a significant biological threat. The sensor relies on toxin-mediated cleavage of a fluorophore-tagged peptide substrate specific for only BoNT/A. Peptide immobilized on beads is recognized and cleaved by the toxin, releasing fluorescent fragments into solution that can be concentrated at an isolated port viaevaporation and detected using microscopy. Evaporative concentration in combination with a specific channel geometry provides up to a 3-fold signal amplification in 35 min, allowing for detection of low levels of fluorophore-labeled peptide—a task not easily accomplished using traditional channel designs. Our bead-based microfluidic platform can sense BoNT/A down to 10 pg of toxin per mL buffer solution in 3.5 h and can be adapted to sensing other toxins that operate via enzymatic cleavage of a known substrate.

Bead-based microfluidic toxin sensor integrating evaporative signal amplification.

We report the development of substrate-cross-linked hydrogels for the specific detection of botulinum neurotoxin type A (BoNT/A), the most poisonous toxin known. Peptide-cross-linked BoNT/A-sensitive hydrogels were photopolymerized within microfluidic channels to create autonomous biosensors capable of sensing toxin enzymatic activity with a visual readout.

Megan L. Frisk

Papers

Biomimetic Soft Multifunctional Miniature Aquabots

Self-assembled peptide monolayers as a toxin sensing mechanism within arrayed microchannels.

A Microfluidic Cell Concentrator

Bead-based microfluidic toxin sensor integrating evaporative signal amplification.

Substrate-Modified Hydrogels for Autonomous Sensing of Botulinum Neurotoxin Type A