Acoustofluidic waveguides for fabrication of localized polymeric microstructure arrays

Particle manipulation and patterning have gained tremendous attention in chemical, biomedical, and manufacturing studies. Hydrogels are usually used for applications in soft robots, biosensing, as well as tissue engineering. In this study, we investigated a nanoparticle manipulation method based on standing surface acoustic waves (SAWs). The SAW device consists of a piezoelectric lithium niobate (LiNbO3) substrate with a pair of interdigital transducers (IDTs). Finite element simulations were performed to understand the mechanisms of the SAW device as well as reveal the acoustic pressure field and electric potential field generated by the device. In addition to numerical studies, proof-of-concept experiments were performed by using a fabricated SAW device for patterning both silicon dioxide (SiO2) nanoparticles and multi-walled carbon nanotubes (MWCNTs) in a hydrogel solution.

Acoustic-assisted 3D printing based on acoustofluidic microparticles patterning for conductive polymer composites fabrication

Surface-acoustic-wave (SAW) aerosol generators are very promising systems for applications requiring small aerosol sources and adjustable droplet-size distributions at low aerosol flowrates. However, the droplet-generation mechanism by SAW is yet unclear and dedicated experiments are necessary to gain further insights. In this work, the atomization zone is investigated experimentally to gain more insight into the droplet-generation mechanism by SAW. Three regions are observed in the atomization zone showing different acoustofluidic effects, including the formation of liquid films and patterns in a standing SAW (SSAW) wavefield and droplet generation. In addition, the influence of the humidity of surrounding air on the measured droplet-size distribution of the aerosols generated by SSAW is investigated. Therefore, the air humidity is adjusted at constant air temperature in the sophisticated wind-tunnel Turbulent Leipzig Aerosol Cloud Interaction Simulator combined with an optical particle spectrometer with resolution down to the sub-\textmu{}m scale. Besides the humidity influence on the droplet size in the measurement region, water-saturated air allowed us to estimate the initial droplet-size distribution at the atomization zone on the chip surface. This parameter is crucial for the determination of the underlying liquid-film disintegration mechanism, i.e., the droplets' origin. Hereby, a bimodal log-normal droplet-size distribution is obtained. The mean diameter of the main droplet fraction decreases from 7.2 to 5.3 \textmu{}m with increasing relative humidity from 8.6 to 97%, apparently due to evaporation of smaller droplets in unsaturated air. A second peak corresponds to a mean diameter as small as 600 nm at high humidity conditions, which is likely to correspond to a droplet-generation mechanism not reported so far and extremely difficult to measure or visualize with conventional techniques. The droplet-size measurement results from the aerosol spectrometer are compared and validated in respect to existing droplet-generation models with the results of a phase-Doppler anemometry, a high-resolution, nonintrusive local aerosol-measurement technique.

Microfluidic technology has great advantages in the precise manipulation of micro and nano particles, and the collection method of micro and nano particles based on ultrasonic standing waves has attracted much attention for its high efficiency and simplicity of structure. This article proposes a two-stage particle separation channel using ultrasound. In the microfluidic channel, two different sound pressure regions are used to achieve the separation of particles with positive acoustic contrast factors. Through numerical simulation, the performance of three common piezoelectric substrate materials was compared qualitatively and quantitatively, and it was found that the output sound pressure intensity of 128°YX-LiNbO3 was high and the output was stable. At the same time, the influence of the number of electrode pairs of the interdigital transducer and the electrode voltage on the output sound wave is studied. Finally, 15 pairs of electrode pairs are selected, and the electrode voltages of the two sound pressure regions are 2.0 V and 3.0 V, respectively. After selecting the corresponding parameters, the separation process was numerically simulated, and the separation of three kinds of particles was successfully achieved. This work has laid a certain theoretical foundation for rapid disease diagnosis and real-time monitoring of the environment in practical applications.

A microfluidicdevice was developed to separate heterogeneous particleor cell mixtures in a continuous flow using acoustophoresis. In thisdevice, two identical surface acoustic waves (SAWs) generated by interdigitaltransducers (IDTs) propagated toward a microchannel, which accordinglybuilt up a standing surface acoustic wave (SSAW) field across thechannel. A numerical model, coupling a piezoelectric effect in thesolid substrate and acoustic pressure in the fluid, was developedto provide a better understanding of SSAW-based particle manipulation.It was found that the pressure nodes across the channel were individualplanes perpendicular to the solid substrate. In the separation experiments,two side sheath flows hydrodynamically focused the injected particleor cell mixtures into a very narrow stream along the centerline. Particlesflowing through the SSAW field experienced an acoustic radiation forcethat highly depends on the particle properties. As a result, dissimilarparticles or cells were laterally attracted toward the pressure nodesat different magnitudes, and were eventually switched to differentoutlets. Two types of fluorescent microspheres with different sizeswere successfully separated using the developed device. In addition, Escherichia coli bacteria premixed in peripheral blood mononuclearcells (PBMCs) were also efficiently isolated using the SSAW-base separationtechnique. Flow cytometric analysis on the collected samples foundthat the purity of separated E. coli bacteria was95.65%.

With the development of in vitro diagnostics, extracting submicron scale particles from mixed body fluids samples is crucial. In recent years, microfluidic separation has attracted much attention due to its high efficiency, label-free, and inexpensive nature. Among the microfluidic-based separation, the separation based on ultrasonic standing waves has gradually become a powerful tool. A microfluid environment containing a tilted-angle ultrasonic standing surface acoustic wave (taSSAW) field has been widely adapted and designed to separate submicron particles for biochemical applications. This paper investigated submicron particle defection in microfluidics using taSSAWs analytically. Particles with 0.1–1 μm diameters were analyzed under acoustic pressure, flow rate, tilted angle, and SSAW frequency. According to different acoustic radiation forces acting on the particles, the motion of large-diameter particles was more likely to deflect to the direction of the nodal lines. Decreasing the input flow rate or increasing acoustic pressure and acoustic wave frequency can improve particle deflection. The tilted angle can be optimized by analyzing the simulation results. Based on the simulation analysis, we experimentally showed the separation of polystyrene microspheres (100 nm) from the mixed particles and exosomes (30–150 nm) from human plasma. This research results can provide a certain reference for the practical design of bioparticle separation utilizing acoustofluidic devices.

Emerging classes of soft robotics with flexible actuation, intelligent sensibility, and biomimetic functionality are driving significant advances in academic researches and commercial applications. Such new generation robotics relies heavily on important breakthroughs in soft matter engineering and flexible actuation systems. Unlike conventional rigid robots, soft robots generally have unique sporting styles and manufacturing strategies. Recently, a series of newly developed soft materials—gallium‐based liquid metals (LMs) are found to display dramatical roles in making soft machines and robotics with their unusual properties in soft robotic actuation and self‐driven field. With superior merits of both high stretchability and electroconductivity over conventional soft materials, LM is increasingly innovated as flexible sensors and actuators in constructing intelligent soft robots. Typical advances in LM enabled soft machines and robotics are summarized and interpreted, herein, with special focus on the driving principles of the LM objects, the preparation methods of LM flexible electronics for making soft robots, and the applications of LM in biomedicines and intelligent robots, together with the consideration of key challenges facing LM‐based soft robotics and its future prospects.

In modeling the acoustic properties of microfluidics driven by surface acoustic waves (SAW), polydimethylsiloxane (PDMS) is usually used as the microchannel material. In this paper, based on a standing surface acoustic wave (SSAW) driven acoutstophoresis chip, the physical field and particle motion behavior in the microchannel are studied through numerical simulation. The major work includes: (a) the distribution of the acoustic pressure field in the microchannel; (b) the dominant behavior change of the drag force and the acoustic radiation force in the acoustic pressure field, which causes the trajectory change of the polystyrene particles. Based on the lossy-wall boundary conditions, it is shown that the model can simulate the acoustic flow and particle movement caused by leaky SAW in the microchannel. The obtained findings can also provide some suggestions for SAW device design and optimization.

Influences of microparticle radius and microchannel height on SSAW-based acoustophoretic aggregation

With advances in new materials and technologies, there has been increasing research focused on flexible sensors. However, in most flexible pressure sensors made using new materials, it is challenging to achieve high detection sensitivity across a wide pressure range. Although traditional silicon-based sensors have good performance, they are not formable and, because of their rigidity and brittleness, they are not suitable for fitting with soft human skin, which limits their application in wearable devices to collect various signals. Silicon nanomembranes are ultra-thin, flexible materials with excellent piezoresistive properties, and they can be applied in various fields, such as in soft robots and flexible devices. In this study, we developed a flexible pressure sensor based on the use of silicon nanomembranes (with a thickness of only 340 nm) as piezoresistive units, which were transferred onto a flexible polydimethylsiloxane (PDMS) substrate. The flexible pressure sensor operated normally in the range of 0-200 kPa, and the sensitivity of the sensor reached 0.0185 kPa-1 in the low-pressure range of 0-5 kPa. In the high-pressure range of 5-200 kPa, the sensitivity of the sensor was maintained at 0.0023 kPa-1. The proposed sensor exhibited a fast response and excellent long-term stability and could recognize human movements, such as the bending of fingers and wrist joints, while maintaining a stable output. Thus, the developed flexible pressure sensor has promising applications in body monitoring and wearable devices.

Since its beginnings in the 1960s, soft robotics has been a steadily growing field that has enjoyed recent growth with the advent of rapid prototyping and the provision of new flexible materials. These two innovations have enabled the development of fully flexible and untethered soft robotic systems. The integration of novel sensors enabled by new manufacturing processes and materials shows promise for enabling the production of soft systems with ‘embodied intelligence’. Here, four experts present their perspectives for the future of the field of soft robotics based on these past innovations. Their focus is on finding answers to the questions of: how to manufacture soft robots, and on how soft robots can sense, move, and think. We highlight industrial production techniques, which are unused to date for manufacturing soft robots. They discuss how novel tactile sensors for soft robots could be created to enable better interaction of the soft robot with the environment. In conclusion this article highlights how embodied intelligence in soft robots could be used to make soft robots think and to make systems that can compute, autonomously, from sensory inputs.

The reduction the operation voltage has been the key challenge to realize of dielectric elastomer actuators (DEA) for many years - especially for the application fields of robotics, lens systems, haptics and future medical implants. Contrary to the approach of manipulating the dielectric properties of the electrically activated polymer (EAP), we intend to realize low-voltage operation by reducing the polymer thickness to the range of a few hundred nanometers. A study recently published presents molecular beam deposition to reliably grow nanometer-thick polydimethylsiloxane (PDMS) films. The curing of PDMS is realized using ultraviolet (UV) radiation with wavelengths from 180 to 400 nm radicalizing the functional side and end groups. The understanding of the mechanical properties of sub-micrometer-thin PDMS films is crucial to optimize DEAs actuation efficiency. The elastic modulus of UV-cured spin-coated films is measured by nano-indentation using an atomic force microscope (AFM) according to the Hertzian contact mechanics model. These investigations show a reduced elastic modulus with increased indentation depth. A model with a skin-like SiO 2 surface with corresponding elastic modulus of (2.29 ± 0.31) MPa and a bulk modulus of cross-linked PDMS with corresponding elastic modulus of (87 ± 7) kPa is proposed. The surface morphology is observed with AFM and 3D laser microscopy. Wrinkled surface microstructures on UV-cured PDMS films occur for film thicknesses above (510 ± 30) nm with an UV-irradiation density of 7.2 10 -4 J cm -2 nm -1 at a wavelength of 190 nm.

Soft materials are widely used in tissue engineering, soft robots, wearable electronics, etc. However, it remains a challenge to fabricate soft materials, such as hydrogels, with both high strength and toughness that are comparable to biological tissues. Inspired by the anisotropic structure of biological tissues, a novel solvent-exchange-assisted wet-stretching strategy is proposed to prepare anisotropic polyvinyl alcohol (PVA) hydrogels by tuning the macromolecular chain movement and optimizing the polymer network. The reinforcing and toughening mechanisms are found to be "macromolecule crystallization and nanofibril formation". These hydrogels exhibit excellent mechanical properties, such as extremely high fracture stress (12.8 ± 0.7 MPa) and fracture strain (1719 ± 77%), excellent modulus (4.51 ± 0.76 MPa), high work of fracture (134.47 ± 9.29 MJ m-3), and fracture toughness (305.04 kJ m-2) compared with other strong hydrogels and even natural tendons. In addition, excellent conductivity, strain sensing capability, water retention, freezing resistance, swelling resistance, and biocompatibility can also be achieved. This work provides a new and effective method to fabricate multifunctional anisotropic hydrogels with high tunable strength and toughness with potential applications in the fields of regenerative medicine, flexible sensors, and soft robotics.

Soft robots actuate themselves and their world through induced pressure and strain, and can often sense these quantities as well. We hypothesize that coordination in a tightly coupled collective of soft robots can be achieved with purely proprioceptive sensing and no direct communication. In this paper, we target a platform of soft pneumatic modules capable of sensing strain on their perimeter, with the goal of using only the robots' own soft actuators and sensors as a medium for distributed coordination. However, methods for modelling, sensing, and controlling strain in such soft robot collectives are not well understood. To address this challenge, we introduce and validate a computationally efficient spring-based model for two-dimensional sheets of soft pneumatic robots. We then translate a classical consensus algorithm to use only proprioceptive data, test in simulation, and show that due to the physical coupling between robots we can achieve consensus-like coordination. We discuss the unique challenges of strain sensors and next steps to bringing these findings to hardware. These findings have promising potential for smart materials and large-scale collectives, because they omit the need for additional communication infrastructure to support coordination.

Flexible and wearable strain sensors for human-computer interaction, health monitoring, and soft robotics have drawn widespread attention to promising applications in the next generation of artificial intelligence devices. However, conventional semiconductor sensors are difficult to meet the requirements of flexibility and stretchability. Here, we reported a kind of novel and simple sensor based on layer-by-layer (LBL) method. Carbon nanotubes (CNTs) layer provides high ductility and stability in the process of tension sensing, while silver layer provides low initial resistance and fast reflecting in the process of tension sensing. LBL method ensures the uniformity of the conductive layer. The sensor has superior sheet resistance of 9.44 Ω/sq., high elongation at break of 104%. For sensing capability, the sensor has wide reflecting range of 60%, high gauge factor (GF) of 1000 up to 60% strain, fast reflecting time of 165 ms. Excellent reliability and stability have also been verified. It is also worth mentioning that the entire process does not require any expensive equipments, complicated processes or harsh experimental conditions. The above features provide an idea for large-scale application of flexible stretchable sensors.