An efficient method for the fabrication of temperature-sensitive hydrogel microactuators

Engineering Smart Composite Hydrogels for Wearable Disease Monitoring

Growing health awareness triggers the public's concern about health problems. People want a timely and comprehensive picture of their condition without frequent trips to the hospital for costly and cumbersome general check-ups. The wearable technique provides a continuous measurement method for health monitoring by tracking a person's physiological data and analyzing it locally or remotely. During the health monitoring process, different kinds of sensors convert physiological signals into electrical or optical signals that can be recorded and transmitted, consequently playing a crucial role in wearable techniques. Wearable application scenarios usually require sensors to possess excellent flexibility and stretchability. Thus, designing flexible and stretchable sensors with reliable performance is the key to wearable technology. Smart composite hydrogels, which have tunable electrical properties, mechanical properties, biocompatibility, and multi-stimulus sensitivity, are one of the best sensitive materials for wearable health monitoring. This review summarizes the common synthetic and performance optimization strategies of smart composite hydrogels and focuses on the current application of smart composite hydrogels in the field of wearable health monitoring.

Linear and nonlinear viscoelasticity of concentrated thermoresponsive microgel suspensions

We present an integrated experimental and theoretical study of the dynamics and rheology of self-crosslinked, slightly charged, temperature responsive soft poly(N-isopropylacrylamide) (pNIPAM) microgels over a wide range of concentration and temperature spanning the sharp change in particle size and intermolecular interactions across the lower critical solution temperature (LCST). Dramatic, non-monotonic changes in viscoelasticity are observed as a function of temperature, with distinct concentration dependence in the dense fluid, glassy, and soft-jammed regimes. Motivated by our experimental observations, we formulate a minimalistic model for the size dependence of a single microgel particle and the change of the interparticle interaction from purely repulsive to attractive upon heating. Using microscopic equilibrium and time-dependent statistical mechanical theories, theoretical predictions are quantitatively compared with experimental measurements of the shear modulus. Good agreement is found for the nonmonotonic temperature behavior that originates as a consequence of the competition between reduced microgel packing fraction and increasing interparticle attractions. Testable predictions are made for nonlinear rheological properties such as the yield stress and strain. To our knowledge, this is the first attempt to quantitatively understand in a unified manner the viscoelasticity of dense, temperature-responsive microgel suspensions spanning a wide range of temperatures and concentrations.

Electrothermally Driven Hydrogel-on-Flex-Circuit Actuator for Smart Steerable Catheters

This paper reports an active catheter-tip device functionalized by integrating a temperature-responsive smart polymer onto a microfabricated flexible heater strip, targeting at enabling the controlled steering of catheters through complex vascular networks. A bimorph-like strip structure is enabled by photo-polymerizing a layer of poly(N-isopropylacrylamide) hydrogel (PNIPAM), on top of a 20 × 3.5 mm² flexible polyimide film that embeds a micropatterned heater fabricated using a low-cost flex-circuit manufacturing process. The heater activation stimulates the PNIPAM layer to shrink and bend the tip structure. The bending angle is shown to be adjustable with the amount of power fed to the device, proving the device’s feasibility to provide the integrated catheter with a controlled steering ability for a wide range of navigation angles. The powered device exhibits uniform heat distribution across the entire PNIPAM layer, with a temperature variation of <2 °C. The operation of fabricated prototypes assembled on commercial catheter tubes demonstrates their bending angles of up to 200°, significantly larger than those reported with other smart-material-based steerable catheters. The temporal responses and bending forces of their actuations are also characterized to reveal consistent and reproducible behaviors. This proof-of-concept study verifies the promising features of the prototyped approach to the targeted application area.

Requirement and Development of Hydrogel Micromotors towards Biomedical Applications

With controllable size, biocompatibility, porosity, injectability, responsivity, diffusion time, reaction, separation, permeation, and release of molecular species, hydrogel microparticles achieve multiple advantages over bulk hydrogels for specific biomedical procedures. Moreover, so far studies mostly concentrate on local responses of hydrogels to chemical and/or external stimuli, which significantly limit the scope of their applications. Tetherless micromotors are autonomous microdevices capable of converting local chemical energy or the energy of external fields into motive forces for self-propelled or externally powered/controlled motion. If hydrogels can be integrated with micromotors, their applicability can be significantly extended and can lead to fully controllable responsive chemomechanical biomicromachines. However, to achieve these challenging goals, biocompatibility, biodegradability, and motive mechanisms of hydrogel micromotors need to be simultaneously integrated. This review summarizes recent achievements in the field of micromotors and hydrogels and proposes next steps required for the development of hydrogel micromotors, which become increasingly important for in vivo and in vitro bioapplications.

Requirement and Development of Hydrogel Micromotors towards Biomedical Applications

With controllable size, biocompatibility, porosity, injectability, responsivity, diffusion time, reaction, separation, permeation, and release of molecular species, hydrogel microparticles achieve multiple advantages over bulk hydrogels for specific biomedical procedures. Moreover, so far studies mostly concentrate on local responses of hydrogels to chemical and/or external stimuli, which significantly limit the scope of their applications. Tetherless micromotors are autonomous microdevices capable of converting local chemical energy or the energy of external fields into motive forces for self-propelled or externally powered/controlled motion. If hydrogels can be integrated with micromotors, their applicability can be significantly extended and can lead to fully controllable responsive chemomechanical biomicromachines. However, to achieve these challenging goals, biocompatibility, biodegradability, and motive mechanisms of hydrogel micromotors need to be simultaneously integrated. This review summarizes recent achievements in the field of micromotors and hydrogels and proposes next steps required for the development of hydrogel micromotors, which become increasingly important for in vivo and in vitro bioapplications.

Toll-Like Receptors and Relevant Emerging Therapeutics with Reference to Delivery Methods

The built-in innate immunity in the human body combats various diseases and their causative agents. One of the components of this system is Toll-like receptors (TLRs), which recognize structurally conserved molecules derived from microbes and/or endogenous molecules. Nonetheless, under certain conditions, these TLRs become hypofunctional or hyperfunctional, thus leading to a disease-like condition because their normal activity is compromised. In this regard, various small-molecule drugs and recombinant therapeutic proteins have been developed to treat the relevant diseases, such as rheumatoid arthritis, psoriatic arthritis, Crohn’s disease, systemic lupus erythematosus, and allergy. Some drugs for these diseases have been clinically approved; however, their efficacy can be enhanced by conventional or targeted drug delivery systems. Certain delivery vehicles such as liposomes, hydrogels, nanoparticles, dendrimers, or cyclodextrins can be employed to enhance the targeted drug delivery. This review summarizes the TLR signaling pathway, associated diseases and their treatments, and the ways to efficiently deliver the drugs to a target site.

Micro 3D Printing of a Temperature-Responsive Hydrogel Using Projection Micro-Stereolithography

Stimuli-responsive hydrogels exhibiting physical or chemical changes in response to environmental conditions have attracted growing attention for the past few decades. Poly(N-isopropylacrylamide) (PNIPAAm), a temperature responsive hydrogel, has been extensively studied in various fields of science and engineering. However, manufacturing of PNIPAAm has been heavily relying on conventional methods such as molding and lithography techniques that are inherently limited to a two-dimensional (2D) space. Here we report the three-dimensional (3D) printing of PNIPAAm using a high-resolution digital additive manufacturing technique, projection micro-stereolithography (PμSL). Control of the temperature dependent deformation of 3D printed PNIPAAm is achieved by controlling manufacturing process parameters as well as polymer resin composition. Also demonstrated is a sequential deformation of a 3D printed PNIPAAm structure by selective incorporation of ionic monomer that shifts the swelling transition temperature of PNIPAAm. This fast, high resolution, and scalable 3D printing method for stimuli-responsive hydrogels may enable many new applications in diverse areas, including flexible sensors and actuators, bio-medical devices, and tissue engineering.

Durable and water-floatable ionic polymer actuator with hydrophobic and asymmetrically laser-scribed reduced graphene oxide paper electrodes

Ionic polymer actuators driven by electrical stimuli have been widely investigated for use in practical applications such as bioinspired robots, sensors, and biomedical devices. However, conventional ionic polymer-metal composite actuators have a serious drawback of poor durability under long-term actuation in open air, mainly because of the leakage of the inner electrolyte and hydrated cations through cracks in the metallic electrodes. Here, we developed a highly durable and water-floatable ionic polymer artificial muscle by employing hydrophobic and asymmetrically laser-scribed reduced graphene oxide paper electrodes (HLrGOP). The highly conductive, flexible, and cost-effective HLrGOP electrodes have asymmetrically smooth hydrophobic outer and rough inner surfaces, resulting in liquid-impermeable and water-floatable functionalities and strong bonding between an ionic polymer and the electrodes. More interestingly, the HLrGOP electrode, which has a unique functionality to prevent the leakage of the vaporized or liquid electrolyte and mobile ions during electrical stimuli, greatly contributes to an exceptionally durable ionic polymer-graphene composite actuator that is a prerequisite for practical applications in active biomedical devices, biomimetic robots, touch-feedback haptic systems, and flexible soft electronics.

Monodisperse and fast-responsive poly(N-isopropylacrylamide) microgels with open-celled porous structure

A simple and efficient method is developed to fabricate monodisperse and fast-responsive poly(N-isopropylacrylamide) (PNIPAM) microgels with open-celled porous structure. First, numerous fine oil droplets are fabricated by homogeneous emulsification method and are then evenly dispersed inside monodisperse PNIPAM microgels as porogens via the combination of microfluidic emulsification and UV-initiated polymerization methods. Subsequently, the embedded fine oil droplets inside the PNIPAM microgels are squeezed out upon stimuli-induced rapid volume shrinkage of the microgels; as a result, a spongelike open-celled porous structure is formed inside the PNIPAM microgels. The open-celled porous structure provides numerous interconnected free channels for the water transferring convectively inward or outward during the volume phase transition process of PNIPAM microgels; therefore, the response rates of the PNIPAM microgels with open-celled porous structure are much faster than that of the normal ones in both thermo-responsive shrinking and swelling processes. Because of the fast-responsive characteristics, the microgels with open-celled porous structure will provide ever better performances in their myriad applications, such as microsensors, microactuators, microvalves, and so on.

Molecular dynamics and experimental study of conformation change of poly(N-isopropylacrylamide) hydrogels in mixtures of water and methanol

The conformation transition of poly(N-isopropylacrylamide) hydrogel as a function of the methanol mole fraction in water/methanol mixtures is studied both experimentally and by atomistic molecular dynamics simulation with explicit solvents. The composition range in which the conformation transition of the hydrogel occurs is determined experimentally at 268.15, 298.15, and 313.15 K. In these experiments, cononsolvency, i.e., collapse at intermediate methanol concentrations while the hydrogel is swollen in both pure solvents, is observed at 268.15 and 298.15 K. The composition range in which cononsolvency is present does not significantly depend on the amount of cross-linker. The conformation transition of the hydrogel is caused by the conformation transition of the polymer chains of its backbone. Therefore, conformation changes of single backbone polymer chains are studied by massively parallel molecular dynamics simulations. The hydrogel backbone polymer is described with the force field OPLS-AA, water with the SPC/E model, and methanol with the model of the GROMOS-96 force field. During simulation, the mean radius of gyration of the polymer chains is monitored. The conformation of the polymer chains is studied at 268, 298, and 330 K as a function of the methanol mole fraction. Cononsolvency is observed at 268 and 298 K, which is in agreement with the present experiments. The structure of the solvent around the hydrogel backbone polymer is analyzed using H-bond statistics and visualization. It is found that cononsolvency is caused by the fact that the methanol molecules strongly attach to the hydrogel's backbone polymer, mainly with their hydroxyl group. This leads to the effect that the hydrophobic methyl groups of methanol are oriented toward the bulk solvent. The hydrogel+solvent shell hence appears hydrophobic and collapses in water-rich solvents. As more methanol is present in the solvent, the effect disappears again.

Stimuli-responsive microwells for formation and retrieval of cell aggregates

Generating cell aggregates is beneficial for various applications ranging from biotechnology to regenerative therapies. Previously, poly(ethylene glycol) (PEG) microwells have been demonstrated as a potentially useful method for generating controlled-size cell aggregates. In addition to controlling cell aggregate size and homogeneity, the ability to confine cell aggregates on glass adhesive substrates and subsequently retrieve aggregates from microwells for further experimentation and analysis could be beneficial for various applications. However, it is often difficult to retrieve cell aggregates from these microwells without the use of digestive enzymes. This study describes the stable formation of cell aggregates in responsive microwells with adhesive substrates and their further retrieval in a temperature dependent manner by exploiting the stimuli responsiveness of these microwells. The responsive polymer structure of the arrays can be used to thermally regulate the microwell diameters causing a mechanical force on the aggregates, subsequently facilitating the retrieval of cell aggregates from the microwells with high efficiency compared to PEG arrays. This approach can be potentially integrated into high-throughput systems and may become a versatile tool for various applications that require aggregate formation and experimentation, such as tissue engineering, drug discovery, and stem cell biology.

Integrated two-step gene synthesis in a microfluidic device

Herein we present an integrated microfluidic device capable of performing two-step gene synthesis to assemble a pool of oligonucleotides into genes with the desired coding sequence. The device comprised of two polymerase chain reactions (PCRs), temperature-controlled hydrogel valves, electromagnetic micromixer, shuttle micromixer, volume meters, and magnetic beads based solid-phase PCR purification, fabricated using a fast prototyping method without lithography process. The fabricated device is combined with a miniaturized thermal cycler to perform gene synthesis. Oligonucleotides were first assembled into genes by polymerase chain assembly (PCA), and the full-length gene was amplified by a second PCR. The synthesized gene was further separated from the PCR reaction mixture by the solid-phase PCR purification. We have successfully used this device to synthesize a green fluorescent protein fragment (GFPuv) (760 bp), and obtained comparable synthesis yield and error rate with experiments conducted in a PCR tube within a commercial thermal cycler. The resulting error rate determined by DNA sequencing was 1 per 250 bp. To our knowledge, this is the first microfluidic device demonstrating integrated two-step gene synthesis.

Autonomously-triggered microfluidic cooling using thermo-responsive hydrogels

We present autonomously-triggered on-chip microfluidic cooling devices that utilize thermo-responsive hydrogels to adapt to local environmental temperatures. An external rotating magnetic stirrer couples with an in situ fabricated nickel impeller in these centrifugal-based microfluidic cooling devices to recirculate cooler water. Temperature-responsive hydrogels, which exhibit volumetric expansion and contraction, are integrated at the axle of the impeller. In this design, the hydrogels behave similar to an automotive clutch, to autonomously control the impeller's rotation as a function of the local environmental temperature. Therefore, the hydrogels act as both sensors and actuators and help take away the necessity for additional temperature sensing, feedback, and/or control units here. Cooling devices capable of on-chip thermal management at multiple predetermined onset operation points are realized by changes to the composition of hydrogel to alter its lowest critical solution temperature (LCST). Furthermore, the effect of magnetic stirrer frequency on the fluid cooling and flowrates for different two-blade nickel impeller designs are presented.

Hydrogel-based reconfigurable components for microfluidic devices

In situ liquid-phase photopolymerization (LP(3)) has been applied to the field of microfluidics to create components within integrated systems. As an extension of LP(3) technology, we present reconfigurable components that utilize the swelling of hydrogels. These components can be conveniently used to enhance microfluidic functions and applications. In order to utilize the swelling characteristic of hydrogels to the fullest, we demonstrate strategies to increase the swelling performance temporally and spatially. To this end, two successful applications using the reconfigurable components were tested: (1) active walls to block or divert flow at different steps in the fabrication or assay process, and (2) delivery pistons to move objects to specific locations within the microchannels after device fabrication.