Hydrogel Microactuators Research Articles

Kinetics of Light-Responsive CNT / PNIPAM Hydrogel Microactuators.

Light-responsive microactuators composed of vertically aligned carbon nanotube (CNT) forests mixed with poly(N-isopropylacrylamide) (PNIPAM) hydrogel composites are studied. The benefit of this composite is that CNTs act as a black absorber to efficiently capture radiative heating and trigger PNIPAM contraction. In addition, CNT forests can be patterned accurately using lithography to span structures ranging from a few micrometers to several millimeters in size, and these CNT-PNIPAM composites can achieve response times as fast as 15ms. The kinetics of these microactuators are investigated through detailed analysis of high-speed videos. These are compared to a theoretical model for the deswelling dynamics, which combines thermal convection and polymer diffusion, and shows that polymer diffusion is the rate-limiting factor in this system. Applications of such CNT/hydrogel actuators as microswimmers are discussed, with light-actuating micro-jellyfish designs exemplified, and >1500 cycles demonstrated.

Dual‐Stimuli Cooperative Responsive Hydrogel Microactuators Via Two‐Photon Lithography

With the development of bionics as well as materials science, intelligent soft actuators have shown promising applications in many fields such as soft robotics, sensing, and remote manipulation. Microfabrication technologies have enabled the reduction of the size of responsive soft actuators to the micron level. However, it is still challenging to construct microscale actuators capable of responding to different external stimuli in complex and diverse conditions. Here, this work demonstrates a dual-stimuli cooperative responsive hydrogel microactuator by asymmetric fabrication via femtosecond laser direct writing. The dual response of the hydrogel microstructure is achieved by employing responsive hydrogel with functional monomer 2-(dimethylamino)ethyl methacrylate. Raman spectra of the hydrogel microstructures suggest that the pH and temperature response of the hydrogel is generated by the changes in tertiary amine groups and hydrogen bonds, respectively. The asymmetric hydrogel microstructures show opposite bending direction when being heated to high temperature or exposed to acid solution, and can independently accomplish the grasp of polystyrene microspheres. Moreover, this work depicts the cooperative response of the hydrogel microactuator to pH and temperature at the same time. The dual-stimuli cooperative responsive hydrogel microactuators will provide a strategy for designing and fabricating controllable microscale actuators with promising applications in microrobotics and microfluidics.

Flytrap Inspired pH‐Driven 3D Hydrogel Actuator by Femtosecond Laser Microfabrication

AbstractWith the development of bionics and nanophotonics, hydrogel microactuators capable of responding to external stimuli to produce controllable deformations have attracted a great deal of interest. These microactuators hold significant promise in areas such as bionic devices, soft robotics, and precision sensors. It is not a trivial task to make such small devices as well as to make them work in a controlled manner. Here, inspired by the intelligent response of flytrap, a smart hydrogel microactuator based on a bionic asymmetric structure is demonstrated. The designed asymmetric microstructure is fabricated by femtosecond laser direct writing with deformation time of 1.2 s and recovery time of 0.3 s. The grasping and releasing behavior of the microactuator for micro‐objects can be realized and tuned by using pH‐triggered shape changes, demonstrating its potential for applications, such as flexible robotics, smart sensors, and microscopic manipulation.

Light-Driven Hydrogel Microactuators for On-Chip Cell Manipulations

We present a driving method for on-chip hydrogel microactuators developed to be used for single-cell manipulations. The hydrogel actuator is composed of a temperatureresponsive gel coated on a glass substrate with light-absorbing micropatterning. The actuator is driven by light irradiation, which increases its temperature. The advantage of this driving method is twofold: allowing local heating of the actuator while decreasing heat dissipation to the substrate and its environment. The local heating induced by light irradiation improves the response characteristics of the hydrogel microactuator because the slight increase in environmental temperature aids cooling of the microactuator when the light is turned off. Furthermore, local heating enables multiple actuators to be located in close proximity without unintentionally actuating neighboring actuators through heat dissipation. In addition, the heating induced by the light irradiation does not require wiring on the chip. Therefore, the developed driving method enables the integration of a large number of actuators that can be independently driven on a single chip.

Light-driven micron-scale 3D hydrogel actuator produced by two-photon polymerization microfabrication

Light-driven micro/nano-actuators are one of the most important topics in the biomedical micro-electromechanical systems (MEMS) field. Currently, their development is hampered due to the difficulties in designing and fabricating biocompatible light-driven microactuators with the dimensions less than one hundred micrometres and a response time in the order of seconds. In this work, gel photoresists were prepared by embedding photothermal surface-modified Fe3O4 nanoparticles (NPs) into a mixture that included a photoinitiator, photosensitizer, monomers, crosslinkers and solvents. Macroscopic poly(N-isopropylacrylamide) (PNIPAM)/nano-Fe3O4 hydrogels were prepared by ultraviolet photopolymerization of gel photoresists, which showed good temperature-responsive volume changes and light-triggered bending deformation. Then the two-photon polymerization (TPP) microfabrication properties of gel photoresists with 0, 0.48 and 0.95 wt% Fe3O4 NPs were investigated in detail. Importantly, after the TPP microfabrication and subsequent solvent-exchange procedure, a double-armed near-infrared (NIR)-light-driven three-dimensional (3D) hydrogel microcantilever with a size of ∼26 μm was successfully fabricated. The hydrogel microactuator had a fast response time of ∼0.033 s in water under NIR radiation and showed good reversibility. Furthermore, the distance between the two arms of the hydrogel microcantilever could be manipulated by controlling the laser focus and incident laser power.

Local electrochemical control of hydrogel microactuators in microfluidics

Due to the complexity of a chemo-electro-mechanical system and the need for a wet environment, to date, few devices fully integrate hydrogels with microelectromechanical systems. In this paper, we demonstrate the use of inkjet-printed gold electrodes integrated in microfluidic channels to alter the morphology of electroactive polymer hydrogels, cross-linked in situ. Microfluidics is a convenient platform for integrating hydrogels with microsystems as it provides a means for encapsulating electrolytic environments, while maintaining UV transparency. Printed electronics provide a new method for rapid prototyping of electrodes on flexible substrates for electrical control of electroactive polymer microsystems. We attribute the observed actuation to electrochemically-induced pH variations in the vicinity of the printed anode and cathode which diffuse into the hydrogel. Response to pH was verified by exposing the hydrogel to various pH conditions in control experiments without applied electrical bias. This work demonstrates a new, integrated, polymer-based, rapid prototyping approach to building flexible electroactive hydrogel systems which can benefit microfluidic valves, biomimetics, electrochemical sensors and artificial muscles.

Rapid actuation of hydrogel micro-actuator with porous structures

Rapid actuation of hydrogel micro-actuator with porous structures

Spring-shaped stimuli-responsive hydrogel microactuator with large deformation

Spring-shaped stimuli-responsive hydrogel microactuator with large deformation

AC frequency-based electrical stimulation of hydrogel microactuators employing Parylene-N coated electrodes

This article demonstrates the first AC-frequency based electrical actuation of hydrogels employing coplanar microelectrodes coated with Parylene-N. For the first time hydrogel microactuators were operated at applied voltages up to 40Vpk-pk without any electrolyte electrolysis or external pumping equipment. An RC circuit model was developed to determine the characteristic frequency required to overcome electrostatic shielding, as the hydrogels were actuated in the polarizable potassium hydroxide (KOH). The characteristic actuation frequency was found to be between 10kHz and 1GHz, with the minimum frequency at an electrolyte concentration of 1mM and Parylene-N thickness of 100nm. This circuit model was also extended to calculate the total capacitance of the hydrogel-based system. To validate the circuit model, the total capacitance of the system was experimentally measured using capacitance and admittance spectroscopy, for Parylene-N thicknesses from 2145±55nm to 565±7nm and from 1mM to 10mM KOH. Good agreement was found at higher frequencies, while at lower frequencies the system demonstrated frequency and concentration dependent behavior. The spectroscopy results were used to calculate the apparent power of the system based off the maximum electric potential of 40Vpk-pk applied during actuation, with an absolute minimum and maximum of ~2×10−4VA and ~2×10−2VA, respectively. To determine actuation dynamics systems were fabricated with Parylene-N thicknesses from 907±28nm to 348±13nm and equilibrated with 1mM or 5mM KOH, providing a system above and a system below the predicted characteristic frequency, respectively. The 1mM KOH system displayed true strains from 18% to 30% with response times from 14.7s to 4.7s, with optimum response at applied electric fields of 16kV/m or 40Vpk-pk with an 80% duty cycle. The 5mM KOH system had a maximum true strain of ~7% and response time of 14.9s. The trend observed while increasing frequency was also observed when a 1mM KOH sample was subjected to increasing applied electric potential, which showed a 674% increase in true strain and a 703% decrease in response time, from 15Vpk-pk to 40Vpk-pk. All systems actuated displayed deformation at all frequencies tested; thus even a minimal frequency can disturb electrostatic shielding, but above the characteristic frequency deformation and response times were optimum. This work overcomes the previous operational challenges associated with electrical hydrogel actuation, and investigates the electrical and actuation characteristics of the hydrogel-based system. Moreover, as hydrogels operated at lower frequencies still displayed actuation, hydrogel actuation could occur at low power zones enabling its integration within ultra-low power portable systems.

Photo-electric biotransducer for activating ionic hydrogel microactuators

An ionic hydrogel microactuator driven by an organic photo-electric transducer is described in this paper. When exposed to visible light, the ultrathin layer of photosensitive bacteriorhodopsin (bR) purple membrane patches adsorbed on a biofunctionalized porous anodic alumina (PAA) gold coated surface acts as an array of proton pumps that transport H + ions from the acidic gel to a separate ionic solution. The movement of ions across the porous membrane creates a pH gradient that causes the hydroxyethyl methacrylate–acrylic acid (HEMA–AA) gel to swell. Experimental studies show that a 13 nm self-assembled photoelectric layer can generate approximately 1.3 mV/(mW cm 2 ) when exposed to an 18 mW, 568 nm light source. The photo-voltage produced by the monolayer is sufficient to change the pH of the surrounding ionic solution with 0.42. The small change in pH around the phase transition point p K a enabled the hydrogel microactuator to swell by more than 80% of its original volume in less than 85 min. The expansion increased the diameter and length of the gel microstructure by 23% and 22%, respectively.

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

In this article a new method for the photolithographical deposition of temperature-sensitive hydrogels is presented. The method can be used in conjunction with standard 365 nm UV-photolithography to accurately dimension and position temperature-sensitive hydrogel microactuators in a highly parallel fashion. A number of characteristics of the hydrogels were investigated. These include: the photolithographical reproduction quality, the effect of the crosslinking density in the hydrogels on their swelling behavior, the swelling hysteresis behavior, the effect of dimensional constraints on the swelling of the hydrogels and the effect of copolymerization with an ionizable comonomer on the temperature behavior of the hydrogels. The method presents a considerable improvement in the microfabrication of temperature-sensitive hydrogel microactuators and has potential for the mass-fabrication of these interesting microactuators.