Bio‐inspired Design of Submerged Hydrogel‐Actuated Polymer Microstructures Operating in Response to pH

Abstract

Responsive and reversibly actuating surfaces have attracted significant attention recently due to their promising applications as dynamic materials [1] that may enable microfluidic mixing, [2] particle propulsion and fluid transport, [3] capture and release systems, [4] and antifouling. [5] Analogs in nature serve as inspiration for the design of such advanced adaptive materials systems⎯microorganisms use flagella for propulsion, [6] cilia line the human respiratory tract to sweep mucus from the lungs and prevent bacterial accumulation, [7] and echinoderms use pedicellariae for body cleaning and food capture. [8] Significant characteristics of these biological systems include functionality in a fluidic environment, controllable actuation direction or pattern, and the ability to translate chemical signals or stimulus into mechanical motion. Researchers have taken various approaches to fabricating biomimetic actuators, among which are biomorph actuators made using Micro Electromechanical Systems (MEMS) technology, [9] magnetically actuated poly(dimethylsiloxane) (PDMS) structures, [10] and artificial cilia or actuators made from responsive gel. [11,12] However, most fabricated actuators, such as MEMS or magneticallyactuated PDMS posts, must be driven by an external force or field and are not responsive to chemical stimuli. Actuating structures that have been made from responsive hydrogel are Submitted to 2 either low aspect ratio and their motion is not patternable, [11] or the movement is irreversible. [12] Microscale actuation systems which exhibit reversible chemo-mechanical response and control of actuation direction or pattern have proven difficult to achieve.Inspired by biological actuators, which can be broadly interpreted as composites consisting of an active "muscle" component coupled with a passive "bone" structure, we recently developed a hybrid actuation system in which a crosslinked acrylamide-based hydrogel, acting as an analog to muscle, drives the movement of embedded silicon [13,14] or polymer [15] microposts, serving as analogs to skeletal elements. Actuation was achieved by the swelling and contraction of the humidity-responsive gel upon hydration/drying. Actuation direction was controlled by modulating the hydrogel thickness; since thicker hydrogel exhibits a greater absolute change in volume than thin hydrogel, the forces exerted on the structures⎯and thus their actuation direction⎯can be patterned.However, a humidity responsive system will not function in the fluidic environment required for many applications such as propulsion, cargo transport, or microfluidics, and it is difficult to pattern uni-directional actuation of posts over large areas due to their lack of preferred bending direction. To enable the actuators to function in various environments, it is possible to alter the chemistry of the hydrogel muscle to make it shrink or swell while submerged in response to a range of stimuli including light, [16] temperature, [17] biomolecules (e.g. glucose), [18] or pH. [19] To gain additional ...

Stimulus-active polymer actuators for next-generation microfluidic devices

“…The previously discussed cilia-inspired actuation scheme has also emerged in connection with hydrogels. In [111], the swelling and shrinkage behavior of hydrogel micropost and microfins was utilized to mimic cilia-based fluid transport in microfluidic structures in response to pH value changes. In their work, they used electrochemically generated pH gradients to visualize the hydrogel volumephase transition and how it translates into movement of the microstructures.…”

Stimuli-Responsive Chemomechanical Actuation: A Hybrid Materials Approach

“…In contrast, asymmetric fin structures possessing a preferred bending orientation in accordance with their geometry generate large areas (cm 2 at least) of highly uniform directional actuation ( Figure 2c). 26 These surfaces can be used to modulate transparency or reflectivity in response to changes in the external environment, such as humidity or temperature, which could be useful for "smart" windows; if the fin structures are coated on one side by shadow evaporation of metal, then when the fins stand upright the surface is highly transparent, but when the gel contracts and the fins bend over, the surface becomes reflective and opaque. 33 The stiffness of the structures relative to the force that can be exerted by the gel dictates whether or not actuation can take place.…”

Effects of Silica Content on the Properties of Poly(N-isopropylacrylamide) / Silica Nanocomposite Hydrogels

“…Due to its particular feedback system [1], that hydrogel can show significant changes in shapes and volumes as a response to variation of external stimuli such as temperature [2], pH [3] and electric field [4], hydrogels have gained its popular application in numerous fields such as biomaterials [5] and sensing applications [6]. Poly(N-isopropylacrylamide)( PNIPA) is a representative thermosensitive polymer with a lower critical solution temperature(LCST) around 34 o C [7], a specific temperature at which the polymer shows phase separation.…”