A Novel Approach to Prepare Porous Poly(N-isopropylacrylamide) Hydrogel with Superfast Shrinking Kinetics

High-Performance Organohydrogel Artificial Muscle with Compartmentalized Anisotropic Actuation Under Microdomain Confinement

Current hydrogel actuators mostly suffer from weak actuation strength and low responsive speed owing to their solvent diffusion-induced volume change mechanism. Here a skeletal muscle-inspired organohydrogel actuator is reported in which solvents are confined in hydrophobic microdomains. Organohydrogel actuator is driven by compartmentalized directional network deformation instead of volume change, avoiding the limitations that originate from solvent diffusion. Organohydrogel actuator has an actuation frequency of 0.11 Hz, 110 times that of traditional solvent diffusion-driven hydrogel actuators (<10^-3  Hz), and can lift more than 85 times their own weight. This design achieves the combination of high responsive speed, high actuation strength, and large material size, proposing a strategy to fabricate hydrogel actuators comparable with skeletal muscle performance.

Anionically modified N-(alkyl)acrylamide-based semi-IPN hybrid gels reinforced with SiO2 for enhanced on-off switching and responsive properties

Semi-interpenetrated (semi-IPN) poly(N-isopropylacrylamide-co-methacrylic acid)/polyacrylamide P(NIPA-MA)/PAAm hybrid gels containing linear poly(acrylamide) PAAm chains were designed by incorporation of different amounts of silica particles (SiP). Formation of temperature-sensitive semi-IPN hybrid gels was evaluated by simultaneous radical polymerization under different polymerization temperatures, and the effect of polymer/particle interfaces on the swelling and elasticity was explained. Nanoparticle-mediated enhancements were studied to understand the effect of addition of SiP to anionically modified semi-IPNs. Hybrid network formation was confirmed by FTIR, with an increase in SiP resulting in an increased Si-O-Si absorption peak in hybrid samples. P(NIPA-MA)/PAAm/SiP gels showed a reduction in the degree of swelling with the addition of SiP. The Flory-Huggins interaction parameter of the semi-IPN hybrid-solvent was estimated using the extended equation. The compression test results showed an improvement in the stiffness and modulus attributed to stress transfer from the hybrid network to nanoparticles. Swelling processes of semi-IPN hybrids prepared by cold polymerization have anomalous diffusion owing to polymer relaxation, while Fickian behavior was observed for the hybrids obtained by warm polymerization. During oscillation shrinking-swelling of semi-IPN hybrids upon ionic-strength switching in NaCl solutions, the gels retained their shape and integrity for 10 cycles of testing. To evaluate the adsorption characteristics of semi-IPN hybrids, methyl violet (MV) was chosen as a model cationic dye. The effects of contact time, silica content and initial dye concentration were studied and the time-dependent adsorption data were fitted with six kinetic models. The MV uptake capacity of semi-IPN hybrids increased with an increase in the initial MV concentration as well as with silica content. The adsorption process followed pseudo-second order type adsorption kinetics and the mechanism of process was better described by intraparticle diffusion. These results will contribute to the understanding of roles of anionic comonomer and linear polymer doping in nanoparticle-mediated synthesis of hybrid gels and to the development of next-generation materials for pharmaceutical and environmental-based applications.

Superfast and Reversible Thermoresponse of Poly( N-isopropylacrylamide) Hydrogels Grafted on Macroporous Poly(vinyl alcohol) Formaldehyde Sponges

Poly( N-isopropylacrylamide) (PNIPAAm), a typical thermoresponsive polymer, exhibits potential application in smart materials. However, bulk PNIPAAm hydrogel monoliths undergo slow volume phase transition at least tens of minutes to hours as determined by the shape and size of polymers due to the formation of the skin layer. In this regard, novel macroporous sponges with rapid thermoresponse are prepared via grafting polymerization of N-isopropylacrylamide (NIPAAm) onto the macroporous poly(vinyl alcohol) formaldehyde (PVF) network as confirmed by attenuated total reflection-infrared (ATR IR) and ¹H NMR spectra. As prepared PVF- g-PNIPAAm sponges display interconnected open-cell structures, and their average pore sizes and porosities are ∼90 μm and >85%, respectively. The equilibrium swelling ratio of PVF- g-PNIPAAm sponges varies from 11 to 50 with temperature. The volume phase transition temperature is at 30-34 °C, as detected in the DSC curves of swollen samples. These features indicate that the existence of the original PVF network exerts almost no influence on the PNIPAAm temperature responsibility. As prepared samples can reach the swelling equilibrium in less than 80 s, and their rapid swelling kinetics can be fitted using the pseudo-first-order rate kinetic equation. Notably, the samples also display rapid deswelling rate in less than 40 s at relative high temperature (48 °C), thereby indicating a superfast responsive behavior to temperature change. The PVF- g-PNIPAAm sponges exhibit rapid and reversible thermoresponse in repeatable swelling-deswelling cycles, which can satisfy the need of special smart materials. In particular, combined with iodine solution (i.e., PVF- g-PNIPAAm/I₂), these sponges can serve as a novel temperature indicator and exhibit excellent antibacterial performances.

Periodicity-controlled two-dimensional crystalline colloidal arrays

We developed a convenient and fast approach to preparing close-packed two-dimensional (2-D) particle arrays on mercury surfaces. Addition of cosolvents, such as alcohols, to aqueous colloidal particle suspensions induces spreading and self-assembly of the particles into 2-D arrays on top of the mercury surface. We can fabricate large-area close-packed 2-D arrays (>70 cm(2)) within 30 s. We attached these 2-D arrays to functional hydrogel films such that the 2-D array spacings were altered by the hydrogel volume response to the environment. We directly observed the hydrogel volume induced 2-D array spacing changes by using confocal laser scanning microscopy to monitor the spacings of fluorescent polystyrene particle 2-D arrays in response to changes in pH, solvent composition, temperature, etc.

A novel two-level microstructured poly(N-isopropylacrylamide) hydrogel for controlled release

The aim of this work was to demonstrate that conventional poly(N-isopropylacrylamide) (PNIPAAm) hydrogels can improve their shrinkage and release properties solely due to the introduction of a heterogeneous density fluctuation-based microstructure. To this end, a novel structurally engineered PNIPAAm hydrogel was designed and compared with a chemically similar, but homogeneous, PNIPAAm hydrogel reference. For the two-step preparation PNIPAAm microgels were firstly synthesized with surface amine groups and further functionalized with polymerizable acrylate groups. In the second step the microgels, themselves acting as crosslinkers, were crosslinked to form a bulk network by inter-connecting the microgels with linear PNIPAAm chains. Although the chemical composition of the newly prepared hydrogel was generally the same as conventional PNIPAAm hydrogels (a relative control), significantly improved shrinkage properties and a more efficient "on-off" switching induced by temperature modulations were observed for the novel gel as compared with the homogeneous reference. These improved shrinkage properties were ascribed to the novel structure, which is believed to enable rapid shrinking of the small microgel crosslinkers and, thereupon, the generation of a sufficient number of diffusion channels for quick water release. Rhodamine B and ibuprofen (IBU) as model compounds were completely released from this novel gel at 20 degrees C, whereas at temperatures above the lower critical solution temperature release stopped after initial 40% and 70% "bursts" for rhodamine B and IBU, respectively, due to shrinkage of the gel network. This approach may provide an avenue to design temperature-sensitive drug delivery systems with state of the art switching properties and fast release kinetics by combining the here presented innovative strategy with complementary enhancements, such as the introduction of porosity.

Responsive hydrogels with poly(N-isopropylacrylamide-co-acrylic acid) colloidal spheres as building blocks

A novel type of microgel-crosslinked hydrogels (MCG), which are constructed with poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAM-co-AAc)) microgels as building blocks, have been successfully prepared by using functionalized P(NIPAM-co-AAc) microgels as crosslinkers to crosslink the P(NIPAM-co-AAc) networks. The prepared MCG hydrogels are featured with both fast response rate and large volume change ratio to environmental temperature stimuli. The cooperative function of all the fast shrinking of microgels, the fast collapse of the grafted dangling chains on the microgel surfaces and the 3-D interconnected water transportation channels between the microgels and polymeric networks inside the MCG hydrogels make the hydrogels respond fast to the environmental temperature change. The response rate and the volume change ratio of the hydrogel increase with decreasing the functional P(NIPAM-co-AAc) microgel concentration in the hydrogel preparation, which means that the volume-phase transition property of the hydrogel is tunable by simply controlling the microgel content in the hydrogel. The proposed MCG hydrogels with improved thermo-responsive phase transition property are attractive for developing efficient stimuli-responsive smart sensors, actuators, bioseparation absorbants, and so on.

New Thermo-responsive Hydrogels Based on Poly (N-isopropylacrylamide)/ Hyaluronic Acid Semi-interpenetrated Polymer Networks: Swelling Properties and Drug Release Studies

New pH and temperature sensitive semi-interpenetrated polymer networks (semi-IPNs) were developed by combining cross-linked PNIPAAm with hyaluronic acid (HA). At pH 7.4, the PNIPAAm/HA semi-IPN hydrogels had significantly greater and faster swelling at 25°C and a more complete deswelling at 37°C than PNIPAAm hydrogels. The temperature-dependent reversibility behavior was analyzed for biomaterial applications. Gentamicin was incorporated as a model drug, and the release profile from the hydrogels was followed under physiological conditions. The presence of HA, even in small quantities, allowed a more complete release of the gentamicin from the hydrogel. The PNIPAAm/HA semi-IPN hydrogels respond to both changes in temperature and pH making it suitable as a delivery system for therapeutics.