This review highlights recent developments in the field of stimuli-responsive hydrogels, focusing primarily on thin films, with a thickness range between 100 nm to 10 lm. The theory and dynamics of hydrogel swelling is reviewed, followed by specific applications. Gels are classified based on the active stimulus-mechanical, chemical, pH, heat, and lightand fabrication methods, design constraints, and novel stimuliresponses are discussed. Often, these materials display large physiochemical reactions to a relatively small stimulus.Noteworthy materials larger than 10 lm, but with response times on the order of seconds to minutes are also discussed. Hydrogels have the potential to advance the fields of medicine and polymer science as useful substrates for "smart" devices.

In this work, we have investigated a quaternary ammonium compound that exhibits excellent antimicrobial activity and can be permanently grafted to substrates containing C−H bonds to form a durable polymeric film within 1 min. The compound consists of a biocidal component, dodecyl-alkylated quaternary ammonium, and a benzophenone moiety that, under mild UV irradiation, generates a densely cross-linked network and covalently attaches to a variety of substrates, including plastics, fabrics, and alkyl-modified glass surfaces. The surface attachment is 1 order of magnitude faster than that of previously reported benzophenone-associated cross-linkers, due to the electronwithdrawing effect of quaternary ammonium on the benzophenone chromophore. The modified surfaces are nonleaching and exhibit contact-killing and highly effective antimicrobial activity against Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) using cell count and live/dead staining methods. The charged ammonium group also promotes photoreaction efficiency with respect to network robustness, leading to a thin film that can sustain high shear forces and abrasion when compared to commercially available silanebased quaternary ammonium compounds. The biocidal activity is also retained after exposure to mechanical stress and abrasion.

Functional, micropatterned and multicomponent polymer brush surfaces can be rapidly fabricated via base-catalyzed thiol-isocyanate ''click'' reactions.

A polyacrylamide hydrogel system that can be liquefied by remote activation using UV irradiation is investigated as a degradable adhesive. The linear polyacrylamide copolymer, formed by conventional free-radical polymerization, contains biomimetic catechol−iron-mediated crosslinkers that are sensitive to pH changes. Hydrogel films and bulk gels are prepared by basic titration of a polymer solution doped with a photoacid generator, diphenyliodonium chloride, generating an ionic cross-linked network via the catechol pendant groups. Irradiation of these hydrogels with UV light affords a viscous liquid solution, demonstrating a gel−sol transition with a subsequent decrease in the adhesive strength of the material. These gels may be prepared in high throughput and require few synthetic steps with commercially available precursors.

Poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) (poly(3HB- co-3HHx)) thermoplastics are a promising biodegradable alternative to traditional plastics for many consumer applications. Biodegradation measured by gaseous carbon loss of several types of poly(3HB- co-3HHx) plastic was investigated under anaerobic conditions and aerobic seawater environments. Under anaerobic conditions, the biodegradation levels of a manufactured sheet of poly(3HB- co-3HHx) and cellulose powder were not significantly different from one another over 85 days with 77.1 ± 6.1 and 62.9 ± 19.7% of the carbon converted to gas, respectively. However, the sheet of poly(3HB- co-3HHx) had significantly higher methane yield ( p ≤ 0.05), 483.8 ± 35.2 mL·g volatile solid (VS), compared to cellulose controls, 290.1 ± 92.7 mL·g VS, which is attributed to a greater total carbon content. Under aerobic seawater conditions (148-195 days at room temperature), poly(3HB- co-3HHx) sheets were statistically similar to cellulose for biodegradation as gaseous carbon loss (up to 83% loss in about 6 months), although the degradation rate was lower than that for cellulose. The microbial diversity was investigated in both experiments to explore the dominant bacteria associated with biodegradation of poly(3HB- co-3HHx) plastic. For poly(3HB- co-3HHx) treatments, Cloacamonales and Thermotogales were enriched under anaerobic sludge conditions, while Clostridiales, Gemmatales, Phycisphaerales, and Chlamydiales were the most enriched under aerobic seawater conditions.

Poly quaternary "-oniums" derived from polyethylenimine (PEI), poly(vinyl-N-alkylpyridinium), or chitosan belong to a class of cationic polymers that are efficient antimicrobial agents. When dissolved in solution, the positively charged polycations are able to displace the divalent cations of the cellular phospholipid bilayer and disrupt the ionic cross-links and structural integrity of the membrane. However, when immobilized to a surface where confinement limits diffusion, poly -oniums still show excellent antimicrobial activity, which implies a different biocidal mode of action. Recently, a proposed mechanism, named phospholipid sponge effect, suggested that surface-bound polycationic networks are capable of recruiting negatively charged phospholipids out of the bacterial cell membrane and sequestering them within the polymer matrix.1 However, there has been insufficient evidence to support this hypothesis. In this study, a surface-bound N,N-dodecyl methyl-co-N,N-methylbenzophenone methyl quaternary PEI (DMBQPEI) was prepared to verify the phospholipid sponge effect. By tuning the irradiation time, the cross-linking densities of surface-bound DMBQPEI films were mediated. The modulus of films was measured by PeakForce Quantitative Nanomechanical Mapping (QNM) to indicate the cross-linking density variation with increasing irradiation time. A negative correlation between the film cross-linking density and the absorption of a negatively charged phospholipid (DPhPG) was observed, but no such correlations were observed with a neutral phospholipid (DPhPC), which strongly supported the action of anionic phospholipid suction proposed in the lipid sponge effect. Moreover, the killing efficiency toward S. aureus and E. coli was inversely affected by the cross-linking density of the films, providing evidence for the phospholipid sponge effect. The relationship between killing efficiency and film cross-linking density is discussed.

In this work, a series of copolymer/silica nanocomposites are investigated that exhibit excellent anti-icing behavior and can be covalently grafted to any substrate containing C–H bonds with high durability.

From July 2015 to November 2016, 96 post-hatchling sea turtles were collected from 118 km of the Atlantic coastline in Florida, USA, including loggerhead, green, and hawksbill sea turtle species. Forty-five of the recovered turtles were rehabilitated and released, but the remaining 52 died and were frozen. At necropsy, the gastrointestinal tracts of most the turtles contained visible plastic, and collected particles of 27 individuals were chemically characterized by Raman microscopy as polyethylene, polypropylene, polyethylene terephthalate, and polystyrene. Mesoparticle plastic fragments 1.0-8.7 mm, microparticle fragments 20-1000 μm, and nanoparticles 5-169 nm were identified in the turtles. Polyethylene and polypropylene were the most common plastics ingested from specimens representing 54.1 and 23.7% of the total observed mesoparticles and 11.7 and 21.0% of the total observed microparticles, respectively. A plastic-to-body mass ratio of 2.07 mg/g was determined for this group. The authors suggest that ingestion of micronizing plastic by post-hatchling sea turtles is likely a substantial risk to survival of these endangered and threatened species. This study also provides some of the first evidence for the formation of nanoscopic plastic particles that we theorize forms in the post-hatchling and juvenile environment and are present post-ingestion.