Crosslinking neat ultrathin films and nanofibres of pH-responsive poly(acrylic acid) by UV radiation

Electrospun polyelectrolyte hydrogel nanofibres are being developed for many applications including artificial muscles, scaffolds for tissue engineering, wound dressings and controlled drug release. For electrospun polyelectrolytes, a post-spinning crosslinking process is necessary for producing a hydrogel. Typically, radiation or thermal crosslinking routines are employed that require multifunctional crosslinking molecules and crosslink reaction initiators (free radical producers). Here, ultraviolet subtype-C (UVC) radiation was employed to crosslink neat poly(acrylic acid) (PAA) nanofibres and films to different crosslink densities. Specific crosslink initiators or crosslinking molecules are not necessary in this fast and simple process providing an advantage for biological applications. Scanning probe microscopy was used for the first time to measure the dry and wet dimensions of hydrogel nanofibres. The diameters of the swollen fibres decrease monotonically with increasing UVC radiation time. The fibres could be reversibly swollen/contracted by treatment with solutions of varying pH, demonstrating their potential as artificial muscles. The surprising success of UVC radiation exposure to achieve chemical crosslinks without a specific initiator molecule exploits the ultrathin dimensions of the PAA samples and will not work with relatively thick samples.

Electrically conductive, tough hydrogels with pH sensitivity

Electrically conductive, mechanically tough hydrogels based on a double network (DN) comprised of poly(ethylene glycol) methyl ether methacrylate (PPEGMA) and poly(acrylic acid) (PAA) were produced. Poly(3,4-ethylenedioxythiophene) (PEDOT) was chemically polymerized within the tough DN gel to provide electronic conductivity. The effects of pH on the tensile and compressive mechanical properties of the fully swollen hydrogels, along with their electrical conductivity and swelling ratio were determined. Compressive and tensile strengths as high as 11.6 and 0.6 MPa, respectively, were obtained for hydrogels containing PEDOT with a maximum conductivity of 4.3 S cm–1. This conductivity is the highest yet reported for hydrogel materials of high swelling ratios. These hydrogels may be useful as soft strain sensors because their electrical resistance changed significantly when cyclically loaded in compression.

Nanostructure and hydrogen bonding in interpolyelectrolyte complexes of poly(ε-caprolactone)-block-poly(2-vinyl pyridine) and poly(acrylic acid)

Nanostructured poly(ε-caprolactone)-block-poly(2-vinyl pyridine) (PCL-b-P2VP)/poly(acrylic acid) (PAA) interpolyelectrolyte complexes (IPECs) were prepared by casting from THF/ethanol solution. The morphological behaviour of this amphiphilic block copolymer/polyelectrolyte complexes with respect to the composition was investigated in a solvent mixture. The phase behaviour, specific interactions and morphology were investigated using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, optical microscopy (OM), dynamic light scattering (DLS) and atomic force microscopy (AFM). Micelle formation occurred due to the aggregation of hydrogen bonded P2VP block and polyelectrolyte (PAA) from non-interacted PCL blocks. It was observed that the hydrodynamic diameter (D_h) of the micelles in solution decreased with increasing PAA content up to 40 wt%. After 50 wt% PAA content, D_h again increased. The micelle formation in PCL-b-P2VP/PAA IPECs was due to the strong intermolecular hydrogen bonding between PAA homopolymer units and P2VP blocks of the block copolymer. The penetration of PAA homopolymers into the shell of the PCL-b-P2VP block copolymer micelles resulted in the folding of the P2VP chains, which in turn reduced the hydrodynamic size of the micelles. After the saturation of the shell with PAA homopolymers, the size of the micelles increased due to the absorption of added PAA onto the surface of the micelles.

A new way to nanostructure hydrogels : electrospun thermo-responsive islands-in-the-sea nanofibres

Islands-in-the-sea nanofibres are a very interesting system: one polymer (islands) is distributed in fibrillar domains within a second polymer (sea). This fibre geometry is often used in microfiber technologies to obtain very fine fibers, by removing the “sea” polymer. This geometry also allows to combine two polymers with very different properties. In this work this geometry is introduced applied to electrospun hydrogel nanofibers, in a novel fashion, and as a way to improve and stabilize the hydrogel nanofibers. Thermo-responsive islands-in-the-sea nanofibers are here produced by electrospinning solutions of a hydrogel-forming thermo-responsive polymer (crosslinked poly(N-isopropylacrylamide), PNIPAM) and a reinforcing polymer (polyetherketone cardo, PEK-c). The two polymers are thermodynamically incompatible in solution and phase separation takes place, which allows the instant formation of islands-in-the-sea nanofibers upon electrospinning. PNIPAM was then crosslinked post-spinning using an oligomeric silsesquioxane. The formed nanocomposite nanofibers showed intrinsic nanostructure, where the fibril-like PNIPAM domains are intimately adjacent to the strong PEK-c domains. Upon contacting with water, the hydrogel domains became instantly highly swollen, while the PEK-c domains did not. As a result, very wrinkly, swollen fibers were obtained, with increased capillary action, as demonstrated through confocal microscopy. The composite nanofibers in water showed excellent swelling ratios and very fast responses to temperature variations (of the order of 1 second) with morphological and optical effects: variations in fiber-diameter were accompanied by optical transitions: transparent-opaque. The produced hydrogel nanofibers also presented improved mechanical properties (even with small amounts of PEK-c), when compared to their crosslinked-PNIPAM-only nanofibers. It will be also shown how these materials can be used as optical actuators and smart hydrogel platforms with tuneable contact angle and morphology. In brief, this work aims to demonstrate a new platform technology which can be applied to several hydrogel systems, to achieve hydrogel-based composites with new and improved properties, while retaining (and improving) the main properties of the hydrogel. Here this was demonstrated by showing that the composite materials showed thermo-responsiveness, and enhanced transition kinetics.

Enzyme-triggered self-assembly of peptide hydrogels via reversed hydrolysis

We demonstrate that proteases can be used to selectively trigger the self-assembly of peptide hydrogels via reversed hydrolysis.

Study on the dyeing properties of hemicyanine dyes. I. Acrylic fabrics

Three hemicyanine dyes were employed in dyeing acrylic fabrics following traditional cationic dyeing procedures. The influence of the substituting groups of the dye molecule on the sorption rate and sorption isotherms was analyzed. The results showed that those dyed acrylic fabrics using hemicyanine dyes had obvious fluorescent effect in the spectra range 550–750 nm. In addition, according to the EN-471 standard (2003), the chromaticity of dyed acrylic fabric was calculated to evaluate whether hemicyanine dye could meet the requirements of the fluorescent dye for high visibility warning clothing.

Monitoring the early stage self-assembly of enzyme-assisted peptide hydrogels

The early stages of the self-assembly of peptide hydrogels largely determine their final material properties. Here we discuss experimental methodologies for monitoring the self-assembly kinetics which underpin peptide hydrogel formation. The early stage assembly of an enzyme-catalysed Fmoc-trileucine based self-assembled hydrogel was examined using spectroscopic techniques (circular dichroism, CD, and solution NMR) as well as chromatographic (HPLC) and mechanical (rheology) techniques. Optimal conditions for enzyme-assisted hydrogel formation were identified and the kinetics examined. A lag time associated with the formation and accumulation of the self-assembling peptide monomer was observed and a minimum hydrogelator concentration required for gelation was identified. Subsequent formation of well defined nano-and microscale structures lead to self-supporting hydrogels at a range of substrate and enzyme concentrations. 1H NMR monitoring of the early self-assembly process revealed trends that were well in agreement with those identified using traditional methods (i.e. HPLC, CD, rheology) demonstrating 1H NMR spectroscopy can be used to non-invasively monitor the self-assembly of peptide hydrogels without damaging or perturbing the system.