No abstract

By the functionalization of poly(N-isopropylacrylamide-co-acrylic acid) microgels with 3-aminophenylboronic acid (APBA) via carbodiimide coupling, nearly monodisperse glucose-sensitive P(NIPAM-PBA) microgels with a diameter of several hundred nanometers were synthesized in aqueous media. Dynamic laser light scattering was used to study the glucose-sensitive and thermosensitive behaviors of the resultant microgels under various conditions. The introduction of the hydrophobic phenylboronic acid (PBA) group significantly decreases the volume phase transition temperature of the resultant microgels. As a result, the P(NIPAM-PBA) microgels with a 10.0 mol % PBA content are in a collapsed state at room temperature. However, the addition of glucose makes the microgels swell dramatically. The glucose-sensitivity of the PBA-containing microgels relies on the stabilization of the charged phenylborate ions by binding with glucose, which can convert more hydrophobic PBA groups to the hydrophilic phenylborate ions. The presence of glucose also induces a two-stage volume phase transition of the P(NIPAM-PBA) microgels, which is explained by the core-shell-like heterogeneous structure of the microgels induced by the formation of the unique glucose-bis(boronate) complex in the "core" area of the microgels. The effects of pH, ionic strength, and PBA content on the glucose sensitivity of the P(NIPAM-PBA) microgels were investigated.

Boronic acid-containing hydrogels are important intelligent materials. With the introduction of boronic acid functionality, these hydrogels exhibit a lot of interesting properties, such as glucose-sensitivity, reversibility and self-healing. These materials have found important applications in many areas, especially in biomedical areas. This paper aims to provide an overview of the current state of the art of the study in this area. We review the synthesis and properties of various boronic acid-containing hydrogels, including macroscopic hydrogels, microgels and layer-by-layer self-assembled films. Their applications were described, with an emphasis on the design of various glucose sensors and self-regulated insulin delivery devices. New development in this area was highlighted. Problems and the new directions were discussed.

Into and out of the blue: The highly ordered structure of a PNIPAM microgel colloidal crystal (MCC) is stabilized by photopolymerization of its surface-bound vinyl groups. The resulting polymerized MCCs can respond reversibly and quickly to external stimuli, including temperature and ionic strength of the surrounding media, allowing the color and band gap to be finely tuned in the whole visible range.

Alginate–dopamine (Alg–DA) conjugate, a polymer with catechol side groups instead of phenol groups, gels in situ in the presence of HRP and H2O2. The resulting hydrogels exhibit significantly improved adhesion properties.

In this work we try to develop a new thermal gelling injectable scaffold for three-dimensional cell culture. Instead of using linear, branched, or grafted macromolecules, thermosensitive microgel particles or microspheres are used as building blocks for the construction of the macroscopic hydrogel scaffold. As a proof of concept, thermosensitive poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) (P(NIPAM-HEMA)) microgel particles were synthesized, which present a volume phase transition temperature (VPTT) at about 29 degrees C. Rheological test shows that the concentrated P(NIPAM-HEMA) microgel dispersion is colloidally stable when heated above its VPTT, indicating hydrophobic interaction alone can not induce thermal gelation of the dispersion. In the presence of a low concentration of CaCl(2), however, with the introduction of additional ionic cross-linking, the microgel dispersion gelates and forms macroscopic hydrogel. Gelation temperature of the microgel dispersion decreases with increasing ionic strength. SEM observation reveals that the resultant bulky gel has an interconnected porous microstructure. 293T cells, a human cell line, were encapsulated inside the hydrogel by simple mixing with the microgel dispersion at room temperature and heating to 37 degrees C. MTT (3-[4,5-dimethylthiazol-2-yl]-3,5-diphenyl tetrazolium bromide) assays reveal that the cells are viable and proliferate inside the 3D scaffold.

Using poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) (P(NIPAM-HEMA)) microgel as an example, we proposed that thermogelable microgel dispersions can be used as a new type of injectable cell scaffolds. However, these in situ formed hydrogels shrink with time, which is undesirable for their use as cell scaffolds. In this work, the syneresis of hydrogels from poly(N-isopropylacrylamide) (PNIPAM) microgel dispersions with various acrylic acid (AA) contents were measured. These concentrated microgel dispersions can all thermally gelate at physiological pH and ionic strength. With increasing AA content in the microgels, the gelation temperature of the dispersion increases, but the degree of syneresis of the resulting hydrogels decreases. The kinetics of shrinkage can be well described by the Tanaka-Fillmore equation, indicating that the deswelling of the hydrogels is governed by the cooperative diffusion of the gel network. These hydrogels present an interconnected porous structure with pore size decreases with an increasing degree of syneresis. HepG2 cells were seeded in these hydrogels. While the cells proliferate and form spheroid-like aggregates in hydrogels with a low degree of syneresis, their growth is inhibited in the one with a high degree of syneresis. These results indicate that the syneresis of the hydrogel has an adverse effect on cell culturing. This effect can be alleviated by adjusting AA content in the microgels. However, cells do not grow either if AA content is too high which results in a lack of cell-substrate interaction.

The layer-by-layer (LbL) method has proved to be a simple but versatile technique for the construction of self-assembled films with controlled thickness and composition since it was first introduced by Decher et al. [1] Using this method, biopolymers, electroactive polymers, and photoactive polymers can be easily introduced into thin films. The resulting films can be bio-, electro-, or photoactive. [2] One advantage of the LbL method is that there is no restriction on the size and morphology of the substrate on which the film is constructed, which implies that the LbL method can be extended from twodimensional (2D) systems (i.e., planar substrates) to threedimensional (3D) systems (for example, spherical or other non-planar substrates). Consequently, a large variety of core± shell particles with finely tuned polymer layer thickness and compositions have been synthesized, [3] and a series of novel hollow microcapsules fabricated by subsequent removal of the sacrificial core of the resulting core±shell particles.[4] Since these supramolecular assemblies offer exciting prospects for the encapsulation of drugs, enzymes, proteins, and other active materials, they are attracting more and more attention.Up to now almost all papers in this area have employed the electrostatic self-assembly method, which is based on the sequential adsorption of oppositely charged polyelectrolytes, except for several cases with some minor modifications.[5] On the other hand, LbL strategies employing other driving forces, such as the hydrogen bond, have been developed.[6] Hydrogen-bonding self-assembly (HSA) was first introduced by Stockton and Rubner in 1997.[6a] Since then, several pairs of polymers have been successfully used in the self-assembly processes.[6b±f] However, no success has been reported in extending the HSA method from 2D to 3D systems and preparing hollow capsules by further removing the sacrificial core. One reason for the increased difficulty of this hydrogen-bonding process is the fact that hydrogen bonding is much weaker than the electrostatic interaction; consequently, rupture may occur when the sacrificial core is being removed. Since the formation and destruction of hydrogen bonds can be controlled easily by external stimuli, such as pH, [6d±f] capsules based on hydrogen bonding may have great merit when being used in controlled drug-release systems. In this report, we demonstrate that it is possible to extend the HSA method from 2D to 3D systems and further to prepare hollow capsules with successful procedures in removing the sacrificial core without rupturing the multilayer shell. Poly(vinylpyrrolidone), PVP, and m-methylphenol-formaldehyde resin (MPR) are employed as hydrogen acceptor and donor, respectively, in this method. Self-assembled films were first fabricated on planar substrates, i.e., quartz and silica slides, to examine the feasibility for HSA. The self-assembly process on quartz is followed by UV-vis spectroscopy (data not shown here). The absorbance of the film increases regularly with increasing bi...