William L. A. Brooks

New directions in thermoresponsive polymers

Interest in thermoresponsive polymers has steadily grown over many decades, and a great deal of work has been dedicated to developing temperature sensitive macromolecules that can be crafted into new smart materials. However, the overwhelming majority of previously reported temperature-responsive polymers are based on poly(N-isopropylacrylamide) (PNIPAM), despite the fact that a wide range of other thermoresponsive polymers have demonstrated similar promise for the preparation of adaptive materials. Herein, we aim to highlight recent results that involve thermoresponsive systems that have not yet been as fully considered. Many of these (co)polymers represent clear opportunities for advancements in emerging biomedical and materials fields due to their increased biocompatibility and tuneable response. By highlighting recent examples of newly developed thermoresponsive polymer systems, we hope to promote the development of new generations of smart materials.

Doubly-dynamic-covalent polymers composed of oxime and oxanorbornene links

Two sets of reversible covalent linkages distributed in series along a polymer backbone were used to prepare a new class of doubly-dynamic-covalent polymers capable of reversibly dissociating via two distinct pathways. These self-repairable linear polymers were prepared via step-growth Diels–Alder polymerization of an AB monomer that contained furfuryl- and maleimido groups linked by an oxime bond. Both the oxime and oxanorbornene links lent orthogonally reversible character to the polymer backbone. The sequentially distributed oxime bonds in the polymer were capable of dynamic oxime exchange in the presence of a competitive monofunctional alkoxyamine under acidic conditions, while the oxanorbornene linkages were susceptible to cleavage via retro-Diels–Alder reactions at elevated temperatures and recombination upon cooling. The self-healing or reversible nature of the dynamic-covalent oxime bonds and oxanorbornene links has potential for designing stimuli-responsive self-repairable materials for sensors and drug delivery applications.

A photonic glucose biosensor for chronic wound prognostics

The ability to monitor glucose levels in chronic wound fluid of diabetic patients is a promising theranostic approach in chronic wound healing. Phenylboronic acid polymers are glucose- and pH-responsive materials. In the presence of glucose, these polymers reversibly form cyclic boronate esters, changing the properties of the polymer and forming the basis of glucose sensing. In this report, poly(4-vinylphenylboronic acid) (PVPBA) was covalently grafted to the pores of porous silicon (pSi) films (pSi-PVPBA). Polymer switching in response to changing pH and glucose concentration was monitored by means of interferometric reflectance spectroscopy (IRS). We observed that a shift of the boronic acid equilibrium between the neutral and anionic form in the polymer translated into refractive index changes that could be detected as a variation of the effective optical thickness (EOT) of the pSi-PVPBA film. The pSi/polymer composite was further investigated as a platform for the detection of glucose. Using this sensing platform, we were able to detect glucose in a buffer solution as low as 0.15 mM and also in a wound fluid sample without encountering interferences.

Synthesis of novel boronic acid-decorated poly(2-oxazoline)s showing triple-stimuli responsive behavior

Boronic acid-functionalized (co)polymers have gained increasing attention in the field of responsive polymers and polymeric materials due to their unique characteristics and responsiveness towards both changes in pH and sugar concentrations. This makes these (co)polymers excellently suited for various applications including responsive membranes, drug delivery applications and sensor materials. Unfortunately, boronic acid-based polymer research is also notorious for its challenging monomer synthesis and polymerization and its overall difficult polymer purification and manipulation. In light of this, many research groups have focused their attention on the optimization of various polymerization techniques in order to expand the field of BA-research including previously unexplored monomers and polymerization techniques. In this paper, a new post-polymerization modification methodology was developed allowing for the synthesis of novel boronic acid-decorated poly(2-alkyl-2-oxazoline) (PAOx) copolymers, utilizing the recently published PAOx methyl ester reaction platform. The developed synthetic pathway provides a straightforward method for the introduction of pH- and glucose-responsiveness, adding this to the already wide variety of possible responsive PAOx-based systems. The synthesized BA-decorated PAOx are based on the thermoresponsive poly(2-n-propyl-2-oxazoline) (PnPropOx). This introduces a pH and glucose dependence on both cloud and clearance point temperatures of the copolymer in aqueous and pH-buffered conditions, yielding a triply-responsive (co)polymer that highlights the wide variety of obtainable properties using this pathway.

Boronic Acid Linear Homopolymers as Effective Emulsifiers and Gelators

We report emulsion studies using poly(vinylphenyl boronic acid) (PVPBA) linear homopolymer as an effective emulsifier and gelator. Two stabilizing regimes were identified depending on the pH of PVPBA aqueous solutions, i.e., emulsions stabilized by the hompolymer nanoparticles (Pickering emulsions) at pH < pKa and emulsions stabilized by the homopolymer unimers at pH > pKa. In both cases, gelled emulsions were obtained from medium to high internal phase volume fractions with the unimers exhibiting more effective emulsification and gelling properties. Hydrogen bonding between the boronic acid units is proposed to account for the high strength of the emulsions. The emulsions were shown to be pH- and sugar-responsive. Finally, the stable emulsions were used as templates to directly prepare PVPBA macroporous materials and to fabricate multilayered capsules. This remarkable observation that a simple homopolymer can serve as an effective emulsifier and gelator may dramatically extend the scope of potential emulsifiers and inspire further research in the design of new types of efficient emulsifying agents.