Abstract
Thermoresponsive gelling materials constructed from natural and synthetic polymers can be used to provide triggered action and therefore customised products such as drug delivery and regenerative medicine types as well as for other industries. Some materials give Arrhenius-type viscosity changes based on coil to globule transitions. Others produce more counterintuitive responses to temperature change because of agglomeration induced by enthalpic or entropic drivers. Extensive covalent crosslinking superimposes complexity of response and the upper and lower critical solution temperatures can translate to critical volume temperatures for these swellable but insoluble gels. Their structure and volume response confer advantages for actuation though they lack robustness. Dynamic covalent bonding has created an intermediate category where shape moulding and self-healing variants are useful for several platforms. Developing synthesis methodology—for example, Reversible Addition Fragmentation chain Transfer (RAFT) and Atomic Transfer Radical Polymerisation (ATRP)—provides an almost infinite range of materials that can be used for many of these gelling systems. For those that self-assemble into micelle systems that can gel, the upper and lower critical solution temperatures (UCST and LCST) are analogous to those for simpler dispersible polymers. However, the tuned hydrophobic-hydrophilic balance plus the introduction of additional pH-sensitivity and, for instance, thermochromic response, open the potential for coupled mechanisms to create complex drug targeting effects at the cellular level.
Highlights
During the past decade or so there has been interest in gelatinous materials that respond to a change in their local environment e.g., specific ligands, pH and temperature among others, some of which are being considered as exploitable in, for example, advanced drug delivery systems
Wang reported a positively responsive hydrogel synthesised by grafting poly(acrylic acid) (PAAc) to maleic anhydride—cyclodextrin (MAH-β-CD) and forming an IPN with polyacrylamide (PAAm)
The focus in this review has been the viscosity changes as these polymeric molecules respond to temperature change, with the accent on upper and lower critical solution temperatures (UCST and Lower Critical Solution Temperature (LCST))
Summary
During the past decade or so there has been interest in gelatinous materials (and their precursors such as xerogels) that respond to a change in their local environment e.g., specific ligands, pH and temperature among others, some of which are being considered as exploitable in, for example, advanced drug delivery systems. Many polymers, including natural ones like xanthan gum, starch, gellan, konjac, carrageenans, as discussed by Gasperini [40] These are often usefully temperature sensitive in terms of viscosity collagen, fibrin, silk fibroin, hyaluronic acid and gelatin [34,35,36,37,38,39], form gels by several main change, but thermoresponsive design will increasingly be combined with synthetic features that relate mechanisms as discussed by Gasperini [40]. The UCST profile on a temperature-composition (volume fraction) plot for a polymer or blend is the less commonly documented transition type and is defined by the gelator coming out of solution below values on a critical separation line known as the spinodal curve, where in addition, a sensitive corridor exists between that and the so-called co-existence (or binodal) curve (Figure 2) These two curves are coincident at their apex, the critical point that represents the infinitely large molecular weight.
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