Binding of Lysozyme to Spherical Poly(styrenesulfonate) Gels

Utilizing a microfluidic platform to investigate drug-eluting beads: Binding and release of amphiphilic antidepressants

Drug-eluting beads made of responsive polyelectrolyte networks are used in the treatment of liver cancer. Aggregates of loaded drugs in complex with the networks dissolve upon release, causing swelling of the network. According to a recent mechanism the release and swelling rates are controlled by the mass transport of drug through a depletion layer created in the microgel. We hypothesise that the mechanism, in which the stability of the drug aggregates and the swelling properties of the network play crucial roles, offers means to control the release profile also for other drugs. To test this, we investigated the loading and release properties of amitriptyline, chlorpromazine and doxepin in polyacrylate, hyaluronate and DCbead™ microgels in a microfluidic setup. Loaded drugs could be released to a medium with physiological ionic strength and pH. The binding strength increased with decreasing critical micelle concentration of the drugs and increasing linear charge density of network chains. Microgels displayed drug-rich core/swollen shell coexistence, and swelled during release at a rate in agreement with the depletion layer mechanism, indicating its generality. The results demonstrate the potential of microgels as vehicles for amphiphilic drugs and the usefulness of the microfluidics method for in vitro studies of such systems.

Volume Transition and Phase Coexistence in Polyelectrolyte Gels Interacting with Amphiphiles and Proteins

Polyelectrolyte gels have the capacity to absorb large amounts of multivalent species of opposite charge from aqueous solutions of low ionic strength, and release them at elevated ionic strengths. The reversibility offers the possibility to switch between “storage” and “release” modes, useful in applications such as drug delivery. The review focuses on systems where so-called volume phase transitions (VPT) of the gel network take place upon the absorption and release of proteins and self-assembling amphiphiles. We discuss the background in terms of thermodynamic driving forces behind complex formation in oppositely charged mixtures, the role played by cross-links in covalent gels, and general aspects of phase coexistence in networks in relation to Gibbs’ phase rule. We also briefly discuss a gel model frequently used in papers covered by the review. After that, we review papers dealing with collapse and swelling transitions of gels in contact with solution reservoirs of macroions and surfactants. Here we describe recent progress in our understanding of the conditions required for VPT, competing mechanisms, and hysteresis effects. We then review papers addressing equilibrium aspects of core–shell phase coexistence in gels in equilibrium. Here we first discuss early observations of phase separated gels and results showing how the phases affect each other. Then follows a review of recent theoretical and experimental studies providing evidence of thermodynamically stable core–shell phase separated states, and detailed analyses of the conditions under which they exist. Finally, we describe the results from investigations of mechanisms and kinetics of the collapse/swelling transitions induced by the loading/release of proteins, surfactants, and amphiphilic drug molecules.

Drug-Eluting Polyacrylate Microgels: Loading and Release of Amitriptyline

We investigated the loading of an amphiphilic drug, amitriptyline hydrochloride (AMT), onto sodium polyacrylate hydrogels at low ionic strength and its release at high ionic strength. The purpose was to show how the self-assembling properties of the drug and the swelling of the gel network influenced the loading/release mechanisms and kinetics, important for the development of improved controlled-release systems for parenteral administration of amphiphilic drugs. Equilibrium studies showed that single microgels (∼100 μm) in a large solution volume underwent a discrete transition between swollen and dense states at a critical drug concentration in the solution. For single macrogels in a small solution volume, the transition progressed gradually with increasing amount of added drug, with swollen and dense phases coexisting in the same gel; in a suspension of microgels, swollen and collapsed particles coexisted. Time-resolved micropipette-assisted microscopy studies showed that drug self-assemblies accumulated in a dense shell enclosing the swollen core during loading and that a dense core was surrounded by a swollen shell during release. The time evolution of the radius of single microgels was determined as functions of liquid flow rate, network size, and AMT concentration in the solution. Mass transport of AMT in the surrounding liquid, and in the dense shell, influenced the deswelling rate during loading. Mass transport in the swollen shell controlled the swelling rate during release. A steady-state kinetic model taking into account drug self-assembly, core-shell phase separation, and microgel volume changes was developed and found to be in semiquantitative agreement with the experimental loading and release data.