Permeation and Selective Retention of Solutes in PEGDA Hydrogel Membranes Containing Free Polymer Chains

We performed permeation experiments using PEGDA-based hydrogel membranes in which we added free polymer chains. We know from previous studies that free polymer chains can be trapped in the PEGDA matrix. Therefore, we use this property to trigger the retention of solutes through noncovalent interactions (heterogeneous bonds and electrostatic interactions) between the solute and the free polymer chains. We show that proton-donor molecules bearing carboxylic acid groups can be retained by hydrogen interactions at low pH with the free PEG (poly(ethylene glycol)) chains, which are proton acceptors. PEG is a nonionic hydrophilic polymer based on ethylene oxide units (−CH₂CH₂O−) which is a proton acceptor and can establish hydrogen bonds with proton-donor groups (such as COOH). The extent of retention increases with the number of acidic groups on the solute. By adding PAA (poly(acrylic acid), based on acrylic acid (−CH₂CHCOOH), these groups are negatively charged at pH > pK_a, i.e., pH > 5) in the PEGDA/PEG membranes, we show that positively charged molecules can be retained inside the membranes at a pH where the PAA is ionized. This retention is reversible, and solute molecules can be flushed out by reducing the pH below the pK_a of PAA. By measuring the adsorption isotherm of molecules in the membranes, we obtain the interaction energy between the solute and PAA, which is consistent with electrostatic interactions. Finally, we photopolymerize our membranes directly in a microfluidic chip to perform tangential filtration of a mixture of fluorescein (negatively charged) and rhodamine B (zwitterionic). Fluorescein is repelled by negatively charged PAA and does not penetrate the membrane, while rhodamine B is retained and eventually permeates through. These findings pave the way to functionalization of hydrogel membranes by trapping free polymer chains for selective retention and filtration.