Reversible Molecular Capture and Release in Microfluidics by Host-Guest Interactions in Hydrogel Microdots.

The integration of microscopic hydrogels with high specific surface area and physically reactive groups into microfluidic systems for selective molecular interactions is attracting increasing attention. Herein, the reversible capture and release of molecules through host-guest interactions of hydrogel dots in a microfluidic device is reported, which translates the supramolecular chemistry to the microscale conditions under continuous flow. Polyacrylamide (PAAm) hydrogel arrays with grafted ß-cyclodextrin (ß-CD)  modified poly(2-methyl-2-oxazoline) (CD-PMOXA) chains are fabricated by photopolymerization and integrated into a polydimethylsiloxane (PDMS)-on-glass chip. The ß-CD/adamantane (ß-CD/Ada) host-guest complex is confirmed by two dimensional Nuclear Overhauser Effect Spectroscopy NMR (2D NOESY NMR) prior to transfer to microfluidics. Ada-modified molecules are successfully captured by host-guest interaction formed between the CD-PMOXA grafted chains in the hydrogel network and the guest molecule in the solution. Furthermore, the captured molecules are released by perfusing free ß-CD with higher binding affinity than those grafted in the hydrogel array. A small guest molecule adamantane-fluorescein-isothiocyanate (Ada-FITC) and a macromolecular guest molecule (Ada-PMOXA-Cyanine 5 (Cy5)) are separately captured and released for three times with a release ratio up to 46% and 92%, respectively. The reproducible capture and release of functional molecules with different sizes demonstrates the stability of this hydrogel system in microfluidics and will provide an opportunity for future applications.

Double cross-linked supramolecular hydrogels with tunable properties based on host-guest interactions.

We report a novel double cross-linked hydrogel system based on polyacrylamide and poly(2-methyl-2-oxazoline) (PMOXA) network chains, as well as on supramolecular host-guest interactions with on-demand tailored mechanical properties. Well-defined vinyl-bearing PMOXA macromonomers, functionalized with either ß-cyclodextrin units (ß-CD-PMOXA) or adamantane units (Ada-PMOXA), were synthesized and confirmed using 1H NMR, MALDI-TOF-MS and GPC measurements. The complexation between adamantane and ß-CD modified macromonomers in solution towards bismacromonomers was confirmed by 2D NOESY NMR and DLS. After introducing these bismacromonomers into the polyacrylamide hydrogel, the supramolecular non-covalent Ada/ß-CD bond was responsible for the presence of PMOXA network chains to form a dense network. Once the interactions broke, the PMOXA chains no longer contributed to the network, but became dangling graft side chains in a predominated polyacrylamide network. Their dissociative nature influenced the physical properties, including the swelling behavior and mechanics of the final hydrogel. Rheological experiments proved that the E-modulus of the network was significantly increased by the supramolecular host-guest interactions. Tuning the lengths of PMOXA network chains even allowed the modification of the changes in mechanical strength, also through the addition of free ß-CD. The tunable properties of the double cross-linked supramolecular hydrogel proved their unique strength for future applications.