ABSTRACT
The flow and transport of polymer solutions through porous media are ubiquitous in myriad scientific and engineering applications. With escalating interest in adaptive polymers, understanding the flow dynamics of their solutions is indispensable (yet lacking). Here, the hydrophobic-effect-driven reversible associations in a self-adaptive polymer (SAP) solution and its flow characteristics in a microfluidic-based "rock-on-a-chip" device have been analyzed. The hydrophobic aggregates were fluorescent labeled; this enabled a direct visualization of the in situ association/disassociation of the polymer supramolecular assemblies in pore spaces and throats. Furthermore, the influence of this adaptation on the macroscopic flow behavior of the SAP solution was analyzed by comparing its flow with that of two partially-hydrolyzed polyacrylamide (the molecular weight (MW)-equivalent HPAM-1 and ultrahigh-MW HPAM-2) solutions in the semi-dilute regime with similar initial viscosities. At low flow rates (with shear predominance), the SAP solution showed a low shear viscosity compared to HPAM-1, indicating a higher shear susceptibility for association than chain entanglement. Although the SAP exhibited the same elastic instability as the non-adaptive polymers above a threshold flow rate, the adaptable structure of the former advanced the onset of its viscoelastic-governed flow, providing a stronger flow resistance, possibly through an extension resistance. Furthermore, 3D-media analysis indicated that the reversible association/disassociation of SAP increased the accessible pore space during nonaqueous-liquid displacement, facilitating oil production.
ABSTRACT
While various static cues such as matrix stiffness have been known to regulate stem cell differentiation, it is unclear whether or not dynamic cues such as degradation rate along with the change of material chemistry can influence cell behaviors beyond simple integration of static cues such as decreased matrix stiffness. The present research is aimed at examining effects of degradation rates on adhesion and differentiation of mesenchymal stem cells (MSCs) in vitro on well-defined synthetic hydrogel surfaces. Therefore, we synthesized macromers by extending both ends of poly(ethylene glycol) (PEG) with oligo(lactic acid) and then acryloyl, and the corresponding hydrogels that were obtained after photopolymerization of the macromers were biodegradable. Combining the unique techniques of block copolymer micelle nanolithography with transfer lithography, we prepared a nanoarray of cell-adhesive arginine-glycine-aspartate peptides on this nonfouling biodegradable hydrogel. The biodegradation is caused by hydrolysis of the ester bonds, and different degradation rates in the cell culture medium were achieved by different stages of accelerated pre-hydrolysis in an acidic medium. For the following cell culture and induction, both the matrix stiffness and degradation rate varied among the examined groups. While adipogenic differentiation of MSCs can be understood by the lowered stiffness, the osteogenic differentiation was contradictory with common sense because we found enhanced osteogenesis on soft hydrogels. Higher degradation rates were suggested to account for this interesting phenomenon in the sole osteogenic/adipogenic induction and even more complicated trends in the co-induction. Hence, the degradation rate is a dynamic cue influencing cell behaviors, which should be paid attention to for degradable biomaterials.
