Abstract
Cross-linked polymeric gels are an important class of materials with applications that broadly range from synthetic wound healing scaffolds to materials used in enhanced oil recovery. To effectively design these materials for each unique applications a deeper understanding of the structure and rheological properties as a function of polymeric interactions is required. Increasing the concentration of polymer in each scaffold increases physical interactions between the molecules that can be reflected in the material structure. To characterize the structure and material properties, we use multiple particle tracking microrheology (MPT) to measure scaffolds during gelation. In MPT, fluorescently labeled probe particles are embedded in the material and the Brownian motion of these particles is captured using video microscopy. Particle motion is related to rheological properties using the Generalized Stokes-Einstein Relation. In this work, we characterize gelation of a photopolymerized scaffold composed of a poly(ethylene glycol) (PEG)-acrylate backbone and a PEG-dithiol cross-linker. Scaffolds with backbone concentrations below and above the overlap concentration, concentration where polymer pervaded volume begins to overlap, are characterized. Using time-cure superposition (TCS) we determine the critical relaxation exponent, n, of each scaffold. The critical relaxation exponent is a quantitative measure of the scaffold structure and is similar to a complex modulus, G*, which is a measure of energy storage and dissipation. Our results show that below the overlap concentration the scaffold is a tightly cross-linked network, navg = 0.40 ± 0.03, which stores energy but can also dissipate energy. As polymeric interactions increase, we measure a step change in the critical relaxation exponent above the overlap concentration to navg = 0.20 ± 0.03. After the overlap concentration the scaffold has transitioned to a more tightly cross-linked network that primarily stores energy. Additionally, continuing to increase concentration results in no change in the scaffold structure. Therefore, we determined that the properties of this scaffold can be tuned above and below the overlap concentration by changing the polymer concentration but the structure will remain the same in each concentration regime. This is advantageous for a wide range of applications that require scaffolds with varying stiffness and the same scaffold architecture.
Highlights
IntroductionSynthetic gels are designed with unprecedented complexity from the bulk material properties down to the scaffold microstructure (Stauffer et al, 1982; Moradi-Araghi et al, 1988; Scanlan and Winter, 1991; Lutolf et al, 2003; Engler et al, 2006; Serra et al, 2006; Zolfaghari et al, 2006; Yamaguchi et al, 2007; He et al, 2009; Schultz et al, 2009a; Schwartz et al, 2010; Tse and Engler, 2010; Zustiak and Leach, 2010; Wylie et al, 2011; Tirrell, 2012; Jung et al, 2013; Tongwa et al, 2013; Purcell et al, 2014; Wang and Heilshorn, 2015; Escobar et al, 2017)
We measure no change in the dilute, c < c∗, and semi-dilute, c > c∗, concentration regimes, but there is a step change in the structure of the scaffold at the transition. This result indicates that the structure remains constant above and below the overlap concentration regardless of poly(ethylene glycol) (PEG)-acrylate concentration. This information can lead to the design of new materials, where the moduli of the material can be tailored for applications, such as the mimic of a tissue or a material to reduce permeability in enhanced oil recovery without changing the scaffold structure
This work characterizes the change in rheological properties and scaffold structure during the photopolymerization of a four-arm star PEG-acrylate:PEG-dithiol hydrogel scaffold as polymeric interactions are added to the system
Summary
Synthetic gels are designed with unprecedented complexity from the bulk material properties down to the scaffold microstructure (Stauffer et al, 1982; Moradi-Araghi et al, 1988; Scanlan and Winter, 1991; Lutolf et al, 2003; Engler et al, 2006; Serra et al, 2006; Zolfaghari et al, 2006; Yamaguchi et al, 2007; He et al, 2009; Schultz et al, 2009a; Schwartz et al, 2010; Tse and Engler, 2010; Zustiak and Leach, 2010; Wylie et al, 2011; Tirrell, 2012; Jung et al, 2013; Tongwa et al, 2013; Purcell et al, 2014; Wang and Heilshorn, 2015; Escobar et al, 2017). This work can be leveraged to design gels with highly-engineered microstructures and properties that can be tailored throughout the phase transition
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