Preferential Control of Forward Reaction Kinetics in Hydrogels Crosslinked with Reversible Conjugate Additions

Molecular substitutions were used to demonstrate preferential control over the kinetic rate constants in a poly(ethylene glycol)-based hydrogel with two different reversible thia-conjugate addition reactions. A strong electron withdrawing nitrile group on the conjugate acceptor showed a 20-fold increase in the forward rate constant over a neutral withdrawing group, while the reverse rate constant only increased 6-fold. Rheometry experiments demonstrated that the hydrogel plateau modulus was primarily dictated by reaction equilibrium, while the stress relaxation characteristics of the hydrogel were dominated by the reverse rate constant. Furthermore, the dynamic crosslinking allowed the hydrogel to rapidly and spontaneously self-heal. These results indicate that decoupling the kinetic rate constants of bond exchange allow systematic control over dynamic covalent hydrogel bulk properties, such as their adaptability, stress relaxation ability, and self-healing properties.This research was supported by the National Science Foundation (NSF MRSEC DMR-1720595, T.M.F., E.V.A., and A.M.R) and by the Burroughs Wellcome Fund (CASI-1015895, A.M.R). The authors acknowledge the use of shared research facilities supported in part by the Texas Materials Institute, the Center for Dynamics and Control of Materials: an NSF MRSEC (DMR- 1720595), and the NSF National Nanotechnology Coordinated Infrastructure (ECCS-1542159).Center for Dynamics and Control of Material

Genetic Control of Radical Crosslinking in a Semi-Synthetic Hydrogel

Enhancing materials with the qualities of living systems, including sensing, computation, and adaptation, is an important challenge in designing next-generation technologies. Living materials address this challenge by incorporating live cells as actuating components that control material function. For abiotic materials, this requires new methods that couple genetic and metabolic processes to material properties. Toward this goal, we demonstrate that extracellular electron transfer (EET) from Shewanella oneidensis can be leveraged to control radical cross-linking of a methacrylate-functionalized hyaluronic acid hydrogel. Cross-linking rates and hydrogel mechanics, specifically storage modulus, were dependent on various chemical and biological factors, including S. oneidensis genotype. Bacteria remained viable and metabolically active in the networks for a least 1 week, while cell tracking revealed that EET genes also encode control over hydrogel microstructure. Moreover, construction of an inducible gene circuit allowed transcriptional control of storage modulus and cross-linking rate via the tailored expression of a key electron transfer protein, MtrC. Finally, we quantitatively modeled hydrogel stiffness as a function of steady-state mtrC expression and generalized this result by demonstrating the strong relationship between relative gene expression and material properties. This general mechanism for radical cross-linking provides a foundation for programming the form and function of synthetic materials through genetic control over extracellular electron transfer.Center for Dynamics and Control of Material

Depletion-Driven Morphological Control of Bundled Actin Networks

The actin cytoskeleton is a semiflexible biopolymer network whose morphology is controlled by a wide range of biochemical and physical factors. Actin is known to undergo a phase transition from a single-filament state to a bundled state by the addition of polyethylene glycol (PEG) molecules in sufficient concentration. While the depletion interaction experienced by these biopolymers is well-known, the effect of changing the molecular weight of the depletant is less well understood. Here, we experimentally identify a phase transition in solutions of actin from networks of filaments to networks of bundles by varying the molecular weight of PEG polymers, while holding the concentration of these PEG polymers constant. We examine the states straddling the phase transition in terms of micro and macroscale properties. We find that the mesh size, bundle diameter, persistence length, and intra-bundle spacing between filaments across the line of criticality do not show significant differences, while the relaxation time, storage modulus, and degree of bundling change between the two states do show significant differences. Our results demonstrate the ability to tune actin network morphology and mechanics by controlling depletant size, a property which could be exploited to develop actin-based materials with switchable rigidity.Comment: 22 pages, 10 figures. Authors James Clarke and Francis Cavanna contributed equally; Changes: Added modeling work, extended dynamic light scattering analysi

Photoresponsive Elastic Properties of Azobenzene-Containing Poly(ethylene-glycol)-Based Hydrogels

The elastic modulus of the extracellular matrix is a dynamic property that changes during various biological processes, such as disease progression or wound healing. Most cell culture platforms, however, have traditionally exhibited static properties, making it necessary to replate cells to study the effects of different elastic moduli on cell phenotype. Recently, much progress has been made in the development of substrates with mechanisms for either increasing or decreasing stiffness in situ, but there are fewer examples of substrates that can both stiffen and soften, which may be important for simulating the effects of repeated ECM injury and resolution. In the work presented here, poly­(ethylene glycol)-based hydrogels reversibly stiffen and soften with multiple light stimuli via photoisomerization of an azobenzene-containing cross-linker. Upon irradiation with cytocompatible doses of 365 nm light (10 mW/cm², 5 min), isomerization to the azobenzene <i>cis</i> configuration leads to a softening of the hydrogel up to 100–200 Pa (shear storage modulus, <i>G</i>′). This change in gel properties is maintained over a time scale of several hours due to the long half-life of the <i>cis</i> isomer. The initial modulus of the gel can be recovered upon irradiation with similar doses of visible light. With applications in mechanobiology in mind, cytocompatibility with a mechanoresponsive primary cell type is demonstrated. Porcine aortic valvular interstitial cells were encapsulated in the developed hydrogels and shown to exhibit high levels of survival, as well as a spread morphology. The developed hydrogels enable a route to the noninvasive control of substrate modulus independent of changes in the chemical composition or network connectivity, allowing for investigations of the effect of dynamic matrix stiffness on adhered cell behavior

Shear Thickening Behavior in Injectable Tetra-PEG Hydrogels Cross-Linked via Dynamic Thia-Michael Addition Bonds

Injectable poly(ethylene glycol) (PEG)-based hydrogels were reversibly cross-linked through thia-conjugate addition bonds and demonstrated to shear thicken at low shear rates. Cross-linking bond exchange kinetics and dilute polymer concentrations were leveraged to tune hydrogel plateau moduli (from 60 to 650 Pa) and relaxation times (from 2 to 8 s). Under continuous flow shear rheometry, these properties affected the onset of shear thickening and the degree of shear thickening achieved before a flow instability occurred. The changes in viscosity were reversible whether the shear rate increased or decreased, suggesting that chain stretching drives this behavior. Given the relevance of dynamic PEG hydrogels under shear to biomedical applications, their injectability was investigated. Injection forces were found to increase with higher polymer concentrations and slower bond exchange kinetics. Altogether, these results characterize the nonlinear rheology of dilute, dynamic covalent tetra-PEG hydrogels and offer insight into the mechanism driving their shear thickening behavior

Tunable Phase Behavior of Polystyrene–Polypeptoid Block Copolymers

Block copolymers with tunable compositions offer the ability to directly control the interaction strength between the two blocks and therefore polymer properties. The miscibility of an A–B block copolymer can be increased by introducing B or B-like comonomers into the A block, and literature has shown that both the amount and the distribution of these comonomers affect the compatibility of the two blocks. Sequence-defined block copolymers in which one can exactly control the composition and comonomer distribution provide a unique opportunity to control the overall strength of segregation. Here, sequence-defined block copolymers have been synthesized via azide–alkyne coupling using polystyrene and sequence-specific polypeptoids (N-substituted glycines) with 2-methoxyethyl side chains. These polystyrene-polypeptoid block copolymers readily self-assemble into hexagonally packed and lamellar morphologies. <i>N</i>-(2-phenylethyl)­glycine residues, which have a styrene-like side chain, were introduced throughout the polypeptoid block to increase the compatibility with the polystyrene block. As the compatibility increased, the strength of segregation and therefore the block copolymer order–disorder transitions decreased. The polystyrene–polypeptoid block copolymers provide a tunable platform for further studies on the effect of composition and sequence design on self-assembly and block copolymer properties

Performance of a quantitative fecal immunochemical test for detecting advanced colorectal neoplasia: a prospective cohort study

Abstract Background The fecal immunochemical test (FIT) is easier to use and more sensitive than the guaiac fecal occult blood test, but it is unclear how to optimize FIT performance. We compared the sensitivity and specificity for detecting advanced colorectal neoplasia between single-sample (1-FIT) and two-sample (2-FIT) FIT protocols at a range of hemoglobin concentration cutoffs for a positive test. Methods We recruited 2,761 average-risk men and women ages 49-75 referred for colonoscopy within a large nonprofit, group-model health maintenance organization (HMO), and asked them to complete two separate single-sample FITs. We generated receiver-operating characteristic (ROC) curves to compare sensitivity and specificity estimates for 1-FIT and 2-FIT protocols among those who completed both FIT kits and colonoscopy. We similarly compared sensitivity and specificity between hemoglobin concentration cutoffs for a single-sample FIT. Results Differences in sensitivity and specificity between the 1-FIT and 2-FIT protocols were not statistically significant at any of the pre-specified hemoglobin concentration cutoffs (10, 15, 20, 25, and 30 μg/g). There was a significant difference in test performance of the one-sample FIT between 50 ng/ml (10 μg/g) and each of the higher pre-specified cutoffs. Disease prevalence was low. Conclusions A two-sample FIT is not superior to a one-sample FIT in detection of advanced adenomas; the one-sample FIT at a hemoglobin concentration cutoff of 50 ng/ml (10 μg/g) is significantly more sensitive for advanced adenomas than at higher cutoffs. These findings apply to a population of younger, average-risk patients in a U.S. integrated care system with high rates of prior screening

Immunomodulatory functions of human mesenchymal stromal cells are enhanced when cultured on HEP/COL multilayers supplemented with interferon-gamma

Human mesenchymal stromal cells (hMSCs) are multipotent cells that have been proposed for cell therapies due to their immunosuppressive capacity that can be enhanced in the presence of interferon-gamma (IFN-γ). In this study, multilayers of heparin (HEP) and collagen (COL) (HEP/COL) were used as a bioactive surface to enhance the immunomodulatory activity of hMSCs using soluble IFN-γ. Multilayers were formed, via layer-by-layer assembly, varying the final layer between COL and HEP and supplemented with IFN-γ in the culture medium. We evaluated the viability, adhesion, real-time growth, differentiation, and immunomodulatory activity of hMSCs on (HEP/COL) multilayers. HMSCs viability, adhesion, and growth were superior when cultured on (HEP/COL) multilayers compared to tissue culture plastic. We also confirmed that hMSCs osteogenic and adipogenic differentiation remained unaffected when cultured in (HEP/COL) multilayers in the presence of IFN-γ. We measured the immunomodulatory activity of hMSCs by measuring the level of indoleamine 2,3-dioxygenase (IDO) expression. IDO expression was higher on (HEP/COL) multilayers treated with IFN-γ. Lastly, we evaluated the suppression of peripheral blood mononuclear cell (PBMC) proliferation when co-cultured with hMSCs on (HEP/COL) multilayers with IFN-γ. hMSCs cultured in (HEP/COL) multilayers in the presence of soluble IFN-γ have a greater capacity to suppress PBMC proliferation. Altogether, (HEP/COL) multilayers with IFN-γ in culture medium provides a potent means of enhancing and sustaining immunomodulatory activity to control hMSCs immunomodulation

PS2-01: A Multimodal Reminder Program Improves Routine Mammogram Screening Rates

Background/Aims: This retrospective cohort study evaluated the effectiveness of implementation and maintenance phases of a multimodal reminder program to improve routine mammography screening

Impact of Helical Chain Shape in Sequence-Defined Polymers on Polypeptoid Block Copolymer Self-Assembly

Controlling the self-assembly of block copolymers with variable chain shape and stiffness is important for driving the self-assembly of functional materials containing nonideal chains as well as for developing materials with new mesostructures and unique thermodynamic interactions. The polymer helix is a particularly important functional motif. In the helical chain, the traditional scaling relationships between local chain stiffness and space-filling properties are not applicable; this in turn impacts the scaling relationships critical for governing self-assembly. Polypeptoids, a class of sequence-defined peptidomimetic polymers with controlled helical secondary structure, were used to systematically investigate the impact of helical chain shape on block copolymer self-assembly in a series of poly­(<i>n</i>-butyl acrylate)-<i>b</i>-polypeptoid block copolymers. Small-angle X-ray scattering (SAXS) of the bulk materials shows that block copolymers form hexagonally packed cylinder domains. By leveraging sequence control, the polypeptoid block was controlled to form a helix only at the part either adjacent to or distant from the block junction. Differences in domain size from SAXS reveal that chain stretching of the helix near the block junction is disfavored, while helical segments at the center of cylindrical domains contribute to unfavorable packing interactions, increasing domain size. Finally, temperature-dependent SAXS shows that helix-containing diblock copolymers disorder at lower temperatures than the equivalent unstructured diblock copolymers; we attribute this to the smaller effective <i>N</i> of the helical structure resulting in a larger entropic gain upon disordering. These results emphasize how current descriptions of rod/coil interactions and conformational asymmetry for coil polymers do not adequately address the behavior of chain secondary structures, where the scalings of space-filling and stiff–elastic properties relative to chain stiffness deviate from those of typical coil, semiflexible, and rodlike polymers