Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C02: Responsive Polymers, Soft Materials, and Hybrids IFocus Live
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Sponsoring Units: DPOLY DSOFT DBIO Chair: Jinhye Bae, University of California, San Diego |
Monday, March 15, 2021 3:00PM - 3:12PM Live |
C02.00001: Explaining the mechanical properties of hygroscopic bacterial spores using water nanoconfinement Steven Harrellson, Michael DeLay, Xi Chen, Ahmet Hamdi Cavusoglu, Ozgur Sahin Nanoconfined water can exhibit unusual properties. Water filled nanopores are ubiquitous in biological materials, but the effects of water nanoconfinement on their mechanical properties is not well understood. Recently, bacterial spores have been used as hygroscopic actuators for energy harvesting and soft robotics applications. Here we show the coupling of the mechanical properties and water nanoconfinement in Bacillus Subtilis spores can be understood by using a simple statistical mechanical model. Using nanomechanical experiments to probe the stiffness of the spore over four frequency decades, we found that the effective elastic modulus increases by up to an order of magnitude, far larger than common models of water transport predict. Force-Indentation experiments reveal a strain-dependent elastic modulus which can be understood using our statistical mechanical model. This work could be used to build intuitive understanding of the mechanical properties of nanoporous hygroscopic materials and could be used as the basis for the development of materials with frequency dependent mechanical properties. |
Monday, March 15, 2021 3:12PM - 3:48PM Live |
C02.00002: Adaptable hydrogels for organoid culture Invited Speaker: Sarah Heilshorn While organoid culture has the potential to revolutionize our understanding of human biology, current protocols rely on the use of Matrigel, a complex, heterogeneous material with large batch-to-batch variations that hinder reproducibility. In response, several groups have begun designing synthetic hydrogel systems to enable the reproducible culture of organoids. Recently, the matrix stress relaxation rate (i.e. the ability of a hydrogel to remodel its network connectivity in response to an applied stress) has been demonstrated to have profound effects on encapsulated cells. To date, the role of matrix stress relaxation on organoid cultures has not been explored. Here we present the design of a family of double-network hydrogels that undergo two stages of crosslinking: the first stage uses reversibly dynamic covalent chemistry bonds, while the second stage reinforces the hydrogel through thermal-induced polymer aggregation. This double-network of physical interactions results in a gel with a broad dynamic range of tunable mechanical properties, where the gel stiffness is set by the number of crosslinks and the gel stress relaxation rate is independently set by the kinetics of the crosslink binding and unbinding. These novel, double-network hydrogels have been used to study the role of mechanotransduction in the culture of patient-derived, human intestinal organoids. In this system, we find that the organoid cultures display strong phenotypic responses to matrix stress relaxation, which are dependent on cell-matrix interactions with both CD44 and integrin cell-surface receptors. |
Monday, March 15, 2021 3:48PM - 4:00PM Live |
C02.00003: Polymer field theory applied to Liquid Crystal Elastomers Pratik Khandagale, Carmel Majidi, Kaushik Dayal Existing continuum and micromechanical elastic models for rubbery polymers do not explicitly account for the inter-segment interaction in a cross-linked network. To address this limitation and understand the physics of chain interactions, we have developed a statistical mechanics-based field theoretic model for elastomers. |
Monday, March 15, 2021 4:00PM - 4:12PM Live |
C02.00004: Computational Study of Mechanochemical Activation in Nanostructured Triblock Copolymers Zijian Huo, Jennifer Laaser, Antonia Statt Designing polymeric materials with targeted mechanochemical responses has attracted great attention in the field of mechanochemistry; however, understanding how these materials respond to external force at the molecular level remains a significant challenge. In this work, we use molecular dynamics simulations to investigate mechanical activation in ABA triblock copolymers. We extend a classical coarse-grained polymer model to include a force-sensitive unit, or mechanophore, at the center of the rubbery region, and we vary the length of the glassy domain to obtain different well-defined nanostructured triblock copolymer morphologies. By monitoring the activation of the mechanophore under strain, we aim to understand the relationship between local activation and chain conformations in the microphase-separated triblock copolymers. We find that the activation of the force-sensitive unit is strongly dependent on the morphology of these materials, as well as on the individual chain conformations. We anticipate this finding will guide the design of force-responsive materials with precisely targeted mechanochemical activation profiles. |
Monday, March 15, 2021 4:12PM - 4:24PM Live |
C02.00005: Computational Study of the AC Susceptibility of Isolated Magnetic Nanoparticles in a Polymer Suspension Patrick Kreissl, Christian Holm, Rudolf Weeber Magnetic nanoparticles immersed in polymeric environments are promising candidates for both technical and bio-medical applications. Unfortunately, in many such systems the details of the coupling mechanisms are still unknown. Experimentally, the magnetic properties are typically studied using frequency-dependent AC susceptibility measurements from which also elastic properties of the surrounding polymer suspension can be inferred using appropriate theory such as the Gemant-DiMarzio-Bishop model [DiMarzio and Bishop, J. Chem. Phys., 1974, 3802–3811]. We present a computational study of isolated magnetic nanoparticles in a polymer suspension, considering only hydrodynamic and excluded volume interactions. AC susceptibility spectra are extracted using a linear-response Green-Kubo approach. We show that hydrodynamic coupling to the rotational behavior of the nanoparticles alone can lead to shifts of the spectra towards lower frequencies with both, increasing polymer concentration and polymer chain length. These findings qualitatively reproduce recent experimental observations [Roeben et al., Colloid Polym. Sci., 2014, 2013–2023]. |
Monday, March 15, 2021 4:24PM - 4:36PM Live |
C02.00006: Microion-Driven Free Swelling of Cylindrical Ionic Microgels Mohammed Alziyadi, Alan Denton Ionic microgels are soft colloidal particles, made of cross-linked polymer networks, that become charged and swollen in solution. Swelling can be controlled by adjusting temperature, pH, and ionic strength, facilitating applications to drug delivery and tissue engineering. Unlike spherical microgels, cylindrical microgels can independently swell in radial and axial directions, triggered by adjusting the balance between electrostatic and gel contributions to the osmotic pressure [1]. Within a cylindrical cell model, we compute the electrostatic osmotic pressure via exact statistical mechanical relations [1], implemented within Poisson-Boltzmann theory and molecular dynamics simulations. Combining electrostatic and gel osmotic pressures, the latter modeled using Flory-Rehner theory, we explore the dependence of swelling on charge density and salt concentration. With increasing density of fixed charge, spread uniformly over the particle surface or volume, we find that cylindrical microgels swell more in the axial than the radial direction. Our approach can help guide the design of responsive, rodlike microgels. |
Monday, March 15, 2021 4:36PM - 4:48PM Live |
C02.00007: A Kinetic Monte Carlo algorithm for swelling drug delivery systems Maxime Ignacio, Gary Slater A major challenge in pharmaceutical science consists in designing drug delivery systems that can maintain the concentration of active principle within a therapeutic window over a given period of time. A popular approach consists in storing the drug molecules inside an hydrogel and selecting its physical properties (such as hydrogel swelling in an aqueous environment) to control the release. The theory of swelling systems is not analytically easy because of the complexity of the evolution equations and thus requires numerical approaches. In this context, we have developed a flexible Kinetic Monte Carlo algorithm (KMCa) that can be used to study the release process for a variety of drug delivery systems. In this talk, we present this algorithm and test it for systems where one can derive an analytical solution, namely a core/shell structures and a swelling system in the adiabatic regime. We show that our KMCa correctly reproduces the analytical predictions. Finally, we introduce a semi-empirical model that captures the main features of swelling systems. |
Monday, March 15, 2021 4:48PM - 5:00PM Live |
C02.00008: Chemical controlled tangential growth leads to autonomous shell morphology Siyu Li, Daniel Matoz Fernandez, Monica Olvera De La Cruz Polymersomes are widely used as biomimetic material and drug carriers. By copolymerizing with functionalized polymers, stimuli-responsive polymersomes can be synthesized, where the external field modifies the microscopic interaction and leads to a macroscopic morphology change. In this work, we study the autonomous shell dynamics undergoing a periodically reduction-oxidation reaction and build a general model to simulate the chemical reaction and the shell elasticity simultaneously. Based on the fact that the building blocks are hydrophobic in the reduced state and hydrophilic in the oxidized form, we design the shell with chemical controlled mechanical properties. We show that the shell undergoes homogeneous swell or heterogeneous deformation depending on the diffusion coefficient and stoichiometric factors. The dynamic morphologies provide a straightforward future design for new materials with desired functionality and shed light on the mechanism of the living system. |
Monday, March 15, 2021 5:00PM - 5:12PM Live |
C02.00009: Polymer Dispersity Affects Conformation of Brushes Grafted on Nanoparticles Tzu-Han Li, Vivek Yadav, Jacinta Conrad, Megan Robertson Polyelectrolyte-grafted nanoparticles have been applied in controlled drug delivery systems, sensors, and water purification membranes. Their efficacy depends, in part, on the conformational response of polyelectrolyte brushes to pH changes. Although some brush properties controlling conformational pH-response, such as brush length lb, are well-studied, the impact of brush dispersity remains underexplored. Here, we show that the effect of dispersity on the conformational pH-response of polyelectrolyte brushes depends on the molecular weight. We synthesized poly(tert-butyl acrylate)-grafted silica nanoparticles with varying dispersities and subsequently hydrolyzed them to poly(acrylic acid) (PAA) brushes. At low weight-average degree of polymerization Nw, lb of low-dispersity PAA brushes was weakly dependent on pH, whereas lb of high-dispersity PAA brushes was greater and drastically increased with pH. At a higher Nw, however, lb of the PAA brushes showed similar pH-response regardless of dispersity. We hypothesize that the differences in lb are related to differences in conformation among low and high dispersity brushes. Variation in response of lb to pH, originating in conformational changes, via control over dispersity can be leveraged to improve dispersion in complex media. |
Monday, March 15, 2021 5:12PM - 5:24PM Live |
C02.00010: Exploration of the ordered morphologies of thermo- and photo-responsive LCOT block copolymer systems Claire Seitzinger, Timothy Lodge The microphase separation of block copolymer systems as a function of temperature is generally well understood. However, not all applications are amenable to changes in temperature, and therefore the use of other stimuli to induce changes in the nanostructured behavior of block copolymers is desired. By incorporating small amounts of the photoactive molecule azobenzene into the polymer sidegroups, we have identified a system that exhibits both thermo- and photo-responsive behavior. Upon introduction into imidazolium-based ionic liquid solvents, poly(methyl methacrylate)-b-(benzyl methacrylate-s-4-phenylazophenyl methacrylate) (MBsA, 70 kDa, <10 mol% azobenzene) displays lower critical ordering transition (LCOT) behavior, where microphase separation occurs upon heating, and can be modulated by light. Using small amplitude oscillatory shear rheology and small angle X-ray scattering, we are able to understand the ordering behavior of the system. By irradiating MBsA in situ while probing with these techniques, we can further understand the photo-responsiveness. Here, we explore the ordered phases achieved by MBsA in various ionic liquids, as well as the potential for photo-induced order-order transitions. |
Monday, March 15, 2021 5:24PM - 5:36PM On Demand |
C02.00011: A biocompatible hydrogel pH sensor to measure tumor acidosis Sachindra Kiridena, Uthpala Wijayaratna, MD Arifuzzaman, Jeffrey Anker Extracellular pH in the tumor is particularly promising because they play a key role in cancer metastasis, chemotherapy resistance, however, measuring in situ chemical pH is currently very challenging. We developed a chemically responsive polyacrylic acid-based injectable hydrogel sensor to noninvasively measure local pH in a tumor location biopsied and used for diagnosis using plain radiography. The sensor is based on a polyacrylic acid-based hydrogel which swells according to pH: contracts at low pH, expands at high pH. The hydrogel contains a radiodense tantalum bead within it, so that the pH-dependent hydrogel length can be measured via X-ray imaging. The hydrogel is placed in a porous casing to be able to inject the sensor into a tumor and prevent direct contact. The entire sensor fits inside a biopsy marker needle which will be used to inject the sensor into the tumor tissue. The hydrogel sensor showed a linear response with a precision of 0.07 pH unit and repeatable response to pH cycling in bovine serum in the pH range 6.5 to 7.5, most relevant to tumor acidosis. In vivo studies of the developed hydrogel-based biosensor will be carried out in future on a rat tumor model. |
Monday, March 15, 2021 5:36PM - 5:48PM On Demand |
C02.00012: Monitoring prosthetic hip infections using a synovial fluid pH sensor Uthpala Wijayaratna, Sachindra Kiridena, MD Arifuzzaman, Jeffrey Anker We developed a hydrogel-based synovial fluid pH sensor which can be attached to prosthetic hips for early detection and monitoring of hip infections using plain radiography. The sensor consists of a pH responsive polyacrylic acid hydrogel, with a radiodense tantalum bead and a metal wire at the two ends. Once the sensor is attached to the prosthetic implant, the change in length of the hydrogel can be monitored via plain radiography by measuring the movement of the tantalum bead with respect to the radio-opaque metal wire. The hydrogel sensor demonstrated a linear response with a pKa of 5.25 and a sensitivity of 0.3 cm/pH unit. It also showed reversible response to pH cycling in the range of pH 6.5-7.5 in both buffer and bovine synovial fluid. An interobserver study of randomized series of radiographs in bovine synovial fluid between pH 6 and 8.5 showed an interobserver precision of 0.03 pH units. Therefore, the developed synovial fluid pH sensor can be used for early detection of post-surgery infections, noninvasively, using plain radiography. |
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