Carbodiimide-Driven Toughening of Interpenetrated Polymer Networks.

Recent work has demonstrated that temporary crosslinks in polymer networks generated by chemical "fuels" afford materials with large, transient changes in their mechanical properties. This can be accomplished in carboxylic-acid-functionalized polymer hydrogels using carbodiimides, which generate anhydride crosslinks with lifetimes on the order of minutes to hours. Here, the impact of the polymer network architecture on the mechanical properties of transiently crosslinked materials was explored. Single networks (SNs) were compared to interpenetrated networks (IPNs). Notably, semi-IPN precursors that give IPNs on treatment with carbodiimide give much higher fracture energies (i.e., resistance to fracture) and superior resistance to compressive strain compared to other network architectures. A precursor semi-IPN material featuring acrylic acid in only the free polymer chains yields, on treatment with carbodiimide, an IPN with a fracture energy of 2400 J/m2, a fourfold increase compared to an analogous semi-IPN precursor that yields a SN. This resistance to fracture enables the formation of macroscopic complex cut patterns, even at high strain, underscoring the pivotal role of polymer architecture in mechanical performance.

Chemically Fueled Reinforcement of Polymer Hydrogels.

Carbodiimide-fueled anhydride bond formation has been used to enhance the mechanical properties of permanently crosslinked polymer networks, giving materials that exhibit transitions from soft gels to covalently reinforced gels, eventually returning to the original soft gels. Temporary changes in mechanical properties result from a transient network of anhydride crosslinks, which eventually dissipate by hydrolysis. Over an order of magnitude increase in the storage modulus is possible through carbodiimide fueling. The time-dependent mechanical properties can be modulated by the concentration of carbodiimide, temperature, and primary chain architecture. Because the materials remain rheological solids, new material functions such as temporally controlled adhesion and rewritable spatial patterns of mechanical properties have been realized.

Heat- and Light-Responsive Materials Through Pairing Dynamic Thiol-Michael and Coumarin Chemistry.

Covalent adaptable networks (CANs) based on the thiol-Michael (TM) linkages can be thermal and pH responsive. Here, a new vinyl-sulfone-based thiol-Michael crosslinker is synthesized and incorporated into acrylate-based CANs to achieve stable materials with dynamic properties. Because of the reversible TM linkages, excellent temperature-responsive re-healing and malleability properties are achieved. In addition, for the first time, a photoresponsive coumarin moiety is incorporated with TM-based CANs to introduce light-mediated reconfigureability and postpolymerization crosslinking. Overall, these materials can be on demand dynamic in response to heat and light but can retain mechanical stability at ambient condition.

Shear stress upregulates regeneration-related immediate early genes in liver progenitors in 3D ECM-like microenvironments.

The role of fluid stresses in activating the hepatic stem/progenitor cell regenerative response is not well understood. This study hypothesized that immediate early genes (IEGs) with known links to liver regeneration will be upregulated in liver progenitor cells (LPCs) exposed to in vitro shear stresses on the order of those produced from elevated interstitial flow after partial hepatectomy. The objectives were: (1) to develop a shear flow chamber for application of fluid stress to LPCs in 3D culture; and (2) to determine the effects of fluid stress on IEG expression in LPCs. Two hours of shear stress exposure at ∼4 dyn/cm2 was applied to LPCs embedded individually or as 3D spheroids within a hyaluronic acid/collagen I hydrogel. Results were compared against static controls. Quantitative reverse transcriptase polymerase chain reaction was used to evaluate the effect of experimental treatments on gene expression. Twenty-nine genes were analyzed, including IEGs and other genes linked to liver regeneration. Four IEGs (CFOS, IP10, MKP1, ALB) and three other regeneration-related genes (WNT, VEGF, EpCAM) were significantly upregulated in LPCs in response to fluid mechanical stress. LPCs maintained an early to intermediate stage of differentiation in spheroid culture in the absence of the hydrogel, and addition of the gel initiated cholangiocyte differentiation programs which were abrogated by the onset of flow. Collectively the flow-upregulated genes fit the pattern of an LPC-mediated proliferative/regenerative response. These results suggest that fluid stresses are potentially important regulators of the LPC-mediated regeneration response in liver.

Progress Towards Computational 3-D Multicellular Systems Biology.

Tumors cannot be understood in isolation from their microenvironment. Tumor and stromal cells change phenotype based upon biochemical and biophysical inputs from their surroundings, even as they interact with and remodel the microenvironment. Cancer should be investigated as an adaptive, multicellular system in a dynamical microenvironment. Computational modeling offers the potential to detangle this complex system, but the modeling platform must ideally account for tumor heterogeneity, substrate and signaling factor biotransport, cell and tissue biophysics, tissue and vascular remodeling, microvascular and interstitial flow, and links between all these sub-systems. Such a platform should leverage high-throughput experimental data, while using open data standards for reproducibility. In this chapter, we review advances by our groups in these key areas, particularly in advanced models of tissue mechanics and interstitial flow, open source simulation software, high-throughput phenotypic screening, and multicellular data standards. In the future, we expect a transformation of computational cancer biology from individual groups modeling isolated parts of cancer, to coalitions of groups combining compatible tools to simulate the 3-D multicellular systems biology of cancer tissues.

Tunable stress relaxation behavior of an alginate-polyacrylamide hydrogel: comparison with muscle tissue.

Factors controlling the time-dependent mechanical properties of interpenetrating network (IPN) hydrogel materials are not well understood. In this study, alginate-polyacrylamide IPN were synthesized to mimic the stress relaxation behavior and elastic modulus of porcine muscle tissue. Hydrogel samples were created with single-parameter chemical concentration variations from a baseline formula to establish trends. The concentration of total monomer material had the largest effect on the elastic modulus, while concentration of the acrylamide cross-linker, N,N-methylenebis(acrylamide) (MBAA), changed the stress relaxation behavior most effectively. The IPN material was then tuned to mimic the mechanical response of muscle tissue using these trends. Swelling the hydrogel samples to equilibrium resulted in a dramatic decrease in both elastic modulus and stress relaxation behavior. Collectively, the results demonstrate that alginate-polyacrylamide IPN hydrogels can be tuned to closely mimic both the elastic and the viscoelastic behaviors of muscle tissue, although swelling detrimentally affects these desired properties.

Use of silicone materials to simulate tissue biomechanics as related to deep tissue injury.

OBJECTIVE: Deep tissue injury (DTI) is caused by prolonged mechanical loading that disrupts blood flow and metabolic clearance. A patient simulator that mimics the biomechanical aspects of DTI initiation, stress and strain in deep muscle tissue, would be potentially useful as a training tool for pressure-relief techniques and testing platform for pressure-mitigating products. As a step toward this goal, this study evaluates the ability of silicone materials to mimic the distribution of stress in muscle tissue under concentrated loading. METHODS: To quantify the mechanical properties of candidate silicone materials, unconfined compression experiments were conducted on 3 silicone formulations (Ecoflex 0030, Ecoflex 0010, and Dragon Skin; Smooth-On, Inc, Easton, Pennsylvania). Results were fit to an Ogden hyperelastic material model, and the resulting shear moduli (G) were compared with published values for biological tissues. Indentation tests were then conducted on Ecoflex 0030 and porcine muscle to investigate silicone's ability to mimic the nonuniform stress distribution muscle demonstrates under concentrated loading. Finite element models were created to quantify stresses throughout tissue depth. Finally, a preliminary patient simulator prototype was constructed, and both deep and superficial "tissue" pressures were recorded to examine stress distribution. RESULTS: Indentation tests showed similar stress distribution trends in muscle and Ecoflex 0030, but stress magnitudes were higher in Ecoflex 0030 than in porcine muscle. All 3 silicone formulations demonstrated shear moduli within the range of published values for biological tissue. For the experimental conditions reported in this work, Ecoflex 0030 exhibited greater stiffness than porcine muscle. CONCLUSION: Indentation tests and the prototype patient simulator trial demonstrated similar trends with high pressures closest to the bony prominence with decreasing magnitude toward the interfacial surface. Qualitatively, silicone mimicked the phenomenon observed in muscle of nonuniform stress under concentrated loading. Although shear moduli were within biological ranges, stress and stiffness values exceeded those of porcine muscle. This research represents a first step toward development of a preclinical model simulating the biomechanical conditions of stress and strain in deep muscle, since local biomechanical factors are acknowledged to play a role in DTI initiation. Future research is needed to refine the capacity of preclinical models to simulate biomechanical parameters in successive tissue layers of muscle, fat, dermis, and epidermis typically intervening between bone and support surfaces, for body regions at risk for DTI.

Silky Liquid Metal Electrodes for On-Skin Health Monitoring.

Next generation on-skin electrodes will require soft, flexible, and gentle materials to provide both high-fidelity sensing and wearer comfort. However, many commercially available on-skin electrodes lack these key properties due to their use of rigid hardware, harsh adhesives, uncomfortable support structures, and poor breathability. To address these challenges, this work presents a new device paradigm by joining biocompatible electrospun spider silk with printable liquid metal to yield an incredibly soft and scalable on-skin electrode that is strain-tolerant, conformable, and gentle on-skin. These electrodes, termed silky liquid metal (SLiM) electrodes, are found to be over five times more breathable than commercial wet electrodes, while the silk's intrinsic adhesion mechanism allows SLiM electrodes to avoid the use of harsh artificial adhesives, potentially decreasing skin irritation and inflammation over long-term use. Finally, the SLiM electrodes provide comparable impedances to traditional wet and other liquid metal electrodes, offering a high-fidelity sensing alternative with increased wearer comfort. Human subject testing confirmed the SLiM electrodes ability to sense electrophysiological signals with high fidelity and minimal irritation to the skin. The unique properties of the reported SLiM electrodes offer a comfortable electrophysiological sensing solution especially for patients with pre-existing skin conditions or surface wounds.

Effect of strain rate on the material properties of human liver parenchyma in unconfined compression.

The liver is one of the most frequently injured organs in abdominal trauma. Although motor vehicle collisions are the most common cause of liver injuries, current anthropomorphic test devices are not equipped to predict the risk of sustaining abdominal organ injuries. Consequently, researchers rely on finite element models to assess the potential risk of injury to abdominal organs such as the liver. These models must be validated based on appropriate biomechanical data in order to accurately assess injury risk. This study presents a total of 36 uniaxial unconfined compression tests performed on fresh human liver parenchyma within 48 h of death. Each specimen was tested once to failure at one of four loading rates (0.012, 0.106, 1.036, and 10.708 s-1) in order to investigate the effects of loading rate on the compressive failure properties of human liver parenchyma. The results of this study showed that the response of human liver parenchyma is both nonlinear and rate dependent. Specifically, failure stress significantly increased with increased loading rate, while failure strain significantly decreased with increased loading rate. The failure stress and failure strain for all liver parenchyma specimens ranged from -38.9 kPa to -145.9 kPa and from -0.48 strain to -1.15 strain, respectively. Overall, this study provides novel biomechanical data that can be used in the development of rate dependent material models and the identification of tissue-level tolerance values, which are critical to the validation of finite element models used to assess injury risk.

Forced labour risk is pervasive in the US land-based food supply.

Social risk assessments and case studies of labour conditions in food production primarily focus on specific subpopulations, regions and commodities. To date, research has not systematically assessed labour conditions against international standards across diverse, complex food products. Here we combine data on production, trade, labour intensity and qualitative risk coding to quantitatively assess the risk of forced labour embedded in the US land-based food supply, building on our previous assessment of fruits and vegetables. We demonstrate that animal-based proteins, processed fruits and vegetables, and discretionary foods are major contributors to forced labour risk and that 62% of total forced labour risk stems from domestic production or processing. Our findings reveal the widespread risk of forced labour present in the US food supply and the necessity of collaborative action across all countries-high, middle and low income-to eliminate reliance on labour exploitation.

Measurement of diets that are healthy, environmentally sustainable, affordable, and equitable: A scoping review of metrics, findings, and research gaps.

Introduction: Research on the impacts of dietary patterns on human and planetary health is a rapidly growing field. A wide range of metrics, datasets, and analytical techniques has been used to explore the role of dietary choices/constraints in driving greenhouse gas (GHG) emissions, environmental degradation, health and disease outcomes, and the affordability of food baskets. Many argue that each domain is important, but few have tackled all simultaneously in analyzing diet-outcome relationships. Methods: This paper reviews studies published between January 2015 and December 2021 (inclusive) that examined dietary patterns in relation to at least two of the following four thematic pillars: (i) planetary health, including, climate change, environmental quality, and natural resource impacts, (ii) human health and disease, (iii) economic outcomes, including diet cost/affordability, and (iv) social outcomes, e.g., wages, working conditions, and culturally relevant diets. We systematically screened 2,425 publications by title and abstract and included data from 42 eligible publications in this review. Results: Most dietary patterns used were statistically estimated or simulated rather than observed. A rising number of studies consider the cost/affordability of dietary scenarios in relation to optimized environmental and health outcomes. However, only six publications incorporate social sustainability outcomes, which represents an under-explored dimension of food system concerns. Discussion: This review suggests a need for (i) transparency and clarity in datasets used and analytical methods; (ii) explicit integration of indicators and metrics linking social and economic issues to the commonly assessed diet-climate-planetary ecology relationships; (iii) inclusion of data and researchers from low- and middle-income countries; (iv) inclusion of processed food products to reflect the reality of consumer choices globally; and (v) attention to the implications of findings for policymakers. Better understanding is urgently needed on dietary impacts on all relevant human and planetary domains simultaneously.

Porohyperviscoelastic model simultaneously predicts parenchymal fluid pressure and reaction force in perfused liver.

Porohyperviscoelastic (PHVE) modeling gives a simplified continuum approximation of pore fluid behavior within the parenchyma of liver tissue. This modeling approach is particularly applicable to tissue engineering of artificial livers, where the inherent complexity of the engineered scaffolds prevents the use of computational fluid dynamics. The objectives of this study were to simultaneously predict the experimental parenchymal fluid pressure (PFP) and compression response in a PHVE liver model. The model PFP matched the experimental measurements (318 Pa) to within 1.5%. Linear regression of both phases of compression, ramp, and hold, demonstrated a strong correlation between the model and the experimental reaction force (p<0.5). The ability of this PVE model to accurately predict both fluid and solid behavior is important due to the highly vascularized nature of liver tissue and the mechanosensitivity of liver cells to solid matrix and fluid flow properties.

Extrusion 3D printing of keratin protein hydrogels free of exogenous chemical agents.

Keratins are a class of intermediate filament proteins that can be obtained from numerous sources including human hair. Materials fabricated from keratins offer desirable characteristics as scaffolds for tissue engineering, including intrinsic cell adhesion sequences and tunable degradation kinetics. The capacity to create 3D printed constructs from keratin-based bio-inks generates unique opportunities for spatial control of scaffold physicochemical properties to direct scaffold functions in ways not readily achieved through other means. The aim of this study was to leverage the controllable rheological properties of keratin hydrogels to create a strategy for extrusion 3D printing of keratin bio-inks without the use of exogenous rheological modifiers, crosslinking agents, or photocurable resins. The rheological properties of keratin hydrogels were tuned by varying two parameters: (a) the ratio of keratose (obtained by oxidative extraction of keratin) to kerateine (obtained by reductive extraction of keratin); and (b) the weight percentage of total keratin protein in the gel. A computational model of the dispensing nozzle for a commercially available extrusion 3D printer was developed to calculate the needed pneumatic printing pressures based on the known rheological properties of the gels. Keratin hydrogel constructs, of varying keratose/kerateine ratios and total keratin weight percentages, were 3D printed in cylindrical geometries via extrusion 3D printing. Rheology and degradation studies showed that gels with greater relative kerateine content exhibited greater flow resistance and slower degradation kinetics when submerged in phosphate buffered saline solution at 37 °C, owing to the presence of cysteine residues in kerateine and the capability of forming disulfide bonds. Total keratin weight percentage was found to influence gel yield stress, with possible implications for tuning filament fidelity. Findings from this work support the use of keratose/kerateine ratio and total keratin weight percentage as handles for modulating rheological characteristics of keratin hydrogels to enhance printability and control scaffold properties.

Revealing global risks of labor abuse and illegal, unreported, and unregulated fishing.

Labor abuse on fishing vessels and illegal, unreported and unregulated (IUU) fishing violate human rights, jeopardize food security, and deprive governments of revenues. We applied a multi-method approach, combining new empirical data with satellite information on fishing activities and vessel characteristics to map risks of labor abuse and IUU fishing, understand their relationships, and identify major drivers. Port risks were globally pervasive and often coupled, with 57% of assessed ports associated with labor abuse or IUU fishing. For trips ending in assessed ports, 82% were linked to labor abuse or IUU fishing risks. At-sea risk areas were primarily driven by fishing vessel flags linked to poor control of corruption by the flag state, high ownership by countries other than the flag state, and Chinese-flagged vessels. Transshipment risk areas were related to the gear type of fishing vessels engaged in potential transshipment and carrier vessel flags. Measures at port offer promise for mitigating risks, through the Port State Measures Agreement for IUU fishing, and ensuring sufficient vessel time at port to detect and respond to labor abuse. Our results highlight the need for coordinated action across actors to avoid risk displacement and make progress towards eliminating these socially, environmentally and economically unsustainable practices.

Aromatic foldamers as molecular springs in network polymers.

Polymer networks crosslinked with spring-like ortho-phenylene (oP) foldamers were developed. NMR analysis indicated the oP crosslinkers were well-folded. Polymer networks with oP-based crosslinkers showed enhanced energy dissipation and elasticity compared to divinylbenzene crosslinked networks. The energy dissipation was attributed to the strain-induced reversible unfolding of the oP units. Energy dissipation increased with the number of helical turns in the foldamer.

Risk of forced labour embedded in the US fruit and vegetable supply.

Sustainable food consumption studies have largely focused on promoting human health within ecological limits. Less attention has been paid to social sustainability, in part because of limited data and models. Globally, agriculture has one of the highest incidences of forced labour, with exploitative conditions enabled by low margins, domestic labour scarcity, inadequate legal protections for workers and high labour requirements. Here we assess the forced labour risk embedded in the US retail supply of fruits and vegetables using distinct datasets and a new forced labour risk scoring method. We demonstrate that there is risk of forced labour in a broad set of fruit and vegetable commodities, with a small number of commodities accounting for a substantial fraction of total risk at the retail supply level. These findings signal potential trade-offs and synergies across dimensions of food system sustainability and the need for novel research approaches to develop evidence-based forced labour risk mitigation strategies.

Impact of tumor-parenchyma biomechanics on liver metastatic progression: a multi-model approach.

Colorectal cancer and other cancers often metastasize to the liver in later stages of the disease, contributing significantly to patient death. While the biomechanical properties of the liver parenchyma (normal liver tissue) are known to affect tumor cell behavior in primary and metastatic tumors, the role of these properties in driving or inhibiting metastatic inception remains poorly understood, as are the longer-term multicellular dynamics. This study adopts a multi-model approach to study the dynamics of tumor-parenchyma biomechanical interactions during metastatic seeding and growth. We employ a detailed poroviscoelastic model of a liver lobule to study how micrometastases disrupt flow and pressure on short time scales. Results from short-time simulations in detailed single hepatic lobules motivate constitutive relations and biological hypotheses for a minimal agent-based model of metastatic growth in centimeter-scale tissue over months-long time scales. After a parameter space investigation, we find that the balance of basic tumor-parenchyma biomechanical interactions on shorter time scales (adhesion, repulsion, and elastic tissue deformation over minutes) and longer time scales (plastic tissue relaxation over hours) can explain a broad range of behaviors of micrometastases, without the need for complex molecular-scale signaling. These interactions may arrest the growth of micrometastases in a dormant state and prevent newly arriving cancer cells from establishing successful metastatic foci. Moreover, the simulations indicate ways in which dormant tumors could "reawaken" after changes in parenchymal tissue mechanical properties, as may arise during aging or following acute liver illness or injury. We conclude that the proposed modeling approach yields insight into the role of tumor-parenchyma biomechanics in promoting liver metastatic growth, and advances the longer term goal of identifying conditions to clinically arrest and reverse the course of late-stage cancer.

3D-Printed Self-Healing Elastomers for Modular Soft Robotics.

Advances in materials, designs, and controls are propelling the field of soft robotics at an incredible rate; however, current methods for prototyping soft robots remain cumbersome and struggle to incorporate desirable geometric complexity. Herein, a vat photopolymerizable self-healing elastomer system capable of extreme elongations up to 1000% is presented. The material is formed from a combination of thiol/acrylate mixed chain/step-growth polymerizations and uses a combination of physical processes and dynamic-bond exchange via thioethers to achieve full self-healing capacity over multiple damage/healing cycles. These elastomers can be three dimensional (3D) printed with modular designs capable of healing together to form highly complex and large functional soft robots. Additionally, these materials show reprogrammable resting shapes and compatibility with self-healing liquid metal electronics. Using these capabilities, subcomponents with multiple internal channel systems were printed, healed together, and combined with functional liquid metals to form a high-wattage pneumatic switch and a humanoid-scale soft robotic gripper. The combination of 3D printing and self-healing elastomeric materials allows for facile production of support-free parts with extreme complexity, resulting in a paradigm shift for the construction of modular soft robotics.

Anilinium Salts in Polymer Networks for Materials with Mechanical Stability and Mild Thermally Induced Dynamic Properties.

Dynamic nucleophilic exchange of quaternary anilinium salts has been incorporated into rehealable and malleable polymeric materials that can be activated under mild (60 °C) thermal stimulus. The mechanism of dynamic exchange between quaternary anilinium salt and free aniline was assessed in small-molecule model experiments. The dynamic exchange was found to be dissociative in nature, due to the indirect SN2 mechanism, where initially the bromide anion attacks the anilinium salt to generate an alkyl bromide which undergoes subsequent attack by a free aniline group. A quaternary anilinium-based cross-linker was synthesized to act as dynamic linkages in the polymer network. Cross-linked polymeric materials showed thermoresponsive rehealing and malleability properties at 60 °C along with being resistant to irreversible creep under ambient conditions. The use of anilinium salts enables dynamic exchange to occur with significantly milder thermal stimulus than other comparable materials, while maintaining mechanical stability.

Chemically fueled covalent crosslinking of polymer materials.

Transiently crosslinked dynamic polymer networks are developed, using carbodiimide hydration to link carboxylic acids as anhydrides. From aqueous polymer solutions, non-equilibrium hydrogels are transiently formed, which dissolve upon anhydride hydrolysis. The materials can be refueled using a subsequent injection of carbodiimide. The gels exhibit higher storage moduli compared to transient supramolecular gels as a result of their covalent crosslinks.