Publications by authors named "Alfred J Crosby"

64 Publications

Autonomous snapping and jumping polymer gels.

Nat Mater 2021 Feb 1. Epub 2021 Feb 1.

Polymer Science & Engineering Department, University of Massachusetts, Amherst, MA, USA.

Snap-through buckling is commonly used in nature for power-amplified movements. While natural examples such as Utricularia and Dionaea muscipula can autonomously reset their snapping structures, bio-inspired analogues require external mediation for sequential snap events. Here we report the design principles for self-repeating, snap-based polymer jumping devices. Transient shape changes during the drying of a polymer gel are exploited to generate mechanical constraint and an internal driving force for snap-through buckling. Snap-induced shape changes alter environmental interactions to realize multiple, self-repeating snap events. The underlying mechanisms are understood through controlled experiments and numerical modelling. Using these lessons, we create snap-induced jumping devices with power density outputs (specific power ≈ 312 W kg) that are similar to high-performing jumping organisms and engineered robots. These results provide the demonstration of an autonomous, self-repeating, high-speed movement, marking an important advance in the development of environmental energy harvesting, high-power motion that is important for microscale robots and actuated devices.
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http://dx.doi.org/10.1038/s41563-020-00909-wDOI Listing
February 2021

Localized characterization of brain tissue mechanical properties by needle induced cavitation rheology and volume controlled cavity expansion.

J Mech Behav Biomed Mater 2021 02 26;114:104168. Epub 2020 Oct 26.

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA; Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA. Electronic address:

Changes in the elastic properties of brain tissue have been correlated with injury, cancers, and neurodegenerative diseases. However, discrepancies in the reported elastic moduli of brain tissue are persistent, and spatial inhomogeneities complicate the interpretation of macroscale measurements such as rheology. Here we introduce needle induced cavitation rheology (NICR) and volume-controlled cavity expansion (VCCE) as facile methods to measure the apparent Young's modulus E of minimally manipulated brain tissue, at specific tissue locations and with sub-millimeter spatial resolution. For different porcine brain regions and sections analyzed by NICR, we found E to be 3.7 ± 0.7 kPa and 4.8 ± 1.0 kPa for gray matter, and white matter, respectively. For different porcine brain regions and sections analyzed by VCCE, we found E was 0.76 ± 0.02 kPa for gray matter and 0.92 ± 0.01 kPa for white matter. Measurements from VCCE were more similar to those obtained from macroscale shear rheology (0.75 ± 0.06 kPa) and from instrumented microindentation of white matter (0.97 ± 0.40 kPa) and gray matter (0.86 ± 0.20 kPa). We attributed the higher stiffness reported from NICR to that method's assumption of a cavitation instability due to a neo-Hookean constitutive response, which does not capture the strain-stiffening behavior of brain tissue under large strains, and therefore did not provide appropriate measurements. We demonstrate via both analytical modeling of a spherical cavity and finite element modeling of a needle geometry, that this strain stiffening may prevent a cavitation instability. VCCE measurements take this stiffening behavior into account by employing an incompressible one-term Ogden model to find the nonlinear elastic properties of the tissue. Overall, VCCE afforded rapid and facile measurement of nonlinear mechanical properties of intact, healthy mammalian brain tissue, enabling quantitative comparison among brain tissue regions and also between species. Finally, accurate estimation of elastic properties for this strain stiffening tissue requires methods that include appropriate constitutive models of the brain tissue response, which here are represented by inclusion of the Ogden model in VCCE.
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http://dx.doi.org/10.1016/j.jmbbm.2020.104168DOI Listing
February 2021

Seeded laser-induced cavitation for studying high-strain-rate irreversible deformation of soft materials.

Soft Matter 2020 Aug 20. Epub 2020 Aug 20.

Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 01003, USA.

Characterizing the high-strain-rate and high-strain mechanics of soft materials is critical to understanding the complex behavior of polymers and various dynamic injury mechanisms, including traumatic brain injury. However, their dynamic mechanical deformation under extreme conditions is technically difficult to quantify and often includes irreversible damage. To address such challenges, we investigate an experimental method, which allows quantification of the extreme mechanical properties of soft materials using ultrafast stroboscopic imaging of highly reproducible laser-induced cavitation events. As a reference material, we characterize variably cross-linked polydimethylsiloxane specimens using this method. The consistency of the laser-induced cavitation is achieved through the introduction of laser absorbing seed microspheres. Based on a simplified viscoelastic model, representative high-strain-rate shear moduli and viscosities of the soft specimens are quantified across different degrees of crosslinking. The quantified rheological parameters align well with the time-temperature superposition prediction of dynamic mechanical analysis. The presented method offers significant advantages with regard to quantifying high-strain rate, irreversible mechanical properties of soft materials and tissues, compared to other methods that rely upon the cyclic dynamics of cavitation. These advances are anticipated to aid in the understanding of how damage and injury develop in soft materials and tissues.
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http://dx.doi.org/10.1039/d0sm00710bDOI Listing
August 2020

Programming Impulsive Deformation with Mechanical Metamaterials.

Phys Rev Lett 2020 Sep;125(10):108002

Polymer Science and Engineering Department, University of Massachusetts Amherst, Amherst, Massachusetts 01003, USA.

Impulsive deformation is widely observed in biological systems to generate movement with high acceleration and velocity. By storing elastic energy in a quasistatic loading and releasing it through an impulsive elastic recoil, organisms circumvent the intrinsic trade-off between force and velocity and achieve power amplified motion. However, such asymmetry in strain rate in loading and unloading often results in reduced efficiency in converting elastic energy to kinetic energy for homogeneous materials. Here, we demonstrate that specific internal structural designs can offer the ability to tune quasistatic and high-speed recoil independently to control energy storage and conversion processes. Experimental demonstrations with mechanical metamaterials reveal that certain internal structures optimize energy conversion far beyond unstructured materials under the same conditions. Our results provide the first quantitative model and experimental demonstration for tuning energy conversion processes through internal structures of metamaterials.
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http://dx.doi.org/10.1103/PhysRevLett.125.108002DOI Listing
September 2020

Low-Voltage Reversible Electroadhesion of Ionoelastomer Junctions.

Adv Mater 2020 Jun 17;32(25):e2000600. Epub 2020 May 17.

Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA, 01003, USA.

Electroadhesion provides a simple route to rapidly and reversibly control adhesion using applied electric potentials, offering promise for a variety of applications including haptics and robotics. Current electroadhesives, however, suffer from key limitations associated with the use of high operating voltages (>kV) and corresponding failure due to dielectric breakdown. Here, a new type of electroadhesion based on heterojunctions between iono-elastomer of opposite polarity is demonstrated, which can be operated at potentials as low as ≈1 V. The large electric field developed across the molecular-scale ionic double layer (IDL) when the junction is placed under reverse bias allows for strong adhesion at low voltages. In contrast, under forward bias, the electric field across the IDL is destroyed, substantially lowering the adhesion in a reversible fashion. These ionoelastomer electroadhesives are highly efficient with respect to the force capacity per electrostatic capacitive energy and are robust to defects or damage that typically lead to catastrophic failure of conventional dielectric electroadhesives. The findings provide new fundamental insight into low-voltage electroadhesion and broaden its possible applications.
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http://dx.doi.org/10.1002/adma.202000600DOI Listing
June 2020

Cavitation in soft matter.

Proc Natl Acad Sci U S A 2020 04 14;117(17):9157-9165. Epub 2020 Apr 14.

Polymer Science & Engineering Department, University of Massachusetts, Amherst, MA 01003;

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http://dx.doi.org/10.1073/pnas.1920168117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7196784PMC
April 2020

Micromechanical Properties of Microstructured Elastomeric Hydrogels.

Macromol Biosci 2020 05 1;20(5):e1900360. Epub 2020 Apr 1.

Polymer Science and Engineering Department, University of Massachusetts Amherst, 120 Governors Drive, Amherst, MA, 01003, USA.

Local, micromechanical environment is known to influence cellular function in heterogeneous hydrogels, and knowledge gained in micromechanics will facilitate the improved design of biomaterials for tissue regeneration. In this study, a system comprising microstructured resilin-like polypeptide (RLP)-poly(ethylene glycol) (PEG) hydrogels is utilized. The micromechanical properties of RLP-PEG hydrogels are evaluated with oscillatory shear rheometry, compression dynamic mechanic analysis, small-strain microindentation, and large-strain indentation and puncture over a range of different deformation length scales. The measured elastic moduli are consistent with volume averaging models, indicating that volume fraction, not domain size, plays a dominant role in determining the low strain mechanical response. Large-strain indentation under a confocal microscope enables the visualization of the microstructured hydrogel micromechanical deformation, emphasizing the translation, rotation, and deformation of RLP-rich domains. The fracture initiation energy results demonstrate that failure of the composite hydrogels is controlled by the RLP-rich phase, and their independence with domain size suggested that failure initiation is controlled by multiple domains within the strained volume. This approach and findings provide new quantitative insight into the micromechanical response of soft hydrogel composites and highlight the opportunities in employing these methods to understand the physical origins of mechanical properties of soft synthetic and biological materials.
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http://dx.doi.org/10.1002/mabi.201900360DOI Listing
May 2020

Control of Astrocyte Quiescence and Activation in a Synthetic Brain Hydrogel.

Adv Healthc Mater 2020 02 15;9(4):e1901419. Epub 2020 Jan 15.

Department of Chemical Engineering, University of Massachusetts, Amherst, MA, 01003, USA.

Bioengineers have designed numerous instructive brain extracellular matrix (ECM) environments with tailored and tunable protein compositions and biomechanical properties in vitro to study astrocyte reactivity during trauma and inflammation. However, a major limitation of both protein-based and synthetic model microenvironments is that astrocytes within fail to retain their characteristic stellate morphology and quiescent state without becoming activated under "normal" culture conditions. Here, a synthetic hydrogel is introduced, which for the first time demonstrates maintenance of astrocyte quiescence and activation on demand. With this synthetic brain hydrogel, the brain-specific integrin-binding and matrix metalloprotease-degradable domains of proteins are shown to control astrocyte star-shaped morphologies, and an ECM condition that maintains astrocyte quiescence with minimal activation can be achieved. In addition, activation can be induced in a dose-dependent manner via both defined cytokine cocktails and low molecular weight hyaluronic acid. This synthetic brain hydrogel is envisioned as a new tool to study the physiological role of astrocytes in health and disease.
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http://dx.doi.org/10.1002/adhm.201901419DOI Listing
February 2020

The effect of size-scale on the kinematics of elastic energy release.

Soft Matter 2019 Nov;15(46):9579-9586

Department of Physics, Harvey Mudd College, Claremont, CA 91711, USA.

Elastically-driven motion has been used as a strategy to achieve high speeds in small organisms and engineered micro-robotic devices. We examine the size-scaling relations determining the limit of elastic energy release from elastomer bands that efficiently cycle mechanical energy with minimal loss. The maximum center-of-mass velocity of the elastomer bands was found to be size-scale independent, while smaller bands demonstrated larger accelerations and shorter durations of elastic energy release. Scaling relationships determined from these measurements are consistent with the performance of small organisms and engineered devices which utilize elastic elements to power motion.
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http://dx.doi.org/10.1039/c9sm00870eDOI Listing
November 2019

Residual strain effects in needle-induced cavitation.

Soft Matter 2019 Sep;15(37):7390-7397

Polymer Science and Engineering Department, University of Massachusetts, 120 Governors Drive, Amherst, MA 01003, USA.

Needle-induced cavitation (NIC) locally probes the elastic and fracture properties of soft materials, such as gels and biological tissues. Current NIC protocols tend to overestimate properties when compared to traditional techniques. New NIC methods are needed in order to address this issue. NIC measurements consist of two distinct processes, namely (1) the needle insertion process and (2) the cavitation process. The cavitation process is hypothesized to be highly dependent on the initial needle insertion process due to the influence of residual strain below the needle. Retracting the needle before pressurization to a state in which a cylindrical, tube-like fracture is left below the needle tip is experimentally demonstrated to reduce the impact of residual strain on NIC. Verification of the critical cavitation pressure equation in this new geometry is necessary before implementing this retraction NIC protocol. Complementary modeling shows that the change in initial geometry has little effect on the critical cavitation pressure. Together, these measurements demonstrate that needle retraction is a viable experimental protocol for reducing the influence of residual strain, thus enabling the confident measurement of local elastic and fracture properties in soft gels and tissues.
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http://dx.doi.org/10.1039/c9sm01173kDOI Listing
September 2019

Macroscopic Geometry-Dominated Orientation of Symmetric Microwrinkle Patterns.

ACS Appl Mater Interfaces 2019 Jul 14;11(26):23741-23749. Epub 2019 Jun 14.

Department of Polymer Science and Engineering , University of Massachusetts Amherst , Amherst , Massachusetts 01003 , United States.

Orientated wrinkle patterns with controlled microarchitectures are highly attractive because of their potential and broad application in technologies ranging from flexible electronic devices to smart windows. Here, we demonstrate a macroscopic, geometry-dominated strategy to fabricate symmetric microwrinkles with precisely controllable pattern dimensions and orientations through a dynamic interfacial release process. The release-induced approach is based on the release of multilayer elastomer composites from polymeric sacrificial layers in solutions combined with crosslinking-induced contraction of the elastomer substrates. Crosslinking-induced contraction provides the driving force for developing and stabilizing surface wrinkle formation, whereas the polymeric sacrificial layer provides a mild and simultaneous release process to form orientated wrinkles through kinetic control of local strain development. The macroscopic shape of the composite controls release kinetics, hence strain history, leading to the generation of photonic reflective surfaces. Moreover, stable wrinkles fabricated from various materials including metals, ceramics, and carbons can be achieved. This versatile, mold-free, and cost-effective platform technology demonstrates how global strain distributions can be harnessed through kinetics to drive local pattern development.
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http://dx.doi.org/10.1021/acsami.9b05264DOI Listing
July 2019

Cross-platform mechanical characterization of lung tissue.

PLoS One 2018 17;13(10):e0204765. Epub 2018 Oct 17.

Department of Chemical Engineering, University of Massachusetts, Amherst, Amherst, MA, United States of America.

Published data on the mechanical strength and elasticity of lung tissue is widely variable, primarily due to differences in how testing was conducted across individual studies. This makes it extremely difficult to find a benchmark modulus of lung tissue when designing synthetic extracellular matrices (ECMs). To address this issue, we tested tissues from various areas of the lung using multiple characterization techniques, including micro-indentation, small amplitude oscillatory shear (SAOS), uniaxial tension, and cavitation rheology. We report the sample preparation required and data obtainable across these unique but complimentary methods to quantify the modulus of lung tissue. We highlight cavitation rheology as a new method, which can measure the modulus of intact tissue with precise spatial control, and reports a modulus on the length scale of typical tissue heterogeneities. Shear rheology, uniaxial, and indentation testing require heavy sample manipulation and destruction; however, cavitation rheology can be performed in situ across nearly all areas of the lung with minimal preparation. The Young's modulus of bulk lung tissue using micro-indentation (1.4±0.4 kPa), SAOS (3.3±0.5 kPa), uniaxial testing (3.4±0.4 kPa), and cavitation rheology (6.1±1.6 kPa) were within the same order of magnitude, with higher values consistently reported from cavitation, likely due to our ability to keep the tissue intact. Although cavitation rheology does not capture the non-linear strains revealed by uniaxial testing and SAOS, it provides an opportunity to measure mechanical characteristics of lung tissue on a microscale level on intact tissues. Overall, our study demonstrates that each technique has independent benefits, and each technique revealed unique mechanical features of lung tissue that can contribute to a deeper understanding of lung tissue mechanics.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0204765PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6192579PMC
March 2019

Micromechanical characterization of soft, biopolymeric hydrogels: stiffness, resilience, and failure.

Soft Matter 2018 May;14(18):3478-3489

Polymer Science and Engineering Department, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, USA.

Detailed understanding of the local structure-property relationships in soft biopolymeric hydrogels can be instrumental for applications in regenerative tissue engineering. Resilin-like polypeptide (RLP) hydrogels have been previously demonstrated as useful biomaterials with a unique combination of low elastic moduli, excellent resilience, and cell-adhesive properties. However, comprehensive mechanical characterization of RLP hydrogels under both low-strain and high-strain conditions has not yet been conducted, despite the unique information such measurements can provide about the local structure and macromolecular behavior underpinning mechanical properties. In this study, mechanical properties (elastic modulus, resilience, and fracture initiation toughness) of equilibrium swollen resilin-based hydrogels were characterized via oscillatory shear rheology, small-strain microindentation, and large-strain puncture tests as a function of polypeptide concentration. These methods allowed characterization, for the first time, of the resilience and failure in hydrogels with low polypeptide concentrations (<20 wt%), as the employed methods obviate the handling difficulties inherent in the characterization of such soft materials via standard mechanical techniques, allowing characterization without any special sample preparation and requiring minimal volumes (as low as 50 μL). Elastic moduli measured from small-strain microindentation showed good correlation with elastic storage moduli obtained from oscillatory shear rheology at a comparable applied strain rate, and evaluation of multiple loading-unloading cycles revealed decreased resilience values at lower hydrogel concentrations. In addition, large-strain indentation-to-failure (or puncture) tests were performed to measure large-strain mechanical response and fracture toughness on length scales similar to biological cells (∼10-50 μm) at various polypeptide concentrations, indicating very high fracture initiation toughness for high-concentration hydrogels. Our results establish the utility of employing microscale mechanical methods for the characterization of the local mechanical properties of biopolymeric hydrogels of low concentrations (<20 wt%), and show how the combination of small and large-strain measurements can provide unique insight into structure-property relationships for biopolymeric elastomers. Overall, this study provides new insight into the effects on local mechanical properties of polypeptide concentration near the overlap polymer concentration c* for resilin-based hydrogels, confirming their unique elastomeric features for applications in regenerative medicine.
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http://dx.doi.org/10.1039/c8sm00501jDOI Listing
May 2018

The principles of cascading power limits in small, fast biological and engineered systems.

Science 2018 04;360(6387)

Department of Biology, Duke University, Durham, NC 27708, USA.

Mechanical power limitations emerge from the physical trade-off between force and velocity. Many biological systems incorporate power-enhancing mechanisms enabling extraordinary accelerations at small sizes. We establish how power enhancement emerges through the dynamic coupling of motors, springs, and latches and reveal how each displays its own force-velocity behavior. We mathematically demonstrate a tunable performance space for spring-actuated movement that is applicable to biological and synthetic systems. Incorporating nonideal spring behavior and parameterizing latch dynamics allows the identification of critical transitions in mass and trade-offs in spring scaling, both of which offer explanations for long-observed scaling patterns in biological systems. This analysis defines the cascading challenges of power enhancement, explores their emergent effects in biological and engineered systems, and charts a pathway for higher-level analysis and synthesis of power-amplified systems.
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http://dx.doi.org/10.1126/science.aao1082DOI Listing
April 2018

Mesoscale Block Copolymers.

Adv Mater 2018 Mar 30;30(13):e1706118. Epub 2018 Jan 30.

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Amherst, MA, 01003-9263, USA.

Materials composed of well-defined mesoscale building blocks are ubiquitous in nature, with noted ability to assemble into hierarchical structures possessing exceptional physical and mechanical properties. Fabrication of similar synthetic mesoscale structures will offer opportunities for precise conformational tuning toward advantageous bulk properties, such as increased toughness or elastic modulus. This requires new materials designs to be discovered to impart such structural control. Here, the preparation of mesoscale polymers is achieved by solution fabrication of functional polymers containing photoinduced chemical triggers. Subsequent photopatterning affords mesoscale block copolymers composed of distinct segments of alternating chemical composition. When dispersed in appropriate solvents, selected segments form helices to generate architectures resembling block copolymers, but on an optically observable size scale. This approach provides a platform for producing mesoscale geometries with structural control and potential for driving materials assembly comparable to examples found in nature.
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http://dx.doi.org/10.1002/adma.201706118DOI Listing
March 2018

How Ligands Affect Resistive Switching in Solution-Processed HfO Nanoparticle Assemblies.

ACS Appl Mater Interfaces 2018 Feb 29;10(5):4824-4830. Epub 2018 Jan 29.

Department of Mechanical and Industrial Engineering, University of Massachusetts Amherst , 160 Governors Drive, 219 Engineering Laboratory I, Amherst, Massachusetts 01003, United States.

Advancement of resistive random access memory (ReRAM) requires fully understanding the various complex, defect-mediated transport mechanisms to further improve performance. Although thin-film oxide materials have been extensively studied, the switching properties of nanoparticle assemblies remain underexplored due to difficulties in fabricating ordered structures. Here, we employ a simple flow coating method for the facile deposition of highly ordered HfO nanoparticle nanoribbon assemblies. The resistive switching character of nanoribbons was determined to correlate directly with the organic capping layer length of their constituting HfO nanoparticles, using oleic acid, dodecanoic acid, and undecenoic acid as model nanoparticle ligands. Through a systematic comparison of the forming process, operating set/reset voltages, and resistance states, we demonstrate a tunable resistive switching response by varying the ligand type, thus providing a base correlation for solution-processed ReRAM device fabrication.
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http://dx.doi.org/10.1021/acsami.7b17376DOI Listing
February 2018

Achieving high aspect ratio wrinkles by modifying material network stress.

Soft Matter 2017 Jun;13(22):4142-4147

Polymer Science and Engineering Department, University of Massachusetts, 120 Governors Drive, Amherst, MA 01003, USA.

Wrinkle aspect ratio, or the amplitude divided by the wavelength, is hindered by strain localization transitions when an increasing global compressive stress is applied to synthetic material systems. However, many examples from living organisms show extremely high aspect ratios, such as gut villi and flower petals. We use three experimental approaches to demonstrate that these high aspect ratio structures can be achieved by modifying the network stress in the wrinkle substrate. We modify the wrinkle stress and effectively delay the strain localization transition, such as folding, to larger aspect ratios by using a zero-stress initial wavy substrate, creating a secondary network with post-curing, or using chemical stress relaxation materials. A wrinkle aspect ratio as high as 0.85, almost three times higher than common values of synthetic wrinkles, is achieved, and a quantitative framework is presented to provide understanding the different strategies and predictions for future investigations.
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http://dx.doi.org/10.1039/c7sm00469aDOI Listing
June 2017

Transferable Memristive Nanoribbons Comprising Solution-Processed Strontium Titanate Nanocubes.

ACS Appl Mater Interfaces 2017 Mar 16;9(12):10847-10854. Epub 2017 Mar 16.

Department of Mechanical and Industrial Engineering, ‡Polymer Science and Engineering Department, and §Department of Chemistry, University of Massachusetts Amherst , Amherst, Massachusetts 01003, United States.

Memristors, often comprising an insulating metal oxide film between two metal electrodes (MIM), constitute a class of two-terminal devices that possesses tunable variations in resistance based on the applied bias history. Intense research remains focused on the metal-insulator interface, which serves as the crux of coupled electronic-ionic interactions and dictates the underpinning transport mechanisms at either electrode. Top-down, ultrahigh-vacuum (UVH) deposition approaches for MIM nanostructures yield highly crystalline, heteroepitaxial interfaces but limit the number of electrode configurations due to a fixed bottom electrode. Here we report on the convective self-assembly, removal, and transfer of individual nanoribbons comprising solution-processed, single-crystalline strontium titanate (STO) perovskite oxide nanocrystals to arbitrary metallized substrates. Nanoribbon transferability enables changes in transport models ranging from interfacial trap-detrap to electrochemical metallization processes. We also demonstrate the endurance of memristive behavior, including switching ratios up to 10, after nanoribbon redeposition onto poly(ethylene terephthalate) (PET) flexible substrates. The combination of ambient, aerobic prepared nanocrystals and convective self-assembly deposition herein provides a pathway for facile, scalable manufacturing of high quality, functional oxide nanostructures on arbitrary surfaces and topologies.
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http://dx.doi.org/10.1021/acsami.7b00220DOI Listing
March 2017

Functional droplets that recognize, collect, and transport debris on surfaces.

Sci Adv 2016 Oct 28;2(10):e1601462. Epub 2016 Oct 28.

Polymer Science and Engineering Department, University of Massachusetts, 120 Governors Drive, Amherst, MA 01003, USA.

We describe polymer-stabilized droplets capable of recognizing and picking up nanoparticles from substrates in experiments designed for transporting hydroxyapatite nanoparticles that represent the principal elemental composition of bone. Our experiments, which are inspired by cells that carry out materials transport in vivo, used oil-in-water droplets that traverse a nanoparticle-coated substrate driven by an imposed fluid flow. Nanoparticle capture is realized by interaction of the particles with chemical functionality embedded within the polymeric stabilizing layer on the droplets. Nanoparticle uptake efficiency is controlled by solution conditions and the extent of functionality available for contact with the nanoparticles. Moreover, in an elementary demonstration of nanoparticle transportation, particles retrieved initially from the substrate were later deposited "downstream," illustrating a pickup and drop-off technique that represents a first step toward mimicking point-to-point transportation events conducted in living systems.
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http://dx.doi.org/10.1126/sciadv.1601462DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5091362PMC
October 2016

Elastic cavitation and fracture via injection.

Soft Matter 2016 Mar;12(9):2557-66

Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, MA, USA.

The cavitation rheology technique extracts soft materials mechanical properties through pressure-monitored fluid injection. Properties are calculated from the system's response at a critical pressure that is governed by either elasticity or fracture (or both); however previous elementary analysis has not been capable of accurately determining which mechanism is dominant. We combine analyses of both mechanisms in order to determine how the full system thermodynamics, including far-field compliance, dictate whether a bubble in an elastomeric solid will grow through either reversible or irreversible deformations. Applying these analyses to experimental data, we demonstrate the sensitivity of cavitation rheology to microstructural variation via a co-dependence between modulus and fracture energy.
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http://dx.doi.org/10.1039/c5sm02055gDOI Listing
March 2016

Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing.

Proc Natl Acad Sci U S A 2016 Feb 19;113(5):1297-302. Epub 2016 Jan 19.

Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, United Kingdom;

Organismal functions are size-dependent whenever body surfaces supply body volumes. Larger organisms can develop strongly folded internal surfaces for enhanced diffusion, but in many cases areas cannot be folded so that their enlargement is constrained by anatomy, presenting a problem for larger animals. Here, we study the allometry of adhesive pad area in 225 climbing animal species, covering more than seven orders of magnitude in weight. Across all taxa, adhesive pad area showed extreme positive allometry and scaled with weight, implying a 200-fold increase of relative pad area from mites to geckos. However, allometric scaling coefficients for pad area systematically decreased with taxonomic level and were close to isometry when evolutionary history was accounted for, indicating that the substantial anatomical changes required to achieve this increase in relative pad area are limited by phylogenetic constraints. Using a comparative phylogenetic approach, we found that the departure from isometry is almost exclusively caused by large differences in size-corrected pad area between arthropods and vertebrates. To mitigate the expected decrease of weight-specific adhesion within closely related taxa where pad area scaled close to isometry, data for several taxa suggest that the pads' adhesive strength increased for larger animals. The combination of adjustments in relative pad area for distantly related taxa and changes in adhesive strength for closely related groups helps explain how climbing with adhesive pads has evolved in animals varying over seven orders of magnitude in body weight. Our results illustrate the size limits of adhesion-based climbing, with profound implications for large-scale bio-inspired adhesives.
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http://dx.doi.org/10.1073/pnas.1519459113DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4747726PMC
February 2016

The functional significance of morphological changes in the dentitions of early mammals.

J R Soc Interface 2016 11;13(124)

Department of Biology, University of Massachusetts Amherst, Amherst, MA 01003, USA.

The Mesozoic marked a time of experimentation in the tooth morphology of early mammals. One particular experiment involved the movement of three points, or cusps, on the surface of a molar tooth from a line into a triangle. This transition is exemplified by two extinct insectivorous mammals, (cusps in a line) and (cusps in a triangle). Here we test whether this difference in cusp arrangement, alongside cusp heights and angles between cusps, is associated with differences in the ability of the teeth to fracture proxy-insect prey. We gathered measurements from molar teeth of both species and used them to create physical models. We then measured the force, time and energy at fracture and peak force, and the amount of damage inflicted by the models on hard and soft gels encased in a tough film that mimicked the material properties of insects. The model required less force and energy to fracture hard gels and reach peak force compared with required a similar time, force and energy to fracture soft gels but reduced the time, force and energy to reach peak force. More importantly, also inflicted more damage to both the hard and the soft gels. These results suggest that changes in dental morphology in some early mammals was driven primarily by selection for maximizing damage, and secondarily for maximizing biomechanical efficiency for a given food material property.
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http://dx.doi.org/10.1098/rsif.2016.0713DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5134021PMC
November 2016

Optimizing Adhesive Design by Understanding Compliance.

ACS Appl Mater Interfaces 2015 Dec 10;7(50):27771-81. Epub 2015 Dec 10.

Polymer Science and Engineering Department, University of Massachusetts Amherst , Amherst, Massachusetts 01003, United States.

Adhesives have long been designed around a trade-off between adhesive strength and releasability. Geckos are of interest because they are the largest organisms which are able to climb utilizing adhesive toepads, yet can controllably release from surfaces and perform this action over and over again. Attempting to replicate the hierarchical, nanoscopic features which cover their toepads has been the primary focus of the adhesives field until recently. A new approach based on a scaling relation which states that reversible adhesive force capacity scales with (A/C)(1/2), where A is the area of contact and C is the compliance of the adhesive, has enabled the creation of high strength, reversible adhesives without requiring high aspect ratio, fibrillar features. Here we introduce an equation to calculate the compliance of adhesives, and utilize this equation to predict the shear adhesive force capacity of the adhesive based on the material components and geometric properties. Using this equation, we have investigated important geometric parameters which control force capacity and have shown that by controlling adhesive shape, adhesive force capacity can be increased by over 50% without varying pad size. Furthermore, we have demonstrated that compliance of the adhesive far from the interface still influences shear adhesive force capacity. Utilizing this equation will allow for the production of adhesives which are optimized for specific applications in commercial and industrial settings.
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http://dx.doi.org/10.1021/acsami.5b08934DOI Listing
December 2015

Smooth Muscle Stiffness Sensitivity is Driven by Soluble and Insoluble ECM Chemistry.

Cell Mol Bioeng 2015 Sep 28;8(3):333-348. Epub 2015 May 28.

Department of Chemical Engineering, University of Massachusetts, 686 N. Pleasant Street, 159 Goessmann Laboratory, Amherst, MA 01003, USA.

Smooth muscle cell (SMC) invasion into plaques and subsequent proliferation is a major factor in the progression of atherosclerosis. During disease progression, SMCs experience major changes in their microenvironment, such as what integrin-binding sites are exposed, the portfolio of soluble factors available, and the elasticity and modulus of the surrounding vessel wall. We have developed a hydrogel biomaterial platform to examine the combined effect of these changes on SMC phenotype. We were particularly interested in how the chemical microenvironment affected the ability of SMCs to sense and respond to modulus. To our surprise, we observed that integrin binding and soluble factors are major drivers of several critical SMC behaviors, such as motility, proliferation, invasion, and differentiation marker expression, and these factors modulated the effect of stiffness on proliferation and migration. Overall, modulus only modestly affected behaviors other than proliferation, relative to integrin binding and soluble factors. Surprisingly, pathological behaviors (proliferation, motility) are not inversely related to SMC marker expression, in direct conflict with previous studies on substrates coupled with single extracellular matrix (ECM) proteins. A high-throughput bead-based ELISA approach and inhibitor studies revealed that differentiation marker expression is mediated chiefly focal adhesion kinase (FAK) signaling, and we propose that integrin binding and FAK drive the transition from a migratory to a proliferative phenotype. We emphasize the importance of increasing the complexity of testing platforms to capture these subtleties in cell phenotypes and signaling, in order to better recapitulate important features of disease and elucidate potential context-dependent therapeutic targets.
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http://dx.doi.org/10.1007/s12195-015-0397-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4610395PMC
September 2015

Geckos as Springs: Mechanics Explain Across-Species Scaling of Adhesion.

PLoS One 2015 2;10(9):e0134604. Epub 2015 Sep 2.

Graduate Program in Organismic and Evolutionary Biology, University of Massachusetts Amherst, Amherst, MA, 01003, United States of America; Department of Biology, University of Massachusetts at Amherst, Amherst, MA, 01003, United States of America.

One of the central controversies regarding the evolution of adhesion concerns how adhesive force scales as animals change in size, either among or within species. A widely held view is that as animals become larger, the primary mechanism that enables them to climb is increasing pad area. However, prior studies show that much of the variation in maximum adhesive force remains unexplained, even when area is accounted for. We tested the hypothesis that maximum adhesive force among pad-bearing gecko species is not solely dictated by toepad area, but also depends on the ratio of toepad area to gecko adhesive system compliance in the loading direction, where compliance (C) is the change in extension (Δ) relative to a change in force (F) while loading a gecko's adhesive system (C = dΔ/dF). Geckos are well-known for their ability to climb on a range of vertical and overhanging surfaces, and range in mass from several grams to over 300 grams, yet little is understood of the factors that enable adhesion to scale with body size. We examined the maximum adhesive force of six gecko species that vary in body size (~2-100 g). We also examined changes between juveniles and adults within a single species (Phelsuma grandis). We found that maximum adhesive force and toepad area increased with increasing gecko size, and that as gecko species become larger, their adhesive systems become significantly less compliant. Additionally, our hypothesis was supported, as the best predictor of maximum adhesive force was not toepad area or compliance alone, but the ratio of toepad area to compliance. We verified this result using a synthetic "model gecko" system comprised of synthetic adhesive pads attached to a glass substrate and a synthetic tendon (mechanical spring) of finite stiffness. Our data indicate that increases in toepad area as geckos become larger cannot fully account for increased adhesive abilities, and decreased compliance must be included to explain the scaling of adhesion in animals with dry adhesion systems.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0134604PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4558017PMC
May 2016

Deformation and shape of flexible, microscale helices in viscous flow.

Phys Rev E Stat Nonlin Soft Matter Phys 2015 Jul 30;92(1):011004. Epub 2015 Jul 30.

PMMH-ESPCI-ParisTech, UMR 7636 CNRS-ESPCI, Université Pierre et Marie Curie, Université Paris Diderot, 10 rue Vauquelin, 75005 Paris, France.

We examine experimentally the deformation of flexible, microscale helical ribbons with nanoscale thickness subject to viscous flow in a microfluidic channel. Two aspects of flexible microhelices are quantified: the overall shape of the helix and the viscous frictional properties. The frictional coefficients determined by our experiments are consistent with calculated values in the context of resistive-force theory. The deformation of helices by viscous flow is well described by nonlinear finite extensibility. Under distributed loading, the pitch distribution is nonuniform, and from this we identify both linear and nonlinear behavior along the contour length of a single helix. Moreover, flexible helices are found to display reversible global to local helical transitions at a high flow rate.
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http://dx.doi.org/10.1103/PhysRevE.92.011004DOI Listing
July 2015

Mechanics of intact bone marrow.

J Mech Behav Biomed Mater 2015 Oct 2;50:299-307. Epub 2015 Jul 2.

Department of Chemical Engineering, University of Massachusetts Amherst, 686 N Pleasant Street, 159 Goessmann Hall, Amherst, MA 01003, USA. Electronic address:

The current knowledge of bone marrow mechanics is limited to its viscous properties, neglecting the elastic contribution of the extracellular matrix. To get a more complete view of the mechanics of marrow, we characterized intact yellow porcine bone marrow using three different, but complementary techniques: rheology, indentation, and cavitation. Our analysis shows that bone marrow is elastic, and has a large amount of intra- and inter-sample heterogeneity, with an effective Young׳s modulus ranging from 0.25 to 24.7 kPa at physiological temperature. Each testing method was consistent across matched tissue samples, and each provided unique benefits depending on user needs. We recommend bulk rheology to capture the effects of temperature on tissue elasticity and moduli, indentation for quantifying local tissue heterogeneity, and cavitation rheology for mitigating destructive sample preparation. We anticipate the knowledge of bone marrow elastic properties for building in vitro models will elucidate mechanisms involved in disease progression and regenerative medicine.
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http://dx.doi.org/10.1016/j.jmbbm.2015.06.023DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4554886PMC
October 2015

Puncture mechanics of soft solids.

Soft Matter 2015 Jun;11(23):4723-30

Polymer Science and Engineering Department, University of Massachusetts, 120 Governors Drive, Amherst, MA 01003, USA.

Gels and other soft elastic networks are a ubiquitous and important class of materials whose unique properties enable special behavior, but generally elude characterization due to the inherent difficulty in manipulating them. An example of such behavior is the stability of gels to large local deformations on their surface. This paper analyzes puncture of model soft materials with particular focus on the force response to deep indentation and the critical load for material failure. Large-strain behavior during deep indentation is described with a neo-hookean contact model. A fracture process zone model is applied to the critical load for puncture. It is found that the indenter geometry influences the size of the fracture process zone, resulting in two distinct failure regimes: stress-limited and energy-limited. The methods outlined in this paper provide a simple means for measuring Young's modulus, E, as well as the material's maximum cohesive stress, σ0, fracture energy, Γ0, and the intrinsic length scale linking the two, l0, all without requiring specialized sample preparation.
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http://dx.doi.org/10.1039/c5sm00230cDOI Listing
June 2015

Rubrene crystal field-effect mobility modulation via conducting channel wrinkling.

Nat Commun 2015 May 5;6:6948. Epub 2015 May 5.

Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governor's Drive, Amherst, Massachusetts 01003, USA.

With the impending surge of flexible organic electronic technologies, it has become essential to understand how mechanical deformation affects the electrical performance of organic thin-film devices. Organic single crystals are ideal for the systematic study of strain effects on electrical properties without being concerned about grain boundaries and other defects. Here we investigate how the deformation affects the field-effect mobility of single crystals of the benchmark semiconductor rubrene. The wrinkling instability is used to apply local strains of different magnitudes along the conducting channel in field-effect transistors. We discover that the mobility changes as dictated by the net strain at the dielectric/semiconductor interface. We propose a model based on the plate bending theory to quantify the net strain in wrinkled transistors and predict the change in mobility. These contributions represent a significant step forward in structure-function relationships in organic semiconductors, critical for the development of the next generation of flexible electronic devices.
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http://dx.doi.org/10.1038/ncomms7948DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4432628PMC
May 2015

Tunable elastic modulus of nanoparticle monolayer films by host-guest chemistry.

Adv Mater 2014 Aug 2;26(29):5056-61. Epub 2014 Jun 2.

Department of Chemistry, University of Massachusetts, 710 North Pleasant Street, Amherst, Massachusetts, 01003, USA.

The elastic modulus of an ultrathin nanoparticle (NP) monolayer film is tuned by modulating the binding strength between the NPs on a molecular level. NP monolayer films constructed by crosslinking NPs of different binding affinities are fabricated at oil/water interfaces. By inducing buckling patterns on these films, the correlation between the binding affinity of the NPs and the elastic modulus is investigated.
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http://dx.doi.org/10.1002/adma.201401226DOI Listing
August 2014