Publications by authors named "Michael D Bartlett"

24 Publications

  • Page 1 of 1

Cancer cell migration in collagen-hyaluronan composite extracellular matrices.

Acta Biomater 2021 08 8;130:183-198. Epub 2021 Jun 8.

Department of Chemical and Biological Engineering, Iowa State University, United States; Department of Genetics, Development and Cell Biology, Iowa State University, United States. Electronic address:

Hyaluronan (HA) is a key component in the tumor microenvironment (TME) that participates in cancer growth and invasiveness. While the molecular weight (MW) dependent properties of HA can cause tumor-promoting and -repressing effects, the elevated levels of HA in the TME impedes drug delivery. The degradation of HA using hyaluronidases (HYALs), resulting in fragments of HA, is a way to overcome this, but the consequences of changes in HA molecular weight and concentration is currently unknown. Therefore, it is critical to understand the MW-dependent biological effects of HA. Here we examine the influence of HA molecular weight on biophysical properties that regulate cell migration and extracellular matrix (ECM) remodeling. In our study, we used vLMW, LMW and HMW HA at different physiologically relevant concentrations, with a particular interest in correlating the mechanical and structural properties to different cell functions. The elastic modulus, collagen network pore size and collagen fiber diameter increased with increasing HA concentration. Although the collagen network pore size increased, these pores were filled with the bulky HA molecules. Consequently, cell migration decreased with increase in HA concentration due to multiple, long-lived and unproductive protrusions, suggesting the influence of steric factors. Surprisingly, even though elastic modulus increased with HA molecular weight and concentration, gel compaction assays showed an increased degree of ECM compaction among HMW HA gels at high concentrations (2 and 4 mg mL [0.2 and 0.4%]). These results were not seen in collagen gels that lacked HA, but had similar stiffness. HA appears to have the effect of decreasing migration and increasing collagen network contraction, but only at high HA molecular weight. Consequently, changes in HA molecular weight can have relatively large effects on cancer cell behavior. STATEMENT OF SIGNIFICANCE: Hyaluronan (HA) is a critical component of the tumor microenvironment (TME). Overproduction of HA in the TME results in poor prognosis and collapse of blood vessels, inhibiting drug delivery. Hyaluronidases have been used to enhance drug delivery. However, they lead to low molecular weight (MW) HA, altering the mechanical and structural properties of the TME and cancer cell behavior. Understanding how HA degradation affects cancer cell behavior is critical for uncovering detrimental effects of this therapy. Very little is known about how HA MW affects cancer cell behavior in tumor-mimicking collagen-HA composite networks. Here we examine how MW and HA content in collagen-HA networks alter structural and mechanical properties to regulate cell migration and matrix remodeling in 3D TME-mimicking environments.
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http://dx.doi.org/10.1016/j.actbio.2021.06.009DOI Listing
August 2021

Liquid assets for soft electronics.

Nat Mater 2021 06;20(6):714-715

Soft Materials and Structures Lab, Department of Mechanical Engineering, Virginia Tech, Blacksburg, VA, USA.

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http://dx.doi.org/10.1038/s41563-021-00939-yDOI Listing
June 2021

Deterministic control of adhesive crack propagation through jamming based switchable adhesives.

Soft Matter 2021 Feb 25;17(7):1731-1737. Epub 2021 Jan 25.

Macromolecules Innovation Institute, Virginia Tech, Blacksburg, VA 24061, USA. and Department of Mechanical Engineering, Soft Materials and Structures Lab, Virginia Tech, Blacksburg, VA 24061, USA.

Controlling delamination across a material interface is a foundation of adhesive science and technology. This ranges from creating permanent, strong adhesives which limit crack propagation to reversible adhesives which initiate cracks for release. Methods which dynamically control cracks can lead to more robust adhesion, however specific control of crack initiation, propagation, and arresting is challenging because time scales of crack propagation are much faster than times scales of mechanisms to arrest cracks. Here we show the deterministic control of crack initiation, propagation, and arresting by integrating a granular jamming layer into adhesive films. This allows for controlled initiation of a propagating crack by reducing rigidity and then rapidly arresting the crack through jamming, with a rise in stiffness and an 11× enhancement in adhesion. This process is highly reversible and programmable, allowing for numerous crack initiation, propagation, and arresting cycles at arbitrary selectable locations in a peeling adhesive. We demonstrate this crack-control approach in single and multiple peel directions under fixed load conditions in response to diverse pressurization input signal profiles (i.e. time varying propagation and arresting scenarios).
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http://dx.doi.org/10.1039/d0sm02129fDOI Listing
February 2021

All-graphene-based open fluidics for pumpless, small-scale fluid transport laser-controlled wettability patterning.

Nanoscale Horiz 2021 01;6(1):24-32

Department of Mechanical Engineering, Iowa State University of Science and Technology, 528 Bissell Rd, Ames, IA 50010, USA.

Open microfluidics have emerged as a low-cost, pumpless alternative strategy to conventional microfluidics for delivery of fluid for a wide variety of applications including rapid biochemical analysis and medical diagnosis. However, creating open microfluidics by tuning the wettability of surfaces typically requires sophisticated cleanroom processes that are unamenable to scalable manufacturing. Herein, we present a simple approach to develop open microfluidic platforms by manipulating the surface wettability of spin-coated graphene ink films on flexible polyethylene terephthalate via laser-controlled patterning. Wedge-shaped hydrophilic tracks surrounded by superhydrophobic walls are created within the graphene films by scribing micron-sized grooves into the graphene with a CO2 laser. This scribing process is used to make superhydrophobic walls (water contact angle ∼160°) that delineate hydrophilic tracks (created through an oxygen plasma pretreatment) on the graphene for fluid transport. These all-graphene open microfluidic tracks are capable of transporting liquid droplets with a velocity of 20 mm s-1 on a level surface and uphill at elevation angles of 7° as well as transporting fluid in bifurcating cross and tree branches. The all-graphene open microfluidic manufacturing technique is rapid and amenable to scalable manufacturing, and consequently offers an alternative pumpless strategy to conventional microfluidics and creates possibilities for diverse applications in fluid transport.
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http://dx.doi.org/10.1039/d0nh00376jDOI Listing
January 2021

Mechanically Cloaked Multiphase Magnetic Elastomer Soft Composites for Wearable Wireless Power Transfer.

ACS Appl Mater Interfaces 2020 Nov 3;12(45):50909-50917. Epub 2020 Nov 3.

Macromolecules Innovation Institute, Virginia Tech, Blacksburg, Virginia 24061, United States.

Wearable electronics allow for new and immersive experiences between technology and the human body, but conventional devices are made from rigid functional components that lack the necessary compliance to safely interact with human tissue. Recently, liquid inclusions have been incorporated into elastomer composites to produce functional materials with high extensibility and ultrasoft mechanical responses. While these materials have shown high thermal and electrical conductivity, there has been an absence of research into compliant magnetic materials through the incorporation of magnetic fluids. Compliant magnetic materials are important for applications in soft matter engineering including sensing, actuation, and power transfer for soft electronics and robotics. In this work, we establish a new class of highly functional soft materials with advanced magnetic and mechanical properties by dispersing magnetic colloidal suspensions as compliant fluid inclusions into soft elastomers. Significantly, the rigid magnetic particles are encapsulated by the fluid. This mechanically cloaks the solid particles and enables a fluid-like mechanical response while imparting high magnetic permeability to the composite. This microstructure reduces the modulus of the composite below that of the initial elastomer to <40 kPa while increasing the permeability by over 100% to greater than 2. We demonstrate the functionality of these materials through conformable magnetic backplanes, which enables a completely soft, coupled inductor system capable of transferring power up to 100% strain and wearable devices for wireless power transfer.
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http://dx.doi.org/10.1021/acsami.0c15909DOI Listing
November 2020

Introduction to liquid composites.

Soft Matter 2020 Jul 17;16(25):5799-5800. Epub 2020 Jun 17.

Materials Science and Engineering, Iowa State University of Science and Technology, Ames, IA 50011, USA.

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http://dx.doi.org/10.1039/d0sm90113jDOI Listing
July 2020

Liquid Metal-Elastomer Soft Composites with Independently Controllable and Highly Tunable Droplet Size and Volume Loading.

ACS Appl Mater Interfaces 2019 May 1;11(19):17873-17883. Epub 2019 May 1.

Soft composites are critical for soft and flexible materials in energy harvesting, actuators, and multifunctional devices. One emerging approach to create multifunctional composites is through the incorporation of liquid metal (LM) droplets such as eutectic gallium indium (EGaIn) in highly deformable elastomers. The microstructure of such systems is critical to their performance; however, current materials lack control of particle size at diverse volume loadings. Here, we present a fabrication approach to create liquid metal-elastomer composites with independently controllable and highly tunable droplet size (100 nm ≤ D ≤ 80 μm) and volume loading (0 ≤ ϕ ≤ 80%). This is achieved through a combination of shear mixing and sonication of concentrated LM/elastomer emulsions to control droplet size and subsequent dilution and homogenization to tune LM volume loading. These materials are characterized utilizing dielectric spectroscopy supported by analytical modeling, which shows a high relative permittivity of 60 (16× the unfilled elastomer) in a composite with ϕ = 80%, a low tan δ of 0.02, and a significant dependence on ϕ and minor dependence on droplet size. Temperature response and stability are determined using dielectric spectroscopy through temperature and frequency sweeps with DSC. These results demonstrate a wide temperature stability of the liquid metal phase (crystallizing at <-85 °C for D < 20 μm). Additionally, all composites are electrically insulating across wide frequency (0.1 Hz-10 MHz) and temperature (-70 to 100 °C) ranges even up to ϕ = 80%. We highlight the benefit of LM microstructure control by creating all-soft-matter stretchable capacitive sensors with tunable sensitivity. These sensors are further integrated into a wearable sensing glove where we identify different objects during grasping motions. This work enables programmable LM composites for soft robotics and stretchable electronics where flexibility and tunable functional response are critical.
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http://dx.doi.org/10.1021/acsami.9b04569DOI Listing
May 2019

Tuning surface functionalization and collagen gel thickness to regulate cancer cell migration.

Colloids Surf B Biointerfaces 2019 Jul 16;179:37-47. Epub 2019 Mar 16.

Department of Chemical and Biological Engineering, Iowa State University, United States; Department of Genetics, Development and Cell Biology, Iowa State University, United States. Electronic address:

Cancer cells have a tremendous ability to sense and respond to extracellular matrix (ECM) stiffness, modulating invasion. The magnitude of the sensed stiffness can either promote or inhibit the migration of cancer cells out of the primary tumor into surrounding tissue. Work has been done on examining the role of stiffness in tuning cancer cell migration by controlling elastic modulus in the bulk. However, a powerful and complementary approach for controlling stiffness is to leverage interactions between stiff-soft (e.g. glass-hydrogel) interfaces. Unfortunately, most work in this area probes cells in 2D environments. Of the reports that probe 3D environments, none have assessed the role of mechanical linkage to the interface as a potential handle in controlling local stiffness and cell behavior. In this paper, we examine the migration of cancer cells embedded in a collagen fiber network between two flat plates. We examine the role of both surface attachment of the collagen network to the stiff interface as well as thickness (50-540 μm) of the collagen gel in driving collagen organization, cell morphology and cell migration. We find that surface attachment and thickness do not operate overlapping mechanisms, because they elicit different cell responses. While thickness and surface chemistry appear to control morphology, only thickness regulates collagen organization and cell migration speed. This suggests that surface attachment and thickness of the collagen gel control cell behavior through both collagen structure and local stiffness in confined fiber-forming networks.
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http://dx.doi.org/10.1016/j.colsurfb.2019.03.031DOI Listing
July 2019

Flexible thermoelectric generators with inkjet-printed bismuth telluride nanowires and liquid metal contacts.

Nanoscale 2019 Mar;11(12):5222-5230

Mechanical Engineering, Iowa State University, Ames, IA 50011, USA.

Solution phase printing of nanomaterials is becoming increasingly important for the creation of scalable flexible electronics including those associated with biomedical and energy harvesting applications. However, the use of solution-phase printed thermoelectric energy generators (TEGs) has been minimally explored. Herein we report a highly flexible inkjet-printed TEG. Bismuth telluride (Bi2Te3) and bismuth antimony telluride (Bi0.5Sb1.5Te3) nanowires (NWs) are inkjet printed onto polyimide to form n-type and p-type legs for the TEGs. A post-print thermal annealing process is used to increase the thermoelectric performance of the printed NWs while eutectic gallium-indium (EGaIn) liquid metal contacts electrically connect the TEG legs in series. Annealing conditions for the combination of p/n legs are examined to maximize the thermoelectric efficiency of the TEG prototype. The maximum power factor was found to be 180 μW m-1 K-2 and 110 μW m-1 K-2 for the Bi2Te3 and Bi0.5Sb1.5Te3 nanowires respectively. A maximum power for the fully printed TEG device measured 127 nW at a 32.5 K temperature difference. The performance of the TEG device does not diminish even after multiple bending experiments (up to 50 times) around a tight radius of curvature (rod-dia. 11 mm). Hence this inkjet-printed flexible TEG is a step towards a fully functional wearable TEG device.
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http://dx.doi.org/10.1039/c8nr09101cDOI Listing
March 2019

An autonomously electrically self-healing liquid metal-elastomer composite for robust soft-matter robotics and electronics.

Nat Mater 2018 07 21;17(7):618-624. Epub 2018 May 21.

Integrated Soft Materials Lab, Carnegie Mellon University, Pittsburgh, PA, USA.

Large-area stretchable electronics are critical for progress in wearable computing, soft robotics and inflatable structures. Recent efforts have focused on engineering electronics from soft materials-elastomers, polyelectrolyte gels and liquid metal. While these materials enable elastic compliance and deformability, they are vulnerable to tearing, puncture and other mechanical damage modes that cause electrical failure. Here, we introduce a material architecture for soft and highly deformable circuit interconnects that are electromechanically stable under typical loading conditions, while exhibiting uncompromising resilience to mechanical damage. The material is composed of liquid metal droplets suspended in a soft elastomer; when damaged, the droplets rupture to form new connections with neighbours and re-route electrical signals without interruption. Since self-healing occurs spontaneously, these materials do not require manual repair or external heat. We demonstrate this unprecedented electronic robustness in a self-repairing digital counter and self-healing soft robotic quadruped that continue to function after significant damage.
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http://dx.doi.org/10.1038/s41563-018-0084-7DOI Listing
July 2018

Extreme Toughening of Soft Materials with Liquid Metal.

Adv Mater 2018 May 16;30(22):e1706594. Epub 2018 Apr 16.

Integrated Soft Materials Lab, Civil & Environmental Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA, 15213, USA.

Soft and tough materials are critical for engineering applications in medical devices, stretchable and wearable electronics, and soft robotics. Toughness in synthetic materials is mostly accomplished by increasing energy dissipation near the crack tip with various energy dissipation techniques. However, bio-materials exhibit extreme toughness by combining multi-scale energy dissipation with the ability to deflect and blunt an advancing crack tip. Here, we demonstrate a synthetic materials architecture that also exhibits multi-modal toughening, whereby embedding a suspension of micron sized and highly deformable liquid metal (LM) droplets inside a soft elastomer, the fracture energy dramatically increases by up to 50x (from 250 ± 50 J m to 11,900 ± 2600 J m ) over an unfilled polymer. For some LM-embedded elastomer (LMEE) compositions, the toughness is measured to be 33,500 ± 4300 J m , which far exceeds the highest value previously reported for a soft elastic material. This extreme toughening is achieved by (i) increasing energy dissipation, (ii) adaptive crack movement, and (iii) effective elimination of the crack tip. Such properties arise from the deformability of the LM inclusions during loading, providing a new mechanism to not only prevent crack initiation, but also resist the propagation of existing tears for ultra tough, soft materials.
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http://dx.doi.org/10.1002/adma.201706594DOI Listing
May 2018

Tunable Mechanical Metamaterials through Hybrid Kirigami Structures.

Sci Rep 2018 02 21;8(1):3378. Epub 2018 Feb 21.

Department of Materials Science and Engineering, Soft Materials and Structures Lab, Iowa State University of Science and Technology, 528 Bissell Rd, Ames, IA, 50011, USA.

Inspired by the art of paper cutting, kirigami provides intriguing tools to create materials with unconventional mechanical and morphological responses. This behavior is appealing in multiple applications such as stretchable electronics and soft robotics and presents a tractable platform to study structure-property relationships in material systems. However, mechanical response is typically controlled through a single or fractal cut type patterned across an entire kirigami sheet, limiting deformation modes and tunability. Here we show how hybrid patterns of major and minor cuts creates new opportunities to introduce boundary conditions and non-prismatic beams to enable highly tunable mechanical responses. This hybrid approach reduces stiffness by a factor of ~30 while increasing ultimate strain by a factor of 2 (up to 750% strain) relative to single incision patterns. We present analytical models and generate general design criteria that is in excellent agreement with experimental data from nanoscopic to macroscopic systems. These hybrid kirigami materials create new opportunities for multifunctional materials and structures, which we demonstrate with stretchable kirigami conductors with nearly constant electrical resistance up to >400% strain and magnetoactive actuators with extremely rapid response (>10,000% strain s) and high, repeatable elongation (>300% strain).
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http://dx.doi.org/10.1038/s41598-018-21479-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5821861PMC
February 2018

Kirigami-Inspired Structures for Smart Adhesion.

ACS Appl Mater Interfaces 2018 Feb 7;10(7):6747-6754. Epub 2018 Feb 7.

Department of Materials Science and Engineering, Soft Materials and Structures Lab, Iowa State University of Science and Technology , 528 Bissell Rd, Ames, Iowa 50011, United States.

Spatially controlled layouts of elasticity can provide enhanced adhesion over homogeneous systems. Emerging techniques in kirigami, where designed cuts in materials impart highly tunable stiffness and geometry, offer an intriguing approach to create well-defined layouts of prescribed elastic regions. Here, we show that kirigami-inspired structures at interfaces provide a new mechanism to spatially control and enhance adhesion strength while providing directional characteristics for smart interfaces. We use kirigami-inspired cuts to define stiff and compliant regions, where above a critical, material-defined length scale, bending rigidity and contact width can be tuned to enhance adhesive force capacity by a factor of ∼100 across a spatially patterned adhesive sheet. The directional nature of these designs also imparts anisotropic responses, where peeling in different directions results in anisotropic adhesion ratios of ∼10. Experimental results are well-supported by theoretical predictions in which the bending rigidity and contact width of kirigami-inspired structures and interconnects control the adhesive capacity. These new interfacial structures and design criteria provide diverse routes for advanced adhesive functionality, including spatially controlled systems, wearable kirigami-inspired electronics, and anisotropic kirigami-inspired bandages that enable strong adhesive capacity while maintaining easy release.
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http://dx.doi.org/10.1021/acsami.7b18594DOI Listing
February 2018

High thermal conductivity in soft elastomers with elongated liquid metal inclusions.

Proc Natl Acad Sci U S A 2017 02 13;114(9):2143-2148. Epub 2017 Feb 13.

Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213;

Soft dielectric materials typically exhibit poor heat transfer properties due to the dynamics of phonon transport, which constrain thermal conductivity () to decrease monotonically with decreasing elastic modulus (). This thermal-mechanical trade-off is limiting for wearable computing, soft robotics, and other emerging applications that require materials with both high thermal conductivity and low mechanical stiffness. Here, we overcome this constraint with an electrically insulating composite that exhibits an unprecedented combination of metal-like thermal conductivity, an elastic compliance similar to soft biological tissue (Young's modulus < 100 kPa), and the capability to undergo extreme deformations (>600% strain). By incorporating liquid metal (LM) microdroplets into a soft elastomer, we achieve a ∼25× increase in thermal conductivity (4.7 ± 0.2 W⋅m⋅K) over the base polymer (0.20 ± 0.01 W⋅m·K) under stress-free conditions and a ∼50× increase (9.8 ± 0.8 W⋅m·K) when strained. This exceptional combination of thermal and mechanical properties is enabled by a unique thermal-mechanical coupling that exploits the deformability of the LM inclusions to create thermally conductive pathways in situ. Moreover, these materials offer possibilities for passive heat exchange in stretchable electronics and bioinspired robotics, which we demonstrate through the rapid heat dissipation of an elastomer-mounted extreme high-power LED lamp and a swimming soft robot.
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http://dx.doi.org/10.1073/pnas.1616377114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5338550PMC
February 2017

Liquid Metals: Stretchable, High-k Dielectric Elastomers through Liquid-Metal Inclusions (Adv. Mater. 19/2016).

Adv Mater 2016 May;28(19):3791

Soft Machines Lab, Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213, USA.

An all-soft-matter composite consisting of liquid metal microdroplets embedded in a soft elastomer matrix is presented by C. Majidi and co-workers on page 3726. This composite exhibits a high dielectric constant while maintaining exceptional elasticity and compliance. The image shows the composite's microstructure captured by 3D X-ray imaging using a nano-computed tomographic scanner.
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http://dx.doi.org/10.1002/adma.201670133DOI Listing
May 2016

Stretchable, High-k Dielectric Elastomers through Liquid-Metal Inclusions.

Adv Mater 2016 May 23;28(19):3726-31. Epub 2016 Mar 23.

Soft Machines Lab, Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213, USA.

An all-soft-matter composite with exceptional electro-elasto properties is demonstrated by embedding liquid-metal inclusions in an elastomer matrix. This material exhibits a unique combination of high dielectric constant, low stiffness, and large strain limit (ca. 600% strain). The elasticity, electrostatics, and electromechanical coupling of the composite are investigated, and strong agreement with predictions from effective medium theory is found.
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http://dx.doi.org/10.1002/adma.201506243DOI Listing
May 2016

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

Creating gecko-like adhesives for "real world" surfaces.

Adv Mater 2014 Jul 17;26(25):4345-51. Epub 2014 Apr 17.

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

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http://dx.doi.org/10.1002/adma.201306259DOI Listing
July 2014

Enhancing adhesion of elastomeric composites through facile patterning of surface discontinuities.

ACS Appl Mater Interfaces 2014 May 25;6(9):6845-50. Epub 2014 Apr 25.

Department of Polymer Science and Engineering, University of Massachusetts , 120 Governors Drive, Amherst, Massachusetts 01003-9265, United States.

Patterning interfaces can provide enhanced adhesion over a projected area. However, careful consideration of the material properties and geometry must be applied to provide successful reversible adhesives. We present a simple method to use patterned, elastomeric fabric composites to enhance the shear adhesion strength by nearly 40% compared to a non-patterned sample. We describe how this enhancement depends on the pattern geometry, the velocity dependence of the adhesive materials, and the controlled displacement rate applied to the interface. Through these observations, we discuss strategies for improving reversible adhesives.
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http://dx.doi.org/10.1021/am5006546DOI Listing
May 2014

High capacity, easy release adhesives from renewable materials.

Adv Mater 2014 Jun 6;26(21):3405-9. Epub 2014 Feb 6.

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

Reversible adhesives composed of renewable materials are presented which achieve high force capacities (810 N) while maintaining easy release (∼ 0.25 N) and reusability. These simple, non-tacky adhesives consist of natural rubber impregnated into stiff natural fiber fabrics, including cotton, hemp, and jute. This versatile approach enables a clear method for designs of environmentally-responsible, reversible adhesives for a wide variety of applications.
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http://dx.doi.org/10.1002/adma.201305593DOI Listing
June 2014

Scaling normal adhesion force capacity with a generalized parameter.

Langmuir 2013 Sep 22;29(35):11022-7. Epub 2013 Aug 22.

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

The adhesive response of a rigid flat cylindrical indenter in contact with a compliant elastic layer of varying confinement is investigated experimentally and described analytically. Using a soft elastic gel with substrate thickness, t, and indenter radius, a, 28 unique combinations of the confinement parameter, a/t, are examined over a range of 0.016 < a/t < 7.2. Continuous force capacity predictions as a function of a/t and material properties are provided through a scaling theory and are found to agree well with the experimental data. We further collapse all of the data over orders of magnitude in adhesive force capacity onto a single line described by a generalized reversible adhesion scaling parameter, A/C, where A is the contact area and C is the compliance. As the scaling analysis does not assume a specific separation mechanism the adhesive force capacity is well described during both axisymmetric edge separation and during interfacial fingering and cavitation instabilities. We discuss how the geometry of the contact, specifically increasing the degree of confinement, allows reversible adhesive materials to be designed that are not "sticky" or "tacky", yet can be very strong and provide high performance.
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http://dx.doi.org/10.1021/la4013526DOI Listing
September 2013

Opportunities with fabric composites as unique flexible substrates.

ACS Appl Mater Interfaces 2012 Dec 21;4(12):6640-5. Epub 2012 Nov 21.

Department of Polymer Science and Engineering, University of Massachusetts, 120 Governors Dr. Amherst, Massachusetts 01003-9265, USA.

Flexible substrates enable new capabilities in applications ranging from electronics to biomedical devices. To provide a new platform for these applications, we investigate a composite material consisting of rigid fiber fabrics impregnated with soft elastomers, offering the ability to create load bearing, yet flexible substrates. We demonstrate an integrated and facile one-step imprint lithographic patterning method on a number of fabrics and resins. Furthermore, the bending and tensile properties were examined to compare the composites to other flexible materials such as PET and cellulose paper. Carbon fiber composites possess a higher tensile modulus than PET while retaining almost an order of magnitude lower bending modulus. Fabric composites can also have anisotropic mechanical properties not observed in homogeneous materials. Finally, we provide a discussion of these anisotropic mechanical responses and their potential use in flexible applications.
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http://dx.doi.org/10.1021/am3017812DOI Listing
December 2012

Looking beyond fibrillar features to scale gecko-like adhesion.

Adv Mater 2012 Feb 26;24(8):1078-83. Epub 2012 Jan 26.

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

Hand-sized gecko-inspired adhesives with reversible force capacities as high as 2950 N (29.5 N cm(-2) ) are designed without the use of fibrillar features through a simple scaling theory. The scaling theory describes both natural and synthetic gecko-inspired adhesives, over 14 orders of magnitude in adhesive force capacity, from nanoscopic to macroscopic length scales.
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http://dx.doi.org/10.1002/adma.201104191DOI Listing
February 2012

Total recoil: perch compliance alters jumping performance and kinematics in green anole lizards (Anolis carolinensis).

J Exp Biol 2012 Jan;215(Pt 2):220-6

Graduate Program in Organismic and Evolutionary Biology, University of Massachusetts Amherst, Amherst, MA 01003, USA.

Jumping is a common form of locomotion for many arboreal animals. Many species of the arboreal lizard genus Anolis occupy habitats in which they must jump to and from unsteady perches, e.g. narrow branches, vines, grass and leaves. Anoles therefore often use compliant perches that could alter jump performance. In this study we conducted a small survey of the compliance of perches used by the arboreal green anole Anolis carolinensis in the wild (N=54 perches) and then, using perches within the range of compliances used by this species, investigated how perch compliance (flexibility) affects the key jumping variables jump distance, takeoff duration, takeoff angle, takeoff speed and landing angle in A. carolinensis in the laboratory (N=11). We observed that lizards lost contact with compliant horizontal perches prior to perch recoil, and increased perch compliance resulted in decreased jump distance and takeoff speed, likely because of the loss of kinetic energy to the flexion of the perch. However, the most striking effect of perch compliance was an unexpected one; perch recoil following takeoff resulted in the lizards being struck on the tail by the perch, even on the narrowest perches. This interaction between the perch and the tail significantly altered body positioning during flight and landing. These results suggest that although the use of compliant perches in the wild is common for this species, jumping from these perches is potentially costly and may affect survival and behavior, particularly in the largest individuals.
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http://dx.doi.org/10.1242/jeb.061838DOI Listing
January 2012
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