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1. Application of Digital Image Correlation (DIC) to the Measurement of Strain Concentration of a PVA Dual-Crosslink Hydrogel Under Large Deformation

   https://doi.org/10.1007/s11340-019-00520-4  

   Liu, M. ; Guo, J. ; Hui, C.-Y. ; Zehnder, A. T. ( January 2019 , Experimental Mechanics)

   Hydrogels are a class of soft, highly deformable materials formed by swelling a network of polymer chains in water. With mechanical properties that mimic biological materials, hydrogels are often proposed for load bearing biomedical or other applications in which their deformation and failure properties will be important. To study the failure of such materials a means for the measurement of deformation fields beyond simple uniaxial tension tests is required. As a non-contact, full-field deformation measurement method, Digital Image Correlation (DIC) is a good candidate for such studies. The application of DIC to hydrogels is studied here with the goal of establishing the accuracy of DIC when applied to hydrogels in the presence of large strains and large strain gradients. Experimental details such as how to form a durable speckle pattern on a material that is 90% water are discussed. DIC is used to measure the strain field in tension loaded samples containing a central hole, a circular edge notch and a sharp crack. Using a nonlinear, large deformation constitutive model, these experiments are modeled using the finite element method (FEM). Excellent agreement between FEM and DIC results for all three geometries shows that the DIC measurements are accurate up to strains of over 10, even in the presence of very high strain gradients near a crack tip. The method is then applied to verify a theoretical prediction that the deformation field in a cracked sample under relaxation loading, i.e. constant applied boundary displacement, is stationary in time even as the stress relaxes by a factor of three. 

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2. Bioinspired Fiber Networks With Tunable Mechanical Properties by Additive Manufacturing

   https://doi.org/10.1115/1.4062451  

   Sarkar, Mainak ; Notbohm, Jacob ( August 2023 , Journal of Applied Mechanics)

   Abstract Soft bioinspired fiber networks offer great potential in biomedical engineering and material design due to their adjustable mechanical behaviors. However, existing strategies to integrate modeling and manufacturing of bioinspired networks do not consider the intrinsic microstructural disorder of biopolymer networks, which limits the ability to tune their mechanical properties. To fill in this gap, we developed a method to generate computer models of aperiodic fiber networks mimicking type I collagen ready to be submitted for additive manufacturing. The models of fiber networks were created in a scripting language wherein key geometric features like connectivity, fiber length, and fiber cross section could be easily tuned to achieve desired mechanical behavior, namely, pretension-induced shear stiffening. The stiffening was first predicted using finite element software, and then a representative network was fabricated using a commercial 3D printer based on digital light processing technology using a soft resin. The stiffening response of the fabricated network was verified experimentally on a novel test device capable of testing the shear stiffness of the specimen under varying levels of uniaxial pretension. The resulting data demonstrated clear pretension-induced stiffening in shear in the fabricated network, with uniaxial pretension of 40% resulting in a factor of 2.65 increase in the small strain shear stiffness. The strategy described in this article addresses current challenges in modeling bioinspired fiber networks and can be readily integrated with advances in fabrication technology to fabricate materials truly replicating the mechanical response of biopolymer networks. 

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3. Multiscale mechanical characterization and computational modeling of fibrin gels

   https://doi.org/10.1016/j.actbio.2023.03.026  

   Jimenez, Julian M. ; Tuttle, Tyler ; Guo, Yifan ; Miles, Dalton ; Buganza-Tepole, Adrian ; Calve, Sarah ( May 2023 , Acta Biomaterialia)

   Fibrin is a naturally occurring protein network that forms a temporary structure to enable remodeling during wound healing. It is also a common tissue engineering scaffold because the structural properties can be controlled. However, to fully characterize the wound healing process and improve the design of regenerative scaffolds, understanding fibrin mechanics at multiple scales is necessary. Here, we present a strategy to quantify both the macroscale (1–10 mm) stress-strain response and the deformation of the mesoscale (10–1000 µm) network structure during unidirectional tensile tests. The experimental data were then used to inform a computational model to accurately capture the mechanical response of fibrin gels. Simultaneous mechanical testing and confocal microscopy imaging of fluorophore-conjugated fibrin gels revealed up to an 88% decrease in volume coupled with increase in volume fraction in deformed gels, and non-affine fiber alignment in the direction of deformation. Combination of the computational model with finite element analysis enabled us to predict the strain fields that were observed experimentally within heterogenous fibrin gels with spatial variations in material properties. These strategies can be expanded to characterize and predict the macroscale mechanics and mesoscale network organization of other heterogeneous biological tissues and matrices. 

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4. Implantable Cardiac Kirigami‐Inspired Lead‐Based Energy Harvester Fabricated by Enhanced Piezoelectric Composite Film

   https://doi.org/10.1002/adhm.202002100  

   Xu, Zhe ; Jin, Congran ; Cabe, Andrew ; Escobedo, Danny ; Gruslova, Aleksandra ; Jenney, Scott ; Closson, Andrew B. ; Dong, Lin ; Chen, Zi ; Feldman, Marc D. ; et al ( January 2021 , Advanced Healthcare Materials)

   Harvesting biomechanical energy to power implantable electronics such as pacemakers has been attracting great attention in recent years because it replaces conventional batteries and provides a sustainable energy solution. However, current energy harvesting technologies that directly interact with internal organs often lack flexibility and conformability, and they usually require additional implantation surgeries that impose extra burden to patients. To address this issue, here a Kirigami inspired energy harvester, seamlessly incorporated into the pacemaker lead using piezoelectric composite films is reported, which not only possesses great flexibility but also requires no additional implantation surgeries. This lead‐based device allows for harvesting energy from the complex motion of the lead caused by the expansion‐contraction of the heart. The device's Kirigami pattern has been designed and optimized to attain greatly improved flexibility which is validated via finite element method (FEM) simulations, mechanical tensile tests, and energy output tests where the device shows a power output of 2.4 µW. Finally, an in vivo test using a porcine model reveals that the device can be implanted into the heart straightforwardly and generates voltages up to ≈0.7 V. This work offers a new strategy for designing flexible energy harvesters that power implantable electronics.

    

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5. Decoding of Facial Strains via Conformable Piezoelectric Interfaces

   Sun, T. ; Tasnim, F. ; McIntosh, R. T. ; Amiri, N. ; Solav, D. ; Anbarani, M. T. ; Sadat, S. ; Zhang, L. ; Gu, Y. ; Karami, M. A. ; et al ( October 2020 , Nature biomedical engineering)

   null (Ed.)

   Devices that facilitate nonverbal communication typically require high computational loads or have rigid and bulky form factors that are unsuitable for use on the face or on other curvilinear body surfaces. Here, we report the design and pilot testing of an integrated system for decoding facial strains and for predicting facial kinematics. The system consists of mass-manufacturable, conformable piezoelectric thin films for strain mapping; multiphysics modelling for analysing the nonlinear mechanical interactions between the conformable device and the epidermis; and three-dimensional digital image correlation for reconstructing soft-tissue surfaces under dynamic deformations as well as for informing device design and placement. In healthy individuals and in patients with amyotrophic lateral sclerosis, we show that the piezoelectric thin films, coupled with algorithms for the real-time detection and classification of distinct skin-deformation signatures, enable the reliable decoding of facial movements. The integrated system could be adapted for use in clinical settings as a nonverbal communication technology or for use in the monitoring of neuromuscular conditions. 

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