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1. Large Biaxial Recovered Strains in Self‐Shrinking 3D Shape‐Memory Polymer Parts Programmed via Printing with Application to Improve Cell Seeding

   https://doi.org/10.1002/admt.202201997  

   Pieri, Katy; Liu, Di; Soman, Pranav; Zhang, Teng; Henderson, James_H (February 2023, Advanced Materials Technologies)

   Abstract Trapping of strain in layers deposited during extrusion‐based (fused filament fabrication) 3D printing has previously been documented. If fiber‐level strain trapping can be understood sufficiently and controlled, 3D shape‐memory polymer parts could be simultaneously fabricated and programmed via printing (programming via printing; PvP), thereby achieving precisely controlled 3D‐to‐3D transformations of complex part geometries. Yet, because previous studies have only examined strain trapping in solid printed parts—such as layers or 3D objects with 100% infill—fundamental aspects of the PvP process and the potential for PvP to be applied to printing of porous 3D parts remain poorly understood. This work examines the extent to which strain can be trapped in individual fibers and in fibers that span negative space and the extent to which infill geometry affects the magnitude and recovery of strain trapped in porous PvP‐fabricated 3D parts. Additionally, multiaxial shape change of porous PvP‐fabricated 3D parts are for the first time studied, modeled, and applied in a proof‐of‐concept application. This work demonstrates the feasibility of strain trapping in individual fibers in 1D, 2D, and 3D PvP‐fabricated parts and illustrates the potential for PvP to provide new strategies to address unmet needs in biomedical and other fields. 

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2. Thermally Drawn Stretchable Electrical and Optical Fiber Sensors for Multimodal Extreme Deformation Sensing

   https://doi.org/10.1002/adom.202001815  

   Zhang, Yujing; Li, Xiyuan; Kim, Jongwoon; Tong, Yuxin; Thompson, Emily_G; Jiang, Shan; Feng, Ziang; Yu, Li; Wang, Jinhua; Ha, Dong_Sam; et al (January 2021, Advanced Optical Materials)

   Abstract Highly stretchable fiber sensors have attracted significant interest recently due to their applications in wearable electronics, human–machine interfaces, and biomedical implantable devices. Here, a scalable approach for fabricating stretchable multifunctional electrical and optical fiber sensors using a thermal drawing process is reported. The fiber sensors can sustain at least 580% strain and up to 750% strain with a helix structure. The electrical fiber sensor simultaneously exhibits ultrahigh stretchability (400%), high gauge factors (≈1960), and excellent durability during 1000 stretching and bending cycles. It is also shown that the stretchable step‐index optical fibers facilitate detection of bending and stretching deformation through changes in the light transmission. By combining both electrical and optical detection schemes, multifunctional fibers can be used for quantifying and distinguishing multimodal deformations such as bending and stretching. The fibers’ utility and functionality in sensing and control applications are demonstrated in a smart glove for controlling a virtual hand model, a wrist brace for wrist motion tracking, fiber meshes for strain mapping, and real‐time monitoring of multiaxial expansion and shrinkage of porcine bladders. These results demonstrate that the fiber sensors can be promising candidates for smart textiles, robotics, prosthetics, and biomedical implantable devices. 

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3. Rendering Polyurethane Hydrophilic for Efficient Cellulose Reinforcement in Melt‐Spun Nanocomposite Fibers

   https://doi.org/10.1002/admi.202201979  

   Redondo, Alexandre; Bast, Livia_K; Djeghdi, Kenza; Airoldi, Martino; Jang, Daseul; Korley, LaShanda_T_J; Steiner, Ullrich; Bruns, Nico; Gunkel, Ilja (February 2023, Advanced Materials Interfaces)

   Abstract Many commodity plastics, such as thermoplastic polyurethanes (PUs), require reinforcement for use as commercial products. Cellulose nanocrystals (CNCs) offer a “green” and scalable approach to polymer reinforcement as they are exceptionally stiff, recyclable, and abundant. Unfortunately, achieving efficient CNC reinforcement of PUs with industrial melt processing techniques is difficult, mostly due to the incompatibility of the hydrophobic PU with hydrophilic CNCs, limiting their dispersion. Here, a hydrophilic PU is synthesized to achieve strong reinforcement in melt‐processed nanocomposite fibers using filter paper‐sourced CNCs. The melt‐spun fibers, exhibiting smooth surfaces even at high CNC loading (up to 25 wt%) indicating good CNC dispersion, are bench‐marked against solvent‐cast films—solvent processing is not scalable but disperses CNCs well and produces strong CNC reinforcement. Mechanical analysis shows the CNC addition stiffens both nanocomposite films and fibers. The stress and strain at break, however, are not significantly affected in films, whereas adding CNCs to fibers increases the stress‐at‐break while reducing the strain‐at‐break. Compared to earlier studies employing a hydrophobic (and stiffer) PU, CNC addition to a hydrophilic PU substantially increases the fiber stiffness and strength. This work therefore suggests that rendering thermoplastics more hydrophilic might pave the way for “greener” polymer composite products using CNCs. 

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4. Melt-Spun Nanocomposite Fibers Reinforced with Aligned Tunicate Nanocrystals

   https://doi.org/10.3390/polym11121912  

   Redondo, Alexandre; Chatterjee, Sourav; Brodard, Pierre; Korley, LaShanda T.; Weder, Christoph; Gunkel, Ilja; Steiner, Ullrich (December 2019, Polymers)

   The fabrication of nanocomposite films and fibers based on cellulose nanocrystals (P-tCNCs) and a thermoplastic polyurethane (PU) elastomer is reported. High-aspect-ratio P-tCNCs were isolated from tunicates using phosphoric acid hydrolysis, which is a process that affords nanocrystals displaying high thermal stability. Nanocomposites were produced by solvent casting (films) or melt-mixing in a twin-screw extruder and subsequent melt-spinning (fibers). The processing protocols were found to affect the orientation of both PU hard segments and the P-tCNCs within the PU matrix and therefore the mechanical properties. While the films were isotropic, both the polymer matrix and the P-tCNCs proved to be aligned along the fiber direction in the fibers, as shown using SAXS/WAXS, angle-dependent Raman spectroscopy, and birefringence analysis. Tensile tests reveal that fibers and films, at similar P-tCNC contents, display Young’s moduli and strain-at-break that are within the same order of magnitude, but the stress-at-break was found to be ten-times higher for fibers, conferring them a superior toughness over films. 

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5. Evolution of Force Chains Explains the Onset of Strain Stiffening in Fiber Networks

   https://doi.org/10.1115/1.4055586  

   Sarkar, Mainak; Notbohm, Jacob (November 2022, Journal of Applied Mechanics)

   Abstract Fiber networks are the primary structural components of many biological structures, including the cell cytoskeleton and the extracellular matrix. These materials exhibit global nonlinearities, such as stiffening in extension and shear, during which the fibers bend and align with the direction of applied loading. Precise details of deformations at the scale of the fibers during strain stiffening are still lacking, however, as prior work has studied fiber alignment primarily from a qualitative perspective, which leaves incomplete the understanding of how the local microstructural evolution leads to the global mechanical behavior. To fill this gap, we studied how axial forces are transmitted inside the fiber network along paths called force chains, which continuously evolve during the course of deformation. We performed numerical simulations on two-dimensional networks of random fibers under uniaxial extension and shear, modeling the fibers using beam elements in finite element software. To quantify the force chains, we identified all chains of connected fibers for which the axial force was larger than a preset threshold and computed the total length of all such chains. To study the evolution of force chains during loading, we computed the derivative of the total length of all force chains with respect to the applied engineering strain. Results showed that the highest rate of evolution of force chains coincided with the global critical strain for strain stiffening of the fiber network. Therefore, force chains are an important factor connecting understanding of the local kinematics and force transmission to the macroscale stiffness of the fiber network. 

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