Shawn A. Chester

Microstructured barbs on the North American porcupine quill enable easy tissue penetration and difficult removal

North American porcupines are well known for their specialized hairs, or quills that feature microscopic backward-facing deployable barbs that are used in self-defense. Herein we show that the natural quill’s geometry enables easy penetration and high tissue adhesion where the barbs specifically contribute to adhesion and unexpectedly, dramatically reduce the force required to penetrate tissue. Reduced penetration force is achieved by topography that appears to create stress concentrations along regions of the quill where the cross sectional diameter grows rapidly, facilitating cutting of the tissue. Barbs located near the first geometrical transition zone exhibit the most substantial impact on minimizing the force required for penetration. Barbs at the tip of the quill independently exhibit the greatest impact on tissue adhesion force and the cooperation between barbs in the 0–2 mm and 2–4 mm regions appears critical to enhance tissue adhesion force. The dual functions of barbs were reproduced with replica molded synthetic polyurethane quills. These findings should serve as the basis for the development of bio-inspired devices such as tissue adhesives or needles, trocars, and vascular tunnelers where minimizing the penetration force is important to prevent collateral damage.

Micro 3D Printing of a Temperature-Responsive Hydrogel Using Projection Micro-Stereolithography

Stimuli-responsive hydrogels exhibiting physical or chemical changes in response to environmental conditions have attracted growing attention for the past few decades. Poly(N-isopropylacrylamide) (PNIPAAm), a temperature responsive hydrogel, has been extensively studied in various fields of science and engineering. However, manufacturing of PNIPAAm has been heavily relying on conventional methods such as molding and lithography techniques that are inherently limited to a two-dimensional (2D) space. Here we report the three-dimensional (3D) printing of PNIPAAm using a high-resolution digital additive manufacturing technique, projection micro-stereolithography (PμSL). Control of the temperature dependent deformation of 3D printed PNIPAAm is achieved by controlling manufacturing process parameters as well as polymer resin composition. Also demonstrated is a sequential deformation of a 3D printed PNIPAAm structure by selective incorporation of ionic monomer that shifts the swelling transition temperature of PNIPAAm. This fast, high resolution, and scalable 3D printing method for stimuli-responsive hydrogels may enable many new applications in diverse areas, including flexible sensors and actuators, bio-medical devices, and tissue engineering.

Printing ferromagnetic domains for untethered fast-transforming soft materials

Soft materials capable of transforming between three-dimensional (3D) shapes in response to stimuli such as light, heat, solvent, electric and magnetic fields have applications in diverse areas such as flexible electronics1,2, soft robotics3,4 and biomedicine5–7. In particular, magnetic fields offer a safe and effective manipulation method for biomedical applications, which typically require remote actuation in enclosed and confined spaces8–10. With advances in magnetic field control11, magnetically responsive soft materials have also evolved from embedding discrete magnets12 or incorporating magnetic particles13 into soft compounds to generating nonuniform magnetization profiles in polymeric sheets14,15. Here we report 3D printing of programmed ferromagnetic domains in soft materials that enable fast transformations between complex 3D shapes via magnetic actuation. Our approach is based on direct ink writing16 of an elastomer composite containing ferromagnetic microparticles. By applying a magnetic field to the dispensing nozzle while printing17, we reorient particles along the applied field to impart patterned magnetic polarity to printed filaments. This method allows us to program ferromagnetic domains in complex 3D-printed soft materials, enabling a set of previously inaccessible modes of transformation, such as remotely controlled auxetic behaviours of mechanical metamaterials with negative Poisson’s ratios. The actuation speed and power density of our printed soft materials with programmed ferromagnetic domains are orders of magnitude greater than existing 3D-printed active materials. We further demonstrate diverse functions derived from complex shape changes, including reconfigurable soft electronics, a mechanical metamaterial that can jump and a soft robot that crawls, rolls, catches fast-moving objects and transports a pharmaceutical dose.Programmed ferromagnetic domains are 3D-printed into soft materials capable of fast transformations between complex three-dimensional shapes via magnetic actuation.

Nonlinear characterization of elasticity using quantitative optical coherence elastography.

Optical coherence elastography (OCE) has been used to perform mechanical characterization on biological tissue at the microscopic scale. In this work, we used quantitative optical coherence elastography (qOCE), a novel technology we recently developed, to study the nonlinear elastic behavior of biological tissue. The qOCE system had a fiber-optic probe to exert a compressive force to deform tissue under the tip of the probe. Using the space-division multiplexed optical coherence tomography (OCT) signal detected by a spectral domain OCT engine, we were able to simultaneously quantify the probe deformation that was proportional to the force applied, and to quantify the tissue deformation. In other words, our qOCE system allowed us to establish the relationship between mechanical stimulus and tissue response to characterize the stiffness of biological tissue. Most biological tissues have nonlinear elastic behavior, and the apparent stress-strain relationship characterized by our qOCE system was nonlinear an extended range of strain, for a tissue-mimicking phantom as well as biological tissues. Our experimental results suggested that the quantification of force in OCE was critical for accurate characterization of tissue mechanical properties and the qOCE technique was capable of differentiating biological tissues based on the elasticity of tissue that is generally nonlinear.

A Thermo-Mechanically Coupled Large-Deformation Theory for Amorphous Polymers Across the Glass Transition Temperature

Amorphous thermoplastic polymers are important engineering materials; however, their nonlinear, strongly temperature- and rate-dependent elastic-viscoplastic behavior is still not very well understood, and is modeled by existing constitutive theories with varying degrees of success. There is no generally agreed upon theory to model the large-deformation, thermo-mechanically-coupled, elastic-viscoplastic response of these materials in a temperature range which spans their glass transition temperature. Such a theory is crucial for the development of a numerical capability for the simulation and design of important polymer processing operations, and also for predicting the relationship between processing methods and the subsequent mechanical properties of polymeric products. In this manuscript we briefly summarize a few results from our own recent research [1–4] which is intended to fill this need.Copyright

Characterization of nonlinear elasticity for biological tissue using quantitative optical coherence elastography

We developed a quantitative optical coherence elastography (qOCE) system for nonlinear mechanical characterization of biological tissues. The fiber-optic probe of the qOCE system had an integrated Fabry-Perot force sensor. To perform mechanical characterization, the tissue was compressed uniaxially by the fiber-optic probe of the qOCE system. Using the optical coherence tomography (OCT) signal detected by a spectral domain OCT engine, we were able to simultaneously quantify the force exerted to the tissue and the displacement of tissue. The quantification of the force was critical for accurate assessment of the elastic behavior of tissue, because most biological tissues have nonlinear elastic behavior. We performed qOCE characterization on tissue mimicking phantoms and biological tissues. Our results demonstrated the capability of the qOCE system for linear and nonlinear assessment of tissue elasticity.

Numerical Simulation of Polymeric Gels

Following [1], a theory for coupled fluid diffusion and large deformation is implemented as a user-element subroutine in the commercial finite element package ABAQUS. The governing equations are summarized along with details of the constitutive theory. A few numerical examples are provided to show the robustness of this methodology in both transient and steady state conditions.Copyright

Numerical simulation of aluminum alloy 6061 micro-mold fabrication for the production of polymeric microstructures by micro-hot-embossing

Micro-molds play an important role in the manufacturing process of polymeric micro-devices, e.g. microfluidic devices, as they determine the product quality and the overall production cost. We report here the applicability of a large-deformation, high-temperature, isotropic elastic-viscoplasticity model for the prediction of micron-scale hot-embossing of AA6061. The material parameters in the constitutive model were determined by fitting the stress–strain curves from compression tests at various temperatures and strain rates. The constitutive theory was implemented in a finite element program, and the numerical simulation capability was validated by predicting the response of AA6061 in some representative macro-scale experiments; these experiments had not been used for the determination of the material parameters in the constitutive model. Additional micron-scale hot-embossing experiments on AA6061 were conducted, and by comparing the numerical simulation results to the corresponding physical experiments, we demonstrate that the deformation evolution of AA6061 during micro-hot-embossing is well predicted. The constitutive model and its numerical implementation open the possibility of optimizing the process of making micro-molds for microfluidic devices from AA6061.

A Large-Deformation Theory for Thermally-Actuated Shape-Memory Polymers and its Application

The most common shape-memory polymers are those in which the shape-recovery is thermally-induced. A body made from such a material may be subjected to large deformations at an elevated temperature above its glass transition temperature ϑg . Cooling the deformed body to a temperature below ϑg under active kinematical constraints fixes the deformed shape of the body. The original shape of the body may be recovered if the material is heated back to a temperature above ϑg without the kinematical constraints. This phenomenon is known as the shape-memory effect. If the shape recovery is partially constrained, the material exerts a recovery force and the phenomenon is known as constrained-recovery.Copyright

Discrete Element Simulations of Floor Pressure due to a Granular Material in a Cylindrical Vessel

At the end of the 19th century, H. A. Janssen discovered that the floor pressure in a cylindrical container of granular material asymptotes exponentially to a value less than the weight of the material i.e., the pressure becomes independent of the fill height of the column. This phenomenon is investigated using discrete element simulations of inelastic, frictional spheres in a cylindrical vessel having a particle‐to‐cylinder diameter ratio at approximately 13.3 or 26.6, with varying bed heights in both cases. The load experienced by a piston that is supporting the granular column are computed. In order to activate frictional forces at the wall contacts either the piston (or equivalently the cylinder wall), is slowly displaced at a constant rate so as to maintain quasi‐static conditions. Various combinations of wall and inter‐particle friction coefficients are examined. The simulated behavior of the load vs. fill level was found to fit well to the functional form of Janssen’s theory. Moreover, quantitative...