Simulation and Validation of Three Dimension Functionally Graded Materials by Material Jetting

The goal of this work is to validate the material models for parts created with a Material Jetting 3-dimensional printer through the comparison of Finite Element Analysis (FEA) simulations and physical tests. The strain maps generated by a video extensometer for multi-material samples are compared to the FEA results based on our material models. Two base materials (ABS-like and rubber-like) and their composites are co-printed in the graded tensile test samples. The graded islands are embedded in the rubber-like test specimens. The simulations were conducted utilizing previously fitted material models, a two-parameter Mooney-Rivlin model for the elastic materials (Tango Black+, DM95, and DM60) and a bilinear model for the rigid material (Vero White+). The results show that the simulation results based on our material models can predict the deformation behaviors of the multi-material samples during a uniaxial tensile test. Our simulation results are able to predict the maximum strain in the matrix material (TB+) within 5% error. Both global deformation pattern and local strain level confirm the validity of the simulated material models

Acoustically-actuated bubble-powered rotational micro-propellers

Bubble-powered acoustic microsystems span a plethora of applications that range from lab-on-chip diagnostic platforms to targeted interventions as microrobots. Numerous studies strategize this bubble-powered mechanism to generate autonomous self-propulsion of microrobots in response to high frequency sound waves. Herein, we present two micro-propeller designs which contain an axis-symmetric distribution of entrapped bubbles that vibrate to induce fast rotational motion. Our micro-propellers are synthesized using 3D Direct Laser Writing and chemically-functionalized to selectively trap air bubbles at their micro-cavities which function as propulsion units. These rotational acoustic micro-propellers offer a dual advantage of being used as mobile microfluidic mixers, and as autonomous microrobots for targeted manipulation. With regards to targeted manipulation, we demonstrate magneto-acoustic actuation of our first propeller design that can be steered to a desired location to perform rotational motion. Furthermore, our second propeller design comprises of a helical arrangement of bubble-filled cavities which makes it suitable for spatial micro-mixing. Our acoustic propellers can reach speeds of up to 400 RPM (rotations per minute) without requiring any direct contact with a vibrating substrate in contrast to the state-of-the-art rotary acoustic microsystems

Intravital three-dimensional bioprinting

Fabrication of three-dimensional (3D) structures and functional tissues directly in live animals would enable minimally invasive surgical techniques for organ repair or reconstruction. Here, we show that 3D cell-laden photosensitive polymer hydrogels can be bioprinted across and within tissues of live mice, using bio-orthogonal two-photon cycloaddition and crosslinking of the polymers at wavelengths longer than 850 nm. Such intravital 3D bioprinting—which does not create by-products and takes advantage of commonly available multiphoton microscopes for the accurate positioning and orientation of the bioprinted structures into specific anatomical sites—enables the fabrication of complex structures inside tissues of live mice, including the dermis, skeletal muscle and brain. We also show that intravital 3D bioprinting of donor-muscle-derived stem cells under the epimysium of hindlimb muscle in mice leads to the de novo formation of myofibres in the mice. Intravital 3D bioprinting could serve as an in vivo alternative to conventional bioprinting

Multitasking smart hydrogels based on the combination of alginate and poly(3,4-ethylenedioxythiophene) properties: A review

Poly(3,4-ethylenedioxythiophene) (PEDOT), a very stable and biocompatible conducting polymer, and alginate (Alg), a natural water-soluble polysaccharide mainly found in the cell wall of various species of brown algae, exhibit very different but at the same complementary properties. In the last few years, the remarkable capacity of Alg to form hydrogels and the electro-responsive properties of PEDOT have been combined to form not only layered composites (PEDOT-Alg) but also interpenetrated multi-responsive PEDOT/Alg hydrogels. These materials have been found to display outstanding properties, such as electrical conductivity, piezoelectricity, biocompatibility, self-healing and re-usability properties, pH and thermoelectric responsiveness, among others. Consequently, a wide number of applications are being proposed for PEDOT-Alg composites and, especially, PEDOT/Alg hydrogels, which should be considered as a new kind of hybrid material because of the very different chemical nature of the two polymeric components. This review summarizes the applications of PEDOT-Alg and PEDOT/Alg in tissue interfaces and regeneration, drug delivery, sensors, microfluidics, energy storage and evaporators for desalination. Special attention has been given to the discussion of multi-tasking applications, while the new challenges to be tackled based on aspects not yet considered in either of the two polymers have also been highlighted.Peer ReviewedPostprint (published version

Intravital three-dimensional bioprinting

Fabrication of three-dimensional (3D) structures and functional tissues directly in live animals would enable minimally invasive surgical techniques for organ repair or reconstruction. Here, we show that 3D cell-laden photosensitive polymer hydrogels can be bioprinted across and within tissues of live mice, using bio-orthogonal two-photon cycloaddition and crosslinking of the polymers at wavelengths longer than 850 nm. Such intravital 3D bioprinting\u2014which does not create by-products and takes advantage of commonly available multiphoton microscopes for the accurate positioning and orientation of the bioprinted structures into specific anatomical sites\u2014enables the fabrication of complex structures inside tissues of live mice, including the dermis, skeletal muscle and brain. We also show that intravital 3D bioprinting of donor-muscle-derived stem cells under the epimysium of hindlimb muscle in mice leads to the de novo formation of myofibres in the mice. Intravital 3D bioprinting could serve as an in vivo alternative to conventional bioprinting

High Temperature Epoxy Composites for Material Extrusion Additive Manufacturing

The geometric design freedom, short lead time, and customization make additive manufacturing (AM) increasingly popular. In addition to rapid prototyping and three-dimensional molds, additive manufacturing has created wind turbine blades, robotic arms, and custom medical implants. Major manufacturing companies such as Porsche and Aetrex are utilizing AM to customize automotive seats and orthopedic footwear. However, available materials limit AM applications. Currently, the high-temperature requirements from the aerospace and automotive industries provide additional, unmet challenges. Many high-temperature epoxies have high pre-polymer viscosities and produce highly exothermic cure reactions, which limits volumetric scaling. Traditionally, fast, high-temperature processing reduces the viscosity, filling a mold before crosslinking initiation; however, this is not possible for AM. Currently, epoxy-fiber composites replace many traditional materials, such as aluminum, in applications where their high strength-to-weight ratios reduce lifetime energy costs. Fiber composites are limited by current fabrication methods, which can be expensive with limited geometric adaptability. Direct ink write (DIW) AM extrudes viscoelastic feedstock, creating parts layer-by-layer. The ink feedstock can readily incorporate fibers while AM produces parts without a mold reducing start-up requirements. This work develops a high temperature, heated cure epoxy feedstock for DIW applications achieving strength and modulus values of 145 MPa and 4.9 GPa, respectively. Two pre-polymers are combined, to maintain a glass transition temperature upwards of 285°C while reducing the viscosity. A heated deposition system requires understanding the thermal viscosity and cure profiles. With a viscosity of 5.4 Pa.s and an 18-hour pot life, 70°C allows for shear flow without premature cure during extrusion. An upper loading limit of 30 vol% glass fibers was determined. The fibers improve the heat deflection temperature by 100°C to 320°C and yield a 160% increase in flexure modulus; however, a 34% reduction in strength occurs. While processing did not decrease the fiber length as observed with carbon, the initial distribution contained 15% of fibers shorter than the critical length. The short fibers and pores that arose from both processing and dissimilar fiber-matrix expansion can account for the reduction. This work aims to develop a hightemperature fiber-filled feedstock while broadly considering print and extrusion parameters of viscous inks

Energy Conversion from Gradients across Bio-Inspired Membranes

Membranes are fundamentally important barriers that enable the processes of life by slowing the dissipation of gradients and transducing usable energy from entropic driving forces. This dissertation presents three investigations of gradient equilibration across bio-inspired membranes, with a focus on permeability and geometric considerations in membrane-based energy transduction systems. The first study models the dynamics of pressure generation from osmotic gradients in an expandable compartment based on mass transport principles. Using an osmotic working fluid composed of aqueous polyethylene glycol inside commercially available dialysis cassettes whose membranes exclude polymer solutes, we validated this model and explored the importance of cassette geometry, restrictions on expansion, and membrane porosity characteristics on pressure generation rates over time. The model made it possible to predict the kinetics of nastic motions caused by osmotically-induced shifts in turgor pressure in plants such as Mimosa pudica; the model’s projections based on plant cell dimensions agree well with published time scales. These cassettes are “waterable” pressure generators available to the general public; we demonstrated their utility by actuating a soft robotic gripper and published our characterization algorithm as an open-source tool. The second study investigates the relationship between the chemical structures of a class of tethered membrane-spanning lipids found in hyperthermoacidophilic archaea and the proton/hydroxide permeability of self-assembled monolayer membranes that the lipids form. We determined permeability values by measuring the fluorescence of solutions of liposomes containing pH-sensitive dyes over time after a step change in the external pH. The presence of isoprenoid methyl groups led to reductions in permeability values, and the length of the transmembrane tether unexpectedly displayed a direct correlation with the permeability, leading us to speculate about the importance of hydrophobic crowding in the membrane interior. Surprisingly, the presence of a transmembrane tether had no significant effect on the permeability at room temperature. We observed a strong positive correlation between the permeability of a membrane and the probability of observing water molecules spontaneously clustering inside the hydrophobic region of a simulated membrane of identical composition using molecular dynamics, providing a predictive parameter obtainable without any “wet” experimentation that may be useful for the design of membrane compositions with specific permeability characteristics. The third study presents a novel hydrogel-based reverse-electrodialytic energy transduction scheme inspired by the electric eel. As in the biological system (but unlike traditional batteries), the “artificial electric organ” presented here is a soft, flexible, potentially biocompatible means of generating electricity using only a repeating arrangement of ionic gradients across selective membranes. The artificial electric organ generates numerous additive voltages at the same time in order to produce a large transient electric signal; while the biological system accomplishes this synchronicity through neural signaling, the artificial setup uses geometries that allow simultaneous mechanical registration of the gels. This scheme is flexible enough that we were able to implement it in three distinct ways: fluidically, through the printing of large arrays (enabling the generation of over 100 V), and using a Miura-ori folding geometry that assembles planar films, which achieved a power density of 27 mW m^-2. The work presented here draws inspiration from biological systems and earlier industrial efforts to extract electrical power from salinity gradients. Membrane-based strategies are well-suited for the local generation of usable energy on small scales and may be useful in the microelectromechanical systems and implantable devices of the future.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/140979/1/tomschro_1.pd