Morteza Vatani

Direct-Write Stretchable Sensors Using Single-Walled Carbon Nanotube/Polymer Matrix

There have been increasing demands and interests in stretchable sensors with the development of flexible or stretchable conductive materials. These sensors can be used for detecting large strain, 3D deformation, and a free-form shape. In this work, a stretchable conductive sensor has been developed using single-walled carbon nanotubes (SWCNTs) and monofunctional acrylate monomers (cyclic trimethylolpropane formal acrylate and acrylate ester). The suggested sensors have been fabricated using a screw-driven microdispensing direct-write (DW) technology. To demonstrate the capabilities of the DW system, effects of dispensing parameters such as the feed rate and material flow rate on created line widths were investigated. Finally, a stretchable conductive sensor was fabricated using proper dispensing parameters, and an experiment for stretchability and resistance change was accomplished. The result showed that the sensor had a large strain range up to 90% with a linear resistance change and gauge factor ∼2.7. Based on the results, it is expected that the suggested DW stretchable sensor can be used in many application areas such as wearable electronics, tactile sensors, 3D structural electronics, etc.

Anthropomorphic finger antagonistically actuated by SMA plates.

Most robotic applications that contain shape memory alloy (SMA) actuators use the SMA in a linear or spring shape. In contrast, a novel robotic finger was designed in this paper using SMA plates that were thermomechanically trained to take the shape of a flexed human finger when Joule heated. This flexor actuator was placed in parallel with an extensor actuator that was designed to straighten when Joule heated. Thus, alternately heating and cooling the flexor and extensor actuators caused the finger to flex and extend. Three different NiTi based SMA plates were evaluated for their ability to apply forces to a rigid and compliant object. The best of these three SMAs was able to apply a maximum fingertip force of 9.01N on average. A 3D CAD model of a human finger was used to create a solid model for the mold of the finger covering skin. Using a 3D printer, inner and outer molds were fabricated to house the actuators and a position sensor, which were assembled using a multi-stage casting process. Next, a nonlinear antagonistic controller was developed using an outer position control loop with two inner MOSFET current control loops. Sine and square wave tracking experiments demonstrated minimal errors within the operational bounds of the finger. The ability of the finger to recover from unexpected disturbances was also shown along with the frequency response up to 7 rad s(-1). The closed loop bandwidth of the system was 6.4 rad s(-1) when operated intermittently and 1.8 rad s(-1) when operated continuously.

Detection of the position, direction and speed of sliding contact with a multi-layer compliant tactile sensor fabricated using direct-print technology

A multi-layer resistance based compliant tactile sensor was fabricated using direct-print (DP) and soft molding processes. The sensor consists of two layers of embedded stretchable sensing elements sandwiched by three layers of a polyurethane rubber material. The sensing elements were created by the DP process using a photopolymer filled with multi-wall carbon nanotubes, which exhibit the property of piezoresistivity. The printed sensing elements were fully cured using ultraviolet light. The sensing elements within each layer of the sensor structure change in electrical resistance when external forces are applied. By processing the measured sensor signals, the fabricated sensor was able to detect the position of contact forces with a 3 mm spatial resolution, as well as their two-dimensional translation directions and speeds. Based on the results, it is concluded that the fabricated sensors are promising in robotic applications and the developed process and material can be a reliable and robust way to build highly stretchable tactile sensors.

Multi-layer stretchable pressure sensors using ionic liquids and carbon nanotubes

A stretchable and pressure sensitive polymer capable of detecting strains was developed through the incorporation of 1-ethyl-3-methylimidazolium tetrafluoroborate as an ionic liquid (IL) into a stretchable photopolymer. The developed IL/polymer composite showed both a field effect characteristic and piezoresistivity by embedding the composite between two layers of carbon nanotube (CNT)-based stretchable electrodes. A multi-layer pressure sensitive taxel was formed using a hybrid manufacturing process, where two electrode layers were fabricated by screen printing and the IL/polymer composite was formed by casting using a mold. A composite material for the electrodes was developed through the dispersion of CNTs into a highly stretchable photo/thermal crosslinkable prepolymer. The fabricated sensor was evaluated with different forces ranging from 0 to 140 g. The experiment results showed that the developed stretchable sensor had good repeatability and reliability in detecting applied pressures.

Direct-write of multi-layer tactile sensors

Direct-write of piezoresistive photopolymers is regarded as a promising means to produce compliant tactile sensors. In this work, a multi-layer compliant tactile sensor was developed using a hybrid manufacturing process including soft molding, micro-dispensing and photopolymerization processes. The working principle of the suggested sensor is to detect changes in resistance as it is deformed. A compliant skin structure was built layer-by-layer using a soft polyurethane material to cover the piezoresistive sensing elements. These sensing elements were created from stretchable photocurable conductive carbon nanotube (CNT)/prepolymer nanocomposites, which were deposited by the micro-dispensing process within the polyurethane skin layers and cured during the molding process. The fabricated tactile sensor consists of two layers of sensing elements within the skin structure; there are eight stretchable straight wires in each layer. The wires in the second layer were orthogonally placed on top of the first layer so that the sensor can detect various external forces/motions in two dimensions. The fabricated sensor was characterized by several experiments such as position, and 2D pattern detection. Finally, it is concluded that the tactile sensor using the hybrid manufacturing method and materials is promising for various applications such as robotics, prosthetics, and wearable electronics.

Direct-print photopolymerization for 3D printing

Purpose This work aims to present a guideline for ink development used in extrusion-based direct-write (DW) (also referred to as direct-print [DP]) technique and combine the extrusion with instant photopolymerization to present a solvent-free DP photopolymerization (DPP) method to fill the gap between 3D printing and printing multi-functional 3D structures. Design/methodology/approach A DP process called DPP was developed by integration of a screw-driven micro-dispenser into XYZ translation stages. The process was equipped with direct photopolymerization to facilitate the creation of 3D structures. The required characteristics of inks used in this technique were simulated through dispersion of fumed silica particles into photocurable resins to transform them into viscoelastic inks. The characterization method of these inks and the required level of shear thinning and thixotropic properties is presented. Findings Shear thinning and thixotropic properties are necessary components of the inks used in DPP process and other DP techniques. These properties are desirable to facilitate printing and filament shape retention. Extrusion of viscoelastic inks out of a nozzle generates a filament capable of retaining its geometry. Likewise, instant photopolymerization of the dispensed filaments prevents deformation due to the weight of filaments or accumulated weight of layers. Originality/value The DPP process with material-reforming methods has been shown, where there remain many shortcomings in realizing a DP-based 3D printing process with instant photopolymerization in existing literature, as well as a standard guideline and material requirements. The suggested method can be extended to develop a new commercial 3D printing system and printable inks to create various functional 3D structures including sensors, actuators and electronics, where nanoparticles are involved for their functionalities. Particularly, an original contribution to the determination of a rheological property of an ink is provided.

Detection of the direction and speed of motion of forces on the surface of a compliant tactile sensor

A compliant tactile sensor (CTS) has been developed with two orthogonal layers of multi-walled carbon nanotubes (MWNTs) and polymer strips. This has enabled the detection of sliding motion along the surface of the sensor. This information is used to detect the motions of forces applied on the surface of the CTS. The speed of sliding motion in each direction can be updated with a resolution of 3mm in both of two perpendicular directions.

Hybrid Additive Manufacturing of 3D Compliant Tactile Sensors

A resistance based conformal, compliant multi-layer tactile sensor was designed and built layer by layer using a hybrid manufacturing process. A highly stretchable, photocurable, piezoresistive sensing material was deposited on a conformal, soft molded structure using a direct printing device. The principle of the sensor is based on detecting the changes in resistance as it is deformed. The fabricated tactile sensor consists of two layers of sensing elements within the 3D skin structure where the sensing elements in the top layer are orthogonally placed atop the bottom layer. Due to the multiple layers of wires, the sensor can potentially detect various external forces/motions in two and/or three dimensions. Piezoresistivity and conductivity was introduced into the nonconductive stretchable prepolymer through dispersion of multi-walled carbon nanotubes (MWNTs). Experiments were performed to characterize the ability of the sensor to detect the location that forces were applied to the surface. Finally, it is expected that the developed conformal tactile sensor using the hybrid manufacturing method and materials could be used for various robotics and electronics applications.Copyright

Development of Direct Printing/Curing Process for 3D Structural Electronics

3D structural electronics is a new paradigm in fabricating electronics with high design complexity. Basically, manufacturing of 3D structural electronics consists of several processes: structure building, wire creation, and pick-and-place of electrical components. In this work, a 3D structure was built in a commercial AM machine, and conductive wires were created on the 3D structure with a predetermined design of an electronic circuit. Generally, 2D wire paths are projected to a 3D surface, and a tool path for the wire is generated in advance. And a direct printing device follows the tool path to draw the conductive wires on the surface, while a direct curing device simultaneously hardens the created wires using thermal/radiation energy. This direct printing/curing device was developed by combining a micro-dispensing device and a light focusing module installed in a motorized xyz stage. Several experiments were accomplished using photocrosslinkable materials filled with carbon nanotubes (CNTs). Finally, a 3D electronics prototype was fabricated to show the compelling evidence that the suggested manufacturing methods and materials would be promising in manufacturing 3D structural electronics.Copyright

An MRI-Compatible Hydrodynamic Simulator of Cerebrospinal Fluid Motion in the Cervical Spine

Goal: Develop and test an MRI-compatible hydrodynamic simulator of cerebrospinal fluid (CSF) motion in the cervical spinal subarachnoid space. Four anatomically realistic subject-specific models were created based on a 22-year-old healthy volunteer and a five-year-old patient diagnosed with Chiari I malformation. Methods : The in vitro models were based on manual segmentation of high-resolution T2-weighted MRI of the cervical spine. Anatomically realistic dorsal and ventral spinal cord nerve rootlets (NR) were added. Models were three dimensional (3-D) printed by stereolithography with 50-μm layer thickness. A computer controlled pump system was used to replicate the shape of the subject specific in vivo CSF flow measured by phase-contrast MRI. Each model was then scanned by T2-weighted and 4-D phase contrast MRI (4D flow). Results: Cross-sectional area, wetted perimeter, and hydraulic diameter were quantified for each model. The oscillatory CSF velocity field (flow jets near NR, velocity profile shape, and magnitude) had similar characteristics to previously reported studies in the literature measured by in vivo MRI. Conclusion: This study describes the first MRI-compatible hydrodynamic simulator of CSF motion in the cervical spine with anatomically realistic NR. NR were found to impact CSF velocity profiles to a great degree. Significance: CSF hydrodynamics are thought to be altered in craniospinal disorders such as Chiari I malformation. MRI scanning techniques and protocols can be used to quantify CSF flow alterations in disease states. The provided in vitro models can be used to test the reliability of these protocols across MRI scanner manufacturers and machines.