Designing Origami-Adapted Deployable Modules for Soft Continuum Arms

TPU/CNTs flexible strain sensor with auxetic structure via a novel hybrid manufacturing process of fused deposition modeling 3D printing and ultrasonic cavitation-enabled treatment

The adhesion in 3D-printed composites between dissimilar layers was investigated in response to demand for multimaterial prints. Previously, we reported on the thermal properties for printability of thermoplastic polyurethane (TPU) blends~\cite{harris001}. Here, we report the qualitative and quantitative adhesion properties between commonly-printed materials, acrylonitrile butadiene styrene (ABS) or acrylonitrile styrene acrylate (ASA), and flexible TPU. ASTM D1002/D3164 lap shear adhesion tests were performed to measure the adhesion strength between the acrylonitrile plastics and TPU, finding values of $4.7~\mathrm{MPa}$ between ABS and TPU and $6.4~\mathrm{MPa}$ between ASA and TPU. These values are on the same order of magnitude of reported commercial adhesives, suggesting that multimaterial 3D printing can provide an automated, industrially-relevant alternative to handmade fabrication. SEM images are reported to show the quality of adhesion between materials in the composites. These images expose qualitative differences in the interlayer adhesion between materials in the composites that depend on the \textit{order of printing}. Thermal (DSC and TGA) and rheological characterizations were performed to investigate the processing conditions of the extruded plastics while they are in their melt states. The role of viscosity is shown to have major implications on the adhesion, as materials with lower viscosities displayed improved filling of the interlayer voids and increased strength of adhesion.

Two designs of flexible triboelectric nanogenerators (TENGs) were successfully fabricated in three different configurations. Typically, TENG consists of anode layer, cathode layer, and electrodes. In design D1, configuration C1, the TENG was fabricated by assembling a flexible polyimide as an anode layer (bottom substrate), a flexible polyethylene terephthalate (PET) as a cathode layer (top substrate), copper (Cu) tapes as electrodes and glass as supporting layers for the top and bottom substrates. TENGs with configurations C2 and C3 were assembled by replacing cathode layer, PET, with a flexible thermoplastic polyurethane (TPU) and a flexible fabric, respectively. In design D2, configurations C1, C2 and C3 were assembled using same materials as in design D1, except anode layer, Kapton, was replaced with the polydimethylsiloxane (PDMS) sheet as bottom substrate. Multiple configurations were considered in order to determine the best material combination that would provide better TENG performance. The capability of the TENG in terms of open circuit voltage (V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</inf> ) and short circuit current (I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</inf> ) was demonstrated by subjecting it to ~ 5-6 Nm force at 10 Hz operating frequency. A maximum V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</inf> of 22.95 V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p-p</inf> , 30.44 V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p-p</inf> , power density of 0.31 μW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , 0.46 μW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were measured for the first and second designs, respectively. The TENG fabricated using PDMS and fabric exhibited a higher open circuit output.

This study addresses the mechanical behavior of lattice materials based on flexible thermoplastic polyurethane (TPU) with honeycomb and gyroid architecture fabricated by 3D printing. Tensile, compression, and three-point bending tests were chosen as mechanical testing methods. The honeycomb architecture was found to provide higher values of rigidity (by 30%), strength (by 25%), plasticity (by 18%), and energy absorption (by 42%) of the flexible TPU lattice compared to the gyroid architecture. The strain recovery is better in the case of gyroid architecture (residual strain of 46% vs. 31%). TPUs with honeycomb architecture are characterized by anisotropy of mechanical properties in tensile and three-point bending tests. The obtained results are explained by the peculiarities of the lattice structure at meso- and macroscopic level and by the role of the pore space.

The development of physiological detection is advancing rapidly, driven largely by the increase in the awareness of sport, healthcare, and biomedical knowledge. Wearable electronics have been integrated into real-world physiological sensing applications, with many recent studies aimed at enhancing their capabilities from both material selection and fabrication perspectives. To create the best fit for specific wearers, three-dimensional (3D) printing is an excellent candidate because of its potential to create structures ranging from simple to highly complex. This work investigates the effect of infill densities (20%, 40%, and 60%) on the electromechanical properties of 3D-printed thermoplastic polyurethane (TPU) using fused deposition modeling (FDM). The printing conditions were consistently controlled throughout the study, specifically using a honeycomb infill pattern. The flexible TPU substrates were successfully 3D-printed, and 1% w/v of multiwalled carbon nanotubes (MWCNTs) were embedded in the 3D-printed samples using an ultrasonic cavitation-enabled treatment and thermal-assisted method. This process aims to prevent CNT fallout while maintaining the compression load-bearing capacity. A compressive load of 10 kN was applied to the samples during electromechanical testing. The results show that a 20% infill density provides the optimum sensitivity of 11.32 MPa-1 at 2V applied voltage due to its appropriate current path, which is confirmed by scanning electron microscope (SEM). The dimension accuracy of the 3D-printed TPU samples tend to increase with higher infill densities and application of the double treatment.

Thermoplastic polyurethane (TPU) is a highly elastic linear polymer composed of soft segments, usually flexible polyester, or polyether. It is widely used in 3D printing technologies, being FFF the most common technique. However, stiffness and hardness values of 3D printed TPU in filament form are higher than would be desirable for some applications, which require softer parts. Therefore, it was seen necessary to find a new methodology for 3D printing soft TPU. In this way, the present study seeks to be the first research study that focuses on the possibility of 3D printing TPU using DIW (Direct Ink Writing) technology with UV light. Firstly, the optimal 3D printing and curing parameters to print soft TPU are determined and then two different TPU formulations are 3D printed. Then, both bulk and 3D printed TPU samples were characterized in terms of shear strength using the Warner-Bratzler shear test and hardness using the Shore hardness test. It was seen that the 3D printed samples showed higher softness than bulk samples compared to TPU filament. Finally, it was concluded that the 3D printing of TPU is challenging due to several points: (1) the viscosity of the TPU; (2) their main issue is their sticky behavior; and (3) the shrinkage which takes place after the thermal treatment and for some applications, it might be a problem. Despite that, TPU appears to be a promising material to be used in different industrial applications.

In the Fused Filament Fabrication (FFF/FDM) technology, the multi-material manufacturing additive method is achieved by a single nozzle or multiple nozzles working simultaneously with different materials. However, the adhesion between different materials at the boundary interface in FDM multi-material printing is a limiting factor. These studies are concerned with improving and study the adhesion between two polymers.Due to the numerous applications and possibilities of 3D printed objects, combining different materials has become a subject of interest. PLA is an alternative to the use of petrochemical-based polymers. Thermoplastic Polyurethane is a flexible material that can achieve different characteristics when combined with a rigid filament, such as PLA. To improve the adhesion between PLA and TPU in multi-material FFF/FDM, we propose the comparison of different processes: post-processing with acetone immersion, surface activation during printing with Acetone, surface activation during printing with tetrahydrofuran, post-processing annealing, and connection of printed parts with tetrahydrofuran.Modifying the 3D printing process improved the quality of the adhesive bond between the two different polymers. Activation of the surface with THF is the treatment method recommended by the authors due to the low impact on the deformation/degradation of the object.In the study, adhesion was considered in relation to the circular pattern of surface development. Further analysis should include other surface development patterns and changes in printing parameters, e.g. process temperatures and layer application speed.3D printing with multi-materials, such as PLA biopolymer and thermoplastic polyurethane, allows for the creation of flexible connections. The strengthening of the biopolymer broadens the possibilities of using polylactide. Examples of applications include: automotive (elements, where flexible TPU absorbs vibrations and protects PLA from cracking), medicine (prostheses with flexible elements ensuring mobility in the joints).Multi-material printing is a new trend in 3D printing research, and this research is aimed at promoting the use and expanding the possibilities of using PLA biopolymer.

This paper presents an antenna-based bidirectional stretch sensor for real-time wireless sensing applications. The proposed design utilizes an antenna structure with two orthogonal dipole elements operating at different frequency bands to independently sense strain in two perpendicular directions. One pair of elements is configured to operate at 2.4 GHz, while the other operate at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{5 GHz}$</tex>. The antenna is fabricated using direct ink writing (DIW) with stretchable silver ink on a flexible thermoplastic polyurethane (TPU) substrate, enabling the mechanical flexibility required for stretch sensing. Under applied stress, the antenna's physical dimensions change, leading to a shift in its resonant frequency. This frequency shift is observed to be linearly correlated with the applied strain along either axis (x-axis or <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$y$</tex>-axis). Experimental results show that for stretch levels ranging from 0 % to 10 % of the original dimensions, the resonant frequency shifts by approximately 6 % in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{x}$</tex>-axis direction and 7 % in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$y$</tex>-axis direction, respectively. The proposed design demonstrates potential for applications requiring simultaneous stretch sensing and wireless communication using the same antenna structure.

Fabric-based pneumatic actuators (FPAs) are extensively employed in the design of lightweight and compliant soft wearable assistive gloves. However, conventional FPAs typically exhibit limited output force, thereby restricting the applications of such gloves. This paper presents the development of a novel honeycomb pneumatic actuator (HPA) constructed using flexible thermoplastic polyurethane (TPU) coating through hot pressing or ultrasonic welding techniques. Compared to the previously utilized double-layer fabric-based pneumatic actuators (DLFPAs), the HPAs yields a remarkable 862% increase in end output force. It can produce a tip force of 13.57 N at a pressure of 150 kPa. The integration of HPAs onto a soft pneumatic glove enables the facilitation of various activities of daily living. A series of trials involving nine patients were conducted to assess the effectiveness of the soft glove. The experimental results indicate that when assisted by the glove, the patients' finger metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints achieved angles of 87.67 ± 19.27° and 64.2 ± 30.66°, respectively. Additionally, the average fingertip force reached 10.16 ± 4.24 N, the average grip force reached 26.04 ± 15.08 N, and the completion rate of daily functions for the patients increased from 39% to 76%. These outcomes demonstrate that the soft glove effectively aids in finger movements and significantly enhances the patients' daily functioning.

Textiles are an essential part of human lives and are associated with them from cradle to grave. There is a limitless choice of materials, processes, designs, and applications of fibers and textiles. However, conventional textile processing uses considerable resources and is not always environment friendly. Three-dimensional (3D) printing offers new avenues and provides a considerable value in addition to textile materials. From simple printing of designs or patterns, complex structures such as wearable electronics and space textiles are made using 3D technology. Similarly, the creation of nanostructures, self-colored clothing, and imparting functional finishes are also done through 3D printing. Innovative smart textiles that contain bioactive agents, carbon nanotubes, etc. which can provide high-performance properties, improve appearance or durability, which can be done using 3D printing. One such example is of temperature-regulated clothing developed using 3D printing. In this chapter, we will provide an overview of the opportunities and the challenges of 3D printing in fashion and textiles. Examples include the application of 3D printing in medical textiles, industrial textiles, jewelry, and fashion. Most 3D-printing 190technologies are done using synthetic polymers such as nylon, polyester, poly(lactic acid), and thermoplastic polyurethane. Possibilities of developing 3D-printed objects using natural polymers or other additives have been discussed.

This study evaluates the three-dimensional (3D) printing materials used in Fused Deposition Modeling (FDM) printing technology. 3D printing technology has been developing strongly, becoming an effective support tool in production and research. The 3D printing process involves many stages, with many parameters affecting the quality and properties of the product, in which 3D printing material is one of many essential factors affecting that process. The study conducts a comprehensive assessment of the most common materials in 3D printing technology to determine the advantages and limitations of precisely five types of materials: Polylactic acid, acrylonitrile butadiene styrene, polyethylene terephthalate glycol-modified, thermoplastic polyurethane, and acrylonitrile styrene acrylate. With 3D printing, parameters such as sintering temperature, printing speed, and layer thickness are kept constant. These parameters are applied equally to all five material samples. The experiment evaluates the tensile strength of materials. The study results provide an overview of the properties and applicability of 3D printing materials, helping to select materials suitable for specific FDM 3D printing technology applications.

Influence of thermoplastic polyurethane (TPU) and printing parameters on the thermal and mechanical performance of polylactic acid (PLA) / thermoplastic polyurethane (TPU) polymer

Additive manufacturing of polymers is gaining momentum in health care industries by providing rapid 3D printing of customizable designs. Yet, little is explored about the cytotoxicity of leachable toxins that the 3D printing process introduced into the final product. We studied three printable materials, which have various mechanical properties and are widely used in stereolithography 3D printing. We evaluated the cytotoxicity of these materials through exposing two fibroblast cell lines (human and mouse derived) to the 3D-printed parts, using overlay indirect contact assays. All the 3D-printed parts were measured toxic to the cells in a leachable manner, with flexible materials more toxic than rigid materials. Furthermore, we attempted to reduce the toxicity of the 3D-printed material by employing three treatment methods (further curing, passivation coating, and Soxhlet solvent extraction).The Soxhlet solvent extraction method was the most effective in removing the leachable toxins, resulting in the eradication of the material’s toxicity. Passivation coating and further curing showed moderate and little detoxification, respectively. Additionally, mechanical testing of the materials treated with extraction methods revealed no significant impacts on its mechanical performances. As leachable toxins are broadly present in 3D-printed polymers, our cytotoxicity evaluation and reduction methods could aid in extending the selections of biocompatible materials and pave the way for the translational use of 3D printing.

Recently, responsive structure color fibers and fabrics have been designed and prepared for colorimetric detecting of volatile organic compounds (VOCs). Fabric substrates can offer greater flexibility and portability than flat and hard substrates such as glass, silicon wafers, etc. At present, one-dimensional photonic crystal (multilayer films) and three-dimensional dense photonic crystal layers are mainly constructed on fabrics to achieve the response to VOCs. However, the binding force between these structural color coatings and the fabrics was poor, and the dense structures inevitably hindered the diffusion of VOCs. Here, thermoplastic polyurethane (TPU) inverse opal (IOs) fabrics were prepared by sacrificing the SiO2 photonic crystal templates to achieve colorimetric detecting of VOCs. The IOs layer of TPU was cured directly on the fabric surface, TPU infiltrated into the fabric yarns, and bonded the fabrics and IOs layer into a whole, which greatly improved the binding force, and the porous structure and large specific surface area of IOs were conducive to the diffusion of VOCs. The results showed that the TPU IOs fabrics have large reflection peak shifts to DMF, THF, toluene and chloroform vapors, and its concentration has a good linear relationship with the maximum reflection peak value of TPU IOs fabrics. The theoretical detection limits are 1.72, 0.89, 0.78 and 1.64 g m−3, respectively. The response times are 105, 62, 75 and 66 seconds, with good stability. Finally, it was calculated that the discoloration of the TPU IOs fabrics in VOCs was due to the joint-effects of lattice spacing and effective refractive index increase.

Additive manufacturing (AM), also known as 3D printing, has been showing significant growth in recent years. Due to the several advantages, these processes provide, such as high process speed, the possibility for non-conventional geometries and a large range of materials; AM is quickly becoming an industry standard for both prototyping and end-user products. There are several 3D printing technologies available in the market, the most prevalent being fused deposition modeling (FDM), also known by fused filament fabrication (FFF). FDM is widely used as it is highly versatile and has the lowest entry level cost of any AM technology. Due to the layer-by-layer nature of the production procedure, 3D printed materials tend to behave with highly anisotropic properties. The main aspect that governs the anisotropy in FDM is the printing orientation. In order to study these effects, as well as select the optimal materials for future work, bulk specimens were produced from three different materials: polylactic acid (PLA), polycarbonate (PC), and thermoplastic polyurethane (TPU). These specimens were then submitted to tensile testing. In this work, the printing orientations used to manufacture the tested materials were 0°, 45°, and 90° for 100% line infill, with the bulk specimens being produced flat on the printing bed. Steps were taken to prevent layer adhesion failure as well as optimize the production of flexible materials, in this case TPU. The results were then compared with the specifications given by the supplier for FDM and with the raw material properties found in the literature. As expected, for all the specimens, infill orientation played a major role on both tensile strength and elongation. With the results presented in this work, it was possible to select TPU and PC as the most interesting materials to combine, given their broadly different properties. As for future work, a multi-material specimen will be produced, varying the percentage of each material as well as the placement and distribution on the testing bulk.