3D printing technologies have advanced significantly in recent years allowing for additive manufacturing of new structured materials, expanding the range, function, and capabilities of manufactured components. In this work, flexible capacitors were produced using additive manufacturing and compared to commercially available capacitance sensors in strain testing. The sensors utilize thermoplastic polyurethane (TPU) printed using fused filament fabrication methods as a dielectric substrate and a combination of flexible inks for production of the conductive surface. Flexible inks were printed using syringe based deposition methods on a custom designed printer using the TPU substrate. Results demonstrated successful capacitor production with capacitance values ranging from 2–70 pF depending on geometry, material, and printing conditions. The 3D printed flexible capacitors were characterized over a frequency range of 100 Hz to 10 kHz and compared to commercial roll-to-roll produced capacitors. Strain testing was conducted from 0–50% strain using a mechanical testing machine for the range of sensors and final capacitance post strain was measured to calculate deviation from original capacitance values. The sensors exhibited a relatively linear increase in capacitance when strained and returned to a resting position upon release of strain with minimal hysteresis effects, demonstrating their utility as 3D printed sensors.

3D printed flexible sensors have demonstrated great potential for utilization in a variety of different applications including healthcare, environmental sensing, and industrial applications. In recent years, research on this topic has increased to meet low-cost sensing needs due to the development of innovative materials and printing techniques that reduce cost, production time, and enhance the electrical and mechanical properties of the sensors. This paper presents computational simulations of 3D printed flexible sensors, capable of producing an output signal based on the deformation caused by external forces. Two different sensors were designed and tested, working based on a capacitance and resistance change produced by structural deformation. The capacitance sensor was designed maximizing the area of the electrodes and distributing the electrodes over a flexible membrane taking advantage of the produced deformation to reduce the distance between the electrodes. The reduction in the distance between the electrodes increases the capacitance value of the structure. The capacitance sensor was able to almost triple its baseline capacitance when 30 kPa of pressure was applied. The resistance sensor was designed with one continuous flexible conductive element attached to a flexible membrane, taking advantage of the distortion induced in the conductive element. The deformation in the conductive element increases the length of the resistor and causes the resistance value of the structure to increase. The resistance sensor was able to increase its resistance by 1200 ω with 30 kPa of applied pressure. Finally, preliminary results of 3D printed sensors were demonstrated.

Inflatable structures provide significant volume and weight savings for future space and soft robotic applications. Structural health monitoring (SHM) of these structures is essential to ensuring safe operation, providing early warnings of damage, and measuring structural changes over time. In this paper, we propose the design of a single flexible strain sensor for distributed monitoring of an inflatable tube, in particular, the detection and localization of a kink should that occur. Several commercially available conductive materials, including 3D-printing filaments, conductive paint, and conductive fabrics are explored for their strain-sensing performance, where the resistance change under uniaxial tension is measured, and the corresponding gauge factor (GF) is characterized. Flexible strain sensors are then fabricated and integrated with an inflatable structure fabric using screen-printing or 3D-printing techniques, depending on the nature of the raw conductive material. Among the tested materials, the conductive paint shows the highest stability, with GF of 15 and working strain range of 2.28%. Finally, the geometry of the sensor is designed to enable distributed monitoring of an inflatable tube. In particular, for a given deformation magnitude, the sensor output shows a monotonic relationship with the location where the deformation is applied, thus enabling the monitoring of the entire tube with a single sensor.

Four dimensional (4D) printing is the convergence of three dimensional (3D) printing, which is an emerging additive manufacturing technology for smart materials. 4D printing is referred to the capability of changing the shape, property, or functionality of a 3D printed structure under a particular external stimulus. This paper presents the structural performance, shape memory behavior and photothermal effect of 4D printed pristine shape memory polymer (SMP) and it’s composite (SMPC) with multi-walled carbon nanotubes (MWCNTs). Both materials have demonstrated the ability to retain a temporary shape and then recover their original. It is revealed that the incorporation of MWCNTs into the SMP matrix has enhanced the light stimulus shape recovery capabilities. Light stimulus shape transformation of 4D printed SMPC is advantageous for space engineering applications as light can be focused onto a particular area at a long distance. Subsequently, a model 4D printed deployable boom, which is applicable for small spacecrafts is presented. The shape fixity and recovery behaviors of the proposed boom have been investigated. Notably, the model boom structure has demonstrated ∼86 % shape recovery ratio. The proposed innovative approach of additive manufacturing based deployable composite structures will shape up the future space technologies.

In the traditional 4D printing method using Shape Memory Polymer (SMP), the design process and preparation of 4d printing are complex. In this research, we proposed a design method of a temperature-driven SMP smart structure and made Realization. This smart structure also a bilayer structure use an SMP material in one printing process to realize the deformation in 4D printing. The design of the smart structure is mainly realized by parameter allocation in the printing process, such as print line width, print line height, print temperature, simulation temperature, and fill the form in Fused Deposition Modelling (FDM). Through experimental determination and analysis of statics and thermodynamics, our method fitting out the model relationship between process parameters and the curvature and strain of smart structure. This bilayer smart structure widely applied to the self-folding. In the example stage, this paper mainly uses PLA as an SMP material for the preparation of structure. Observing that the motion behaviors of the smart structure conformed to the model measured in this paper, the average accuracy of the strategy reaches 95%.

Additive manufacturing is an enabling technology that is rapidly advancing with the development of new printers, materials, and processes. The purpose of this research was to design a part that could function similar to a pneumatic piston-cylinder producing small force outputs between 5 and 10 N. The research presented in this paper looks at various types of 3D printing methods to produce flexible linear bellows actuators to achieve this functionality. In particular, stereolithography, fused deposition modeling, digital light processing, and Polyjet printing were examined to produce a variety of test actuators. A successful flexible part was designed and produced using Polyjet printing, the steady state and dynamic responses of constructed actuators were measured and characterized at various loading conditions. The displacement trends at different load conditions followed a non-linear path, exhibiting highly elastic deformation typical of the flexible resins used in this project.

A key aspect of color change is altering perceived value or intensity. This paper presents a methodology to achieve value change through mechanical means via the deflection of bistable structures. We create mechanical pixel-based, reversible color change using 3D printed switchable bistability. Switchable bistability arises from the combination of pre-strain and shape memory, enabling us to access multiple elastically programmed shapes at elevated temperatures with fast morphing and low actuation forces, while retaining high stiffness at room temperature. Building on our previous study that achieved bistability through FDM printing with directional pre-stress, finite element analysis is conducted to design a pixel-like structure that acts as a unit cell with color change capabilities. Quantitative and qualitative analysis is conducted through image processing techniques in order to prove the viability of this approach to creating value change through geometric deformation of bistable structures. By leveraging this technique, there are numerous potential applications in fields including robotics, architecture, and interior design.

Additive manufacturing is an emerging method to produce customized parts with functional materials without big investments. As one of the common additive manufacturing methods, fused deposition modeling (FDM) uses thermoplastic-based feedstock. It has been recently adapted to fabricate composite materials too. Acrylonitrile butadiene styrene (ABS) is the most widely used material as FDM feedstock. However, it is an electrically insulating polymer. Carbon Nanotubes (CNTs) on the other hand are highly conductive. They are attractive fillers because of their high aspect ratio, and excellent mechanical and physical properties. Therefore, a nanocomposite of these two materials can give an electrically conductive material that is potentially compatible with FDM printing. This work focuses on the investigation of the relationships between the FDM process parameters and the electrical conductivity of the printed ABS/CNT nanocomposites. Nanocomposite filaments with CNT contents up to 10wt% were produced using a twin-screw extruder followed by 3D printing using FDM method. The starting material was pellets from a masterbatch containing 15 wt% CNT. Compression-molded samples of ABS/CNT were also prepared as the bulk baselines. The effects of CNT content and nozzle size on the through-layer and in-layer electrical conductivity of the printed nanocomposites were analyzed. Overall, a higher percolation threshold was observed in the printed samples, compared to that of the compression-molded counterparts. This resulted in the conductivity of the printed samples that is at least one order of magnitude lower. Moreover, at CNT contents up to 5 wt%, the in-layer conductivity of the printed samples was almost two orders of magnitudes higher than that in the through-layer direction. In ABS/3 wt% CNT samples, the through-layer conductivity continuously decreased as the nozzle diameter was decreased from 0.8 mm to 0.35 mm. These variations in the electrical conductivity were explained in terms of the CNT alignment, caused by the extrusion process during the print, quality of interlayer bonding during deposition, and the voids created due to the discrete nature of the printing process.

Additive manufacturing has emerged as an alternative to traditional manufacturing technologies. In particular, industries like fluid power, aviation and robotics have the potential to benefit greatly from this technology, due to the design flexibility, weight reduction and compact size that can be achieved. In this work, the design process and advantages of using 3D printing to make soft linear actuators were studied and highlighted. This work explored the limitations of current additive manufacturing tolerances to fabricate a typical piston-cylinder assembly, and how enclosed bellow actuators could be used to overcome high leakage and friction issues experienced with a piston-cylinder type actuator. To do that, different 3D printing technologies were studied and evaluated (stereolithorgraphy and fused deposition modeling) in the pursuit of high-fidelity, cost-effective 3D printing. The initial attempt consisted of printing the soft actuators directly using flexible materials in a stereolithography-type 3D printer. However, these actuators showed low durability and poor performance. The lack of a reliable resin resulted in the replacement of this material by EcoFlex ® 00-30 silicone and the use of a 3D printed mold to cast the actuators. These molds included a 3-D printed dissolvable core inside the cast actuator in order to finish the manufacturing process in one single step. An experimental setup to evaluate the capabilities of these actuators was developed. Results are shown to assess the steady-state and the dynamic characteristics of these actuators. These tests resulted into the stroke-pressure and stroke-time responses for a specific load given different proportional valve inputs.

Fused deposition modeling (FDM) is highly commercialized Rapid Prototyping (RP) technology for its ability to build complex parts with low cost in a short period of time. The process parameters in the FDM play a vital role in the mechanical properties of the polymeric parts. Most of the research studies show that the variable parameters such as orientation, layer thickness, raster angle, raster width, and air gap are some of the key parameters that affect the mechanical properties of FDM-processed polymeric parts. However, no reports have been made regarding the influence of nozzle diameter with raster width on the tensile properties of FDM fabricated polymeric parts. This work was devoted to achieving improved and isotropic mechanical properties in polycarbonate (PC) and PC/carbon nanotube (PC/CNT) nanocomposites by investigating the effect of printing parameters in FDM process. The nozzle diameter to raster width ratio, α was found to significantly affect the mechanical properties. The printing direction dependency in tensile properties were studied with the ratio α < 1 and α≥ 1 at three different raster angles of 0°, 45°/−45° and 90°. For α < 1, Ultimate tensile strength and modulus of elasticity were higher for 0°, compared to 45°/−45° and 90° raster angles. However, for α ≥ 1, the ultimate tensile strength and the modulus of elasticity showed little dependency to print direction. This certainly determines the decrease in anisotropy at higher values of α. Mesostructure characterization with microscopy and image analysis were used to further explain the printing behavior and the resultant properties of the printed samples.

Electroactive polymers are a class of materials capable of reallocating their shape in response to an electric field while also having the ability to harvest electrical energy when the materials are mechanically deformed. Electroactive polymers can therefore be used as sensors, actuators, and energy harvesters. The parameters for manufacturing flexible electroactive polymers are complex and rate limiting due to number of steps, their necessity, and time intensity of each step. Successful 3D printing manufacturing processes for electroactive polymers will allow for scalability and flexibility beyond current limitations, improving the field, opening additional manufacturing possibilities, and increasing output. The goal for this research is to use additive manufacturing techniques to print conductive and dielectric substrates for building flexible circuits and sensors. Printing flexible conductive layers and substrates together allows for added creativity in design and application. In this work we have successfully demonstrated additive production of a simple flexible circuit using exclusively additive manufacturing.

Shape-memory polymers (SMPs) as stimuli-responsive shape-changing materials gained significant interest in recent years. Their developments have challenged the conventional understanding of the polymer effect and have further enhanced and broadened the applications of the smart materials. Nowadays, 4D printing is seen as an emerging technology that combines smart materials and additive manufacturing, which can be used to design active mechanical structures. It provides tremendous potential for engineering applications which is capable of producing complex, stimuli-responsive 3D structures. While many “ad hoc” designs of 4D printed solutions have been progressively developed for a specific process, the general approach of additive manufacturing that integrates smart materials in real time across an entire product development process is not pervasive in the industry. To solve this issue, the authors propose a general 4D printing oriented framework for the design of multi-functional SMPs architectures. This framework is not intended to be an exhaustive and specific instruction but is instead a means to motivate these designers to seek the process of applying these unique functional materials to their own designs and applications. It will be useful and give more insight into the design process of the SMP device.

Dielectric electroactive polymers are materials capable of mechanically adjusting their volume in response to an electrical stimulus. However, currently these materials require multi-step manufacturing processes which are not additive. This paper presents a novel 3D printed flexible dielectric material and characterizes its use as a dielectric electroactive polymer (DEAP) actuator. The 3D printed material was characterized electrically and mechanically and its functionality as a dielectric electroactive polymer actuator was demonstrated. The flexible 3-D printed material demonstrated a high dielectric constant and ideal stress-strain performance in tensile testing making the 3-D printed material ideal for use as a DEAP actuator. The tensile stress-strain properties were measured on samples printed under three different conditions (three printing angles 0°, 45° and 90°). The results demonstrated the flexible material presents different responses depending on the printing angle. Based on these results, it was possible to determine that the active structure needs low pre-strain to perform a visible contractive displacement when voltage is applied to the electrodes. The actuator produced an area expansion of 5.48% in response to a 4.3 kV applied voltage, with an initial pre-strain of 63.21% applied to the dielectric material.

Robotic Materials are materials that have sensing, computation and, possibly actuation, distributed throughout the bulk of the material. In such a material, we envision semiconducting polymer based sensing, actuation, and information processing for on-board decision making to be designed, in tandem, with the smart product that will be implemented with the smart material. Prior work in printing polymer semiconductors for sensing and cognition have focused on highly energetic inkjet printing. Alternatively, we are developing liquid polymer extrusion processes to work hand-in-hand with existing solid polymer extrusion processes (such as Fused Deposition Manufacturing - FDM) to simultaneously deposit sensing, computation, actuation and structure. We demonstrate the successful extrusion printing of conductors and capacitors to impedance-match a new, higher-performance organic transistor design that solves the cascading problem of the device previously reported and is more amenable to liquid extrusion printing. Consequently, these printed devices are integrated into a sheet material that is folded into a 3-D, six-legged walking machine with attached electric motor.

This paper introduces a 4D printing method to program shape memory polymers (SMPs) during fabrication process. Fused deposition modeling is employed to program SMPs during depositing the material. This approach is implemented to fabricate complicated polymeric structures by self-bending features without need of any post-programming. Experiments are conducted to demonstrate feasibility of one-dimensional (1D)-to 2D and 2D-to-3D self-bending. It is shown that 4D printed plate structures can transform into 3D curved shell structures by simply heating. A 3D macroscopic constitutive model is developed to predict thermo-mechanical behaviors of the printed SMPs. Governing equations are also established to simulate programming mechanism during printing process and shape change of self-bending structures. In this respect, a finite element formulation is developed considering von-Kármán geometric non-linearity and solved by implementing iterative Newton-Raphson scheme. The accuracy of the computational approach is checked with experimental results. It is shown that the structural-material model is capable of replicating the main features observed in the experiments.

In the field of Additive Manufacturing (AM), one of the major applications of laser-based 3D metal printing is the creation of custom implants for medical purposes. However, a significant challenge in the manufacturing of implants using Selective Laser Melting (SLM) is the formation of partially melted particles on the surface of medical implants. These particles result in a multitude of issues including plurality of structurally weak points on the designed implants, obstruction of important design features, and possibility of dislodgement over the service life span, thereby posing a threat to the recipient. To address the above challenges, it is imperative to develop a simple but effective surface cleaning method to remove partially melted particles from the surface without damage to the designed medical implants. In this work, a comparative study was conducted to investigate the effect of both chemical and electro-plasma based cleaning processes on the removal of partially melted particles from the surfaces of 3D printed Ti-6Al-4V medical screw implants. These techniques include chemically polishing implants with HF-HNO 3 acid solutions and using an electro-plasma based cleaning process. With the field of additive manufacturing rapidly expanding, this work offers valuable insight on proper post-process treatment of 3D printed parts for future medical purposes in biomedical fields.

Printing technologies are attractive methods for high-throughput additive manufacturing of nanomaterials-based thin film electronics. Roll-to-roll (R2R) compatible techniques such as gravure printing can operate at high-speed (1–10 m/s) and high-resolution (< 10 μm) to drive down manufacturing costs and produce higher quality flexible electronic devices. However, large-scale deployment of printed wireless sensors, flexible displays, and wearable electronics, will require greater understanding of the printing physics of nanomaterial-based inks in order to improve the resolution, reliability, and uniformity of printed systems. In this study, we designed and constructed a custom sheet-fed gravure printer which features registered multilayer printing for nanomaterial exploration and thin film device development. The design allows precise, independent control of the speeds and forces of each of the subprocesses of gravure (ink filling, wiping, and transfer), enabling novel experimental controls for dissecting the printing process fluid mechanics. We use these new capabilities to investigate the primary artifacts which distort printed nanomaterial patterns, such as dragout tails, edge roughness, and pinholes. These artifacts are studied as a function of print parameters such as contact pressure, wiping speed, and transfer speed, by printing silver nanoparticle ink to form continuous features with dimensions in the range of 100 μm to 10 mm. We found that the contact mechanics of the ink transfer process have a strong influence on the formation of dragout artifacts, indicating the presence of a transfer-driven squeezing flow which distorts the trailing edges of features. By engineering the transfer contact mechanics with varying rubber substrate backing stiffness, we found it is also possible to suppress this artifact formation for a particular nanomaterial ink. The improved areal uniformity and print quality achieved using these methods highlight the potential for gravure printing to be a versatile nano-manufacturing tool for patterning a variety of thin film smart materials. We also hope that the open-source printer designs presented here can serve to accelerate the development of high-speed nanomaterial printing.

Total Knee Replacement (TKR) is a very common procedure in the United States, especially with the aging population. However, despite high numbers of procedures and advancing technology, about 20% of patients with TKR are unsatisfied with the level of discomfort they experience with their replacement. Prevailing theories suggest that this is due to gradual misalignment of the knee. Multiple methods have been attempted to detect the cause of mechanical failure in replacements. One possible method for performing state detection in knees is the embedding of piezoelectric transducers (PZTs) into the bearing component. Preliminary testing of PZT’s embedded in simplified plastic components has shown that this method contains promise. With this said, further testing on realistic knee implant components is still needed to solidify the method’s validity. Commercial knee implant bearings utilize medical grade Ultra-High Molecular Weight Polyethylene (UHMW) and manufacturers utilize proprietary processing technology to develop the final components. This work focuses on the development of surrogate knee implant prototypes that replicate the material and geometric properties of actual knee implants to provide a convenient and economical solution to evaluate the performance of embedded PZTs. In this work, scans of an original knee bearing are taken and used to create a 3D model. From there, a variety of processes including 3D printing and Computer Numerical Controlled (CNC) machining are used to develop surrogate prototypes that are compared for accuracy to a benchmark. This benchmark is taken as a polished CNC machined non-medical grade UHMW prototype. Standards that the prototypes must meet include cost and time effectiveness as well as similarity in geometry and material property to the benchmark. The performance of the prototypes is experimentally compared through mechanical load testing by using pressure sensitive films placed between the femoral and bearing components of the implant as well as measuring piezoelectric output. In addition, the measured voltage output is compared to predictions from an analytical model for validation of the piezoelectric performance. These two experiments help to derive information about the applied load distribution and location, allowing comparisons to be made to the benchmark. This study shows that, while some types of 3D printing, such as fused deposition modeling, provide fast and cheap prototypes, other options such as stereolithography printing produce higher quality and more replicative components. Results of this study can be used in the development of useful surrogates for the advancement of biomedical sensors.

Direct write (DW) technology offers a simple method of rapid manufacturing technology for printing electronic, optoelectronic devices, and complex functional devices. The key component of DW technology is the functional inks, which are colloidal suspensions of functional nanoparticles in various solvents such as aerosol or liquid form. With a DW approach, patterns or structures can be easily deposited on flexible substrates such as paper, plastics, and composites, once the solvent volatilizes or is driven off via conventional, laser, or microwave sintering. In this work, polymer-assisted silver (Ag) nanoinks were synthesized by silver salt and polymer in the water solution at relatively high silver precursor concentrations and relatively low concentration of polymers. The silver nanoparticle dispersion and morphology was examined by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The results showed that the size of Ag nanoparticles was in nanoscale (∼20 nm) with a narrow distribution of Ag nanoparticle sizes. The viscosity and thermal properties of synthesized silver nanoinks were characterized to determine their applicability and the lifetime. It has been shown that the synthesized silver nanoink can be printed on a flexible plastic substrate or glass substrate. The morphology of the Ag nanoink line printed on the substrate was observed by optical microscopy and scanning electron microscopy (SEM) to understand the relationship between the microstructure and wettability. Uniaxial tension tests of silver nanoink line on a Kapton film indicate that the ink can be stretched ∼20% without failure. The resistance of silver nanoink line printed on the Kapton films was also measured by four probe conductivity measurement system to assess the electrical performance. The resistivity is about 7.5 × 10 −5 Ω-cm by thermal treatment at 250°C for 30 min, which is about half that of bulk silver (1.6 × 10 −6 Ω-cm). Overall, the performance of the synthesized silver nanoink is comparable to a commercially available ink with lower Ag weight content at relatively low cost.

Different routes for electrode processing which fulfill the requirements of piezoelectric transducer will be presented. One attempt is the electrode deposition via inkjet printing, another is the deposition via an air-brush technique. For the preparation of electrodes via inkjet printing, different inks such as a silver composite or the semiconducting poly(3,4-ethylene-dioxythiophene): polystyrenesulfonate (PEDOT:PSS) are used. A further attempt is the deposition of carbon nano tubes (CNT’s) via an air-brush technique. For all three systems the ink or solution formulation, the deposition techniques, suitable parameter and partly additional encapsulation steps will be discussed in detail accompanied by a description of electrode properties, e.g. the conductivity, as well as by the characterization of the materials poling behavior in particular.