Direct-write Process Research Articles

Highly conductive and sensitive alginate hydrogel strain sensors fabricated using near-field electrohydrodynamic direct-writing process

Hydrogel flexible sensors have attracted considerable attention because of their wearability, biocompatibility, and precision signal transmission capability. However, the hydrogel strain sensors fabricated by conventional printing or hand-injection methods have difficulty balancing their mechanical strength and sensing characteristics, limiting the application of hydrogel strain sensors. Herein, polyvinyl alcohol and polyacrylamide were loosely crosslinked with sodium alginate through chemical cross-linking. Subsequently, MXene nanosheets were introduced for doping, the crosslinked hydrogel conductive network was constructed, and the hydrogel strain sensors were fabricated using the electrohydrodynamic (EHD) printing method. The ions in the EHD-printed hydrogel undergo directional movement under an externally enhanced electric field, causing the formation of more uniform and dense porous conductive networks inside the hydrogel, and high electrical conductivity (0.49 S m−1) is obtained. These hydrogel strain sensors have excellent mechanical properties (tensile strength: 0.17 MPa at 787 % strain), high sensitivity (gauge factor: 1.54 at 0–100 % strain), and low detection limits (1 % strain). Furthermore, demonstrations of real-time Morse code tapping information transmission, handwriting recognition during writing, and human physiological behavior monitoring demonstrations using the fabricated sensors indicate that the EHD-printed hydrogel strain sensor method has significant potential for wearable devices and human-computer interaction applications.

Direct Writing of SiC-polymer nanocomposites for humidity sensing with enhanced performance

Humidity sensors are essential for monitoring humidity levels in various fields. This paper investigates the development and optimization of humidity sensitive SiC-Fe3O4 polymer nanocomposites with varying Fe3O4 concentrations fabricated using Direct Writing (DW) process. The inks were evaluated for flow behavior to ensure proper extrudability through the nozzle. The rheological behavior was then correlated with the printing parameters to optimize the printing accuracy of the nanocomposites. It was observed that all the inks exhibited shear-thinning behavior, enabling a smooth extrusion process. To ensure high-resolution features, a new image analysis method was introduced to quantify corner rounding in printed features, enabling the optimization of printing parameters. In terms of humidity sensing performance, all the inks displayed a change in electrical properties with humidity adsorption. The fabricated nanocomposites showed a decrease in electrical conductivity with increasing humidity, suggesting potential for humidity-sensing applications. Notably, the humidity sensitivity of the nanocomposites was highly dependent on the Fe3O4 concentration in the inks. These findings provide valuable insights into the DW of SiC-Fe3O4 polymer nanocomposites for humidity sensing applications.

Three-Dimensional-Printed Composite Structures: The Effect of LSCF Slurry Solid Loading, Binder, and Direct-Write Process Parameters.

In this research, a direct-write 3D-printing method was utilized for the fabrication of inter-digitized solid oxide fuel cells (SOFCs) using ceramic materials. The cathode electrode was fabricated using the LSCF (La0.6Sr0.2Fe0.8Co0.2O3-δ) slurry loading and the Polyvinyl butyral (PVB) binder. The rheological parameters of slurries with varying LSCF slurry loading and PVB binder concentration were evaluated to determine their effect on the cathode trace performance in terms of microstructure, size, and resistance. Additionally, the dimensional shrinkage of LSCF lines after sintering was investigated to realize their influence on cathode line width and height. Moreover, the effect of the direct-write process parameters such as pressure, distance between the nozzle and substrate, and speed on the cathode line dimensions and resistance was evaluated. LSCF slurry with 50% solid loading, 12% binder, and 0.2% dispersant concentration was determined to be the optimal value for the fabrication of SOFCs using the direct-write method. The direct-write process parameters, in addition to the binder and LSCF slurry concentration ratios, had a considerable impact on the microstructure of cathode lines. Based on ANOVA findings, pressure and distance had significant effects on the cathode electrode resistance. An increase in the distance between the nozzle and substrate, speed, or extrusion pressure of the direct writing process increased the resistance of the cathode lines. These findings add to the ongoing effort to refine SOFC fabrication techniques, opening the avenues for advanced performance and efficiency of SOFCs in energy applications.

Understanding the Effect of Dispersant Rheology and Binder Decomposition on 3D Printing of a Solid Oxide Fuel Cell.

Solid oxide fuel cells (SOFCs) are a green energy technology that offers a cleaner and more efficient alternative to fossil fuels. The efficiency and utility of SOFCs can be enhanced by fabricating miniaturized component structures within the fuel cell footprint. In this research work, the parallel-connected inter-digitized design of micro-single-chamber SOFCs (µ-SC-SOFCs) was fabricated by a direct-write microfabrication technique. To understand and optimize the direct-write process, the cathode electrode slurry was investigated. Initially, the effects of dispersant Triton X-100 on LSCF (La0.6Sr0.2Fe0.8Co0.2O3-δ) slurry rheology was investigated. The effect of binder decomposition on the cathode electrode lines was evaluated, and further, the optimum sintering profile was determined. Results illustrate that the optimum concentration of Triton X-100 for different slurries was around 0.2-0.4% of the LSCF solid loading. A total of 60% of solid loading slurries had high viscosities and attained stability after 300 s. In addition, 40-50% solid loading slurries had relatively lower viscosity and attainted stability after 200 s. Solid loading and binder affected not only the slurry's viscosity but also its rheology behavior. Based on the findings of this research, a slurry with 50% solid loading, 12% binder, and 0.2% dispersant was determined to be the optimal value for the fabricating of SOFCs using the direct-write method. This research work establishes guidelines for fabricating the micro-single-chamber solid oxide fuel cells by optimizing the direct-write slurry deposition process with high accuracy.

Structure and Validation of a Kinematic Surface Simulation Model for the Direct Laser Writing Process

The article introduces the structure and validation of a kinematic surface simulation model for the replication of laser‐structured surfaces and their impressions after hot embossing. The surface replication is accomplished by the stepwise Boolean intersection of a structured and digitized point ablation with the workpiece. The resulting surface is characterized according to ISO 25178. Resulting from the kinematic simulation principle, the model can be applied to a wide range of material–machine combinations in the field of laser structuring and enables a time–efficient, nanometer‐resolved replication of representative surfaces with a size up to several square millimeters. Within the validation process it was proven that the main surface characteristics can be predicted by the model. Compared to AI approaches and numerical or multiphysical simulations, preliminary studies are only needed to a very small extent. Furthermore, complex influences such as material and temperature‐related properties do not need separate consideration as they are taken into account by the digitized point ablation.

Flexible multimode sensors based on hierarchical microstructures enable non-destructive grading of fruits in cold chain logistics

In practical applications, flexible pressure sensors must demonstrate adequate sensitivity, durability, and the ability to detect both dynamic and static forces across a wide range. The objective of this study is to develop a flexible dual-mechanism piezoelectric/piezoresistive sensor (FDMPS) based on layered microstructures to enable versatile detection in real fruit sorting operations. The FDMPS consists of an upper layer featuring planar MXene electrodes on a PDMS film, a mid-layer comprising microstructured Ag electrodes on a PVDF-TrFE/Silica gel/ZnO film, and a lower layer with microstructured MXene electrodes on a PDMS film composition. To ensure a secure fit, the three-layer structure is treated with APTES and plasma. All electrodes are produced using a pneumatic direct-write process, while the PDMS and piezoelectric films are created via a spin-coating process, making them suitable for large-scale production. The flexible FDMPS, with a 10 wt% ZnO content, achieves an optimal piezoelectric output of 3.6 V. Additionally, the FDMPS demonstrates excellent linearity (0.997), resistive sensitivity (23.65 kpa−1), and stability (5000 cycles). The incorporation of microstructures significantly enhances the performance of piezoelectric/piezoresistive sensing. Moreover, the FDMPS can accurately measure bending strain rate and angle within the ranges of 0–90°/s (with a sensitivity of 0.014 V/(°·s−1)) and 0–110° (with a sensitivity of 0.216/°), respectively. In a wireless, real-time mode, the FDMPS proves effective in monitoring the reciprocating motion of a robotic arm and assessing fruit ripeness during the grading process. This advancement promotes the development and application of precision agriculture and wearable sensing technologies.

Stability Analysis of Jet Deposition in Metal Melt Direct Writing Process

Stability Analysis of Jet Deposition in Metal Melt Direct Writing Process

Facile preparation of patterned anode substrate for solid oxide fuel cells by direct-writing 3D printing technology

3D printing has revolutionized solid oxide fuel cell (SOFC) by introducing complicated structure and new functionalities that cannot be realized using traditional preparation process. This work provides guide and insights into the application of direct-writing 3D printing to fabricate the patterned electrolyte|electrode interface for the anode-supported SOFC using 3 mol % yttria-stabilized zirconia-nickel oxide (3YSZ-NiO) as the anode substrate material. The parameters that affected the shaping and performance of SOFC including the rheological properties of 3YSZ-NiO anode paste, the printing parameters, and the heat treatment process of the pre-sintered anode substrate were evaluated. The prepared 3YSZ-NiO paste exhibited a typical shear-thinning characteristic, “yielding” solid-like behavior, and time- and temperature-independent stability performance, which ensured the paste suitable for extrusion while allowing the shape of the printed body to be maintained after extrusion during the direct-writing printing process. The as-prepared anode paste, combined with the obtained values of 0.1 MPa, 100 mm s−1, 100 rpm, and 0.3 mm for printing parameters of the extrusion pressure, the printing speed, the screw speed, and the nozzle diameter, respectively, as well as the devised heat treatment program for the pre-sintered 3YSZ-NiO anode substrate, successfully produced defect-free corrugated anode substrates. The cell made from the as-mentioned anode substrate showed an open circuit voltage of 1.02 V and a maximum power density of 619.44 mW cm−2 at 850 °C, which was comparable to the other identical cells fabricated by traditional and other 3D printing processes. Using the 3D printing process shown here, the preparation time of patterned anode substrate can be reduced and simplified.

(Invited) Laser-Induced Graphene on Polymers: From Process Science to Applications

Nanocarbons like graphene and carbon nanotubes (CNTs) are promising for various applications including advanced electronic devices, novel energy systems, and next-generation healthcare diagnostics. This is owing to the excellent physical, chemical and electrochemical properties arising from the ordered atomic structure, the hierarchical nanoscale morphology, and tunable chemistry of nanocarbons. In particular, low-dimensional carbon electrodes for biosensors and neural interfaces have consistently been shown to have superior performance when compared to state-of-the-art metal electrodes. Nevertheless, major manufacturing challenges still hinder our ability to scalably produce graphene-based electrodes with tailored morphology and surface chemistry, especially on flexible substrates. While chemical vapor deposition (CVD) processes enable the synthesis of high quality graphene and CNTs, the extreme environments of high temperatures and hydrocarbon-rich gaseous atmosphere in such reactors limit the choice of substrates to silicon, quartz, metals or other rigid temperature-resistant materials. On the other hand, many emerging flexible devices require the fabrication of such nanocarbon electrodes on the surface of polymer substrates. Unlike different transfer technique of CVD-grown nanocarbons, this talk will focus on a unique bottom-up approach for directly growing different types of graphenic nanocarbons on polymer films by laser irradiation. The speaker will show how this direct-write process, often referred to as laser-induced graphene (LIG), can be controlled to produce spatially-varying morphologies and chemical compositions of LIG electrodes, by leveraging gradients of laser fluence. Moreover, a method will be introduced to control the heteroatom doping of these LIG electrodes based on controlling the molecular structure of the polymer being lased. Finally, a demonstration of these functional LIG electrodes as electrochemical biosensors will be presented for the detection of the neurotransmitter dopamine with nanomolar sensitivity.

Sub-5 nm Contacts and Induced p-n Junction Formation in Individual Atomically Precise Graphene Nanoribbons.

This paper demonstrates the fabrication of nanometer-scale metal contacts on individual graphene nanoribbons (GNRs) and the use of these contacts to control the electronic character of the GNRs. We demonstrate the use of a low-voltage direct-write STM-based process to pattern sub-5 nm metallic hafnium diboride (HfB2) contacts directly on top of single GNRs in an ultrahigh-vacuum scanning tunneling microscope (UHV-STM), with all the fabrication performed on a technologically relevant semiconductor silicon substrate. Scanning tunneling spectroscopy (STS) data not only verify the expected metallic and semiconducting character of the contacts and GNR, respectively, but also show induced band bending and p-n junction formation in the GNR due to the metal-GNR work function difference. Contact engineering with different work function metals obviates the need to create GNRs with different characteristics by complex chemical doping. This is a demonstration of the successful fabrication of precise metal contacts and local p-n junction formation on single GNRs.

Fabrication of Panel-Level Glass Substrates with Complete Design Freedom using LIDE

In this work, we present examples of highly reproducible, accurate and extremely cost-effective wafer- and panel-level fabrication of glass interposers, free of any process-dependent design limitations where any distribution and density of Through-Glass Vias (TGV) with any cross-section geometries and aspect-ratios, is possible. Glass has recently garnered significant interest for its many applications in the semiconductor packaging industry, particularly as a core substrate material for interposer applications. Glass has a unique set of material properties with great relevance for this industry, such as very high electrical resistivity, low loss tangent and low dielectric constant. Glass also exhibits low warpage due to its relatively low Young‘s Modulus while still being compatible with Silicon in terms of CTE and, when processed using a technology that doesn’t induce the generation of any defects such as microcracks or thermally-induced stress, it is highly reliable during processing and operation. Laser Induced Deep Etching (LIDE®) technology is a 2.5D glass processing technology with complete design freedom purposedly developed for cost-effective, high-volume fabrication of defect-free glass components. Despite its many advantages as an interposer material in relation to Si, its application has been hindered by the lack of reliable, precise, and cost-effective processing technologies. LIDE® consists of a two-step process: i) a very fast maskless and direct-write laser process induces subtle and very localized modification in the glass substrate, requiring a single laser pulse to modify the whole glass thickness; and ii) a wet-etching process done in batch, where the whole glass substrate is subjected to the action of an etchant and where the removal of glass occurs at an accelerated rate in the laser-modified regions. This process flow allows LIDE to fabricate several thousands of TGV per second, completely free of any defects, with a sub-micron point-to-point accuracy and ± 5 µm accuracy (cumulative) across a 510 mm x 515 mm glass panel (Cp > 1.33), in any glass type, with aspect-ratios from 1:10 to 1:50 (diameter:depth).

Development of a Photopolymer-Based Dielectric Nanocomposite for High Resolution Direct-Write Processes

A UV curable dielectric ink composition, which enables adaptable and high-resolution direct ink writing, is presented. The UV curable dielectric ink composition is designed using non-hazardous, low cost, commercially available ingredients. The ink manufacturing procedure was devised to require only fundamental lab equipment, without the need for complex reaction conditions or harmful processes. The UV dielectric ink composition was engineered to meet a range of direct ink writing application specific needs. These needs include a tailored rheology, cured film rigidity, dielectric performance, and adhesive ability. The versatility of this ink composition is demonstrated through the fabrication of dielectric bridges for printed coplanar waveguides, bare die chip integration, and interconnects between printed circuit board regions.

Fully 3D-printed organic electrochemical transistors

Organic electrochemical transistors (OECTs) are being researched for various applications, ranging from sensors to logic gates and neuromorphic hardware. To meet the requirements of these diverse applications, the device fabrication process must be compatible with flexible and scalable digital techniques. Here, we report a direct-write additive process to fabricate fully 3D-printed OECTs, using 3D printable conducting, semiconducting, insulating, and electrolyte inks. These 3D-printed OECTs, which operate in the depletion mode, can be fabricated on flexible substrates, resulting in high mechanical and environmental stability. The 3D-printed OECTs have good dopamine biosensing capabilities (limit of detection down to 6 µM without metal gate electrodes) and show long-term (~1 h) synapse response, indicating their potential for various applications such as sensors and neuromorphic hardware. This manufacturing strategy is suitable for applications that require rapid design changes and digitally enabled direct-write techniques.

Atom-by-Atom Direct Writing.

Direct-write processes enable the alteration or deposition of materials in a continuous, directable, sequential fashion. In this work, we demonstrate an electron beam direct-write process in an aberration-corrected scanning transmission electron microscope. This process has several fundamental differences from conventional electron-beam-induced deposition techniques, where the electron beam dissociates precursor gases into chemically reactive products that bond to a substrate. Here, we use elemental tin (Sn) as a precursor and employ a different mechanism to facilitate deposition. The atomic-sized electron beam is used to generate chemically reactive point defects at desired locations in a graphene substrate. Temperature control of the sample is used to enable the precursor atoms to migrate across the surface and bond to the defect sites, thereby enabling atom-by-atom direct writing.

CO2 Laser Direct-Write Process for Micro-Gradient-Patterned Carbon Composed of Graphene-like and Disordered Carbon Forms for a Robust Anode-Free Li–Metal Battery

CO₂ Laser Direct-Write Process for Micro-Gradient-Patterned Carbon Composed of Graphene-like and Disordered Carbon Forms for a Robust Anode-Free Li–Metal Battery

Utilizing a photosensitive dry film resist in proton beam writing

Dry film resists (DFRs) are suitable for the fabrication of large volume devices as the thickness of the film can be easily controlled. Here, the DFR microstructures were patterned using the proton beam writing (PBW) technique by taking advantages of the direct-write process, straight trajectories of protons, and large processing depth. The results show that the required irradiation dose of 15 μm DFR was 10 nC mm−2 for 1 MeV protons. In summary, we have optimized the PBW conditions to create smooth surface micropatterns with a vertical wall in the DFR.

High-fidelity glass micro-axicons fabricated by laser-assisted wet etching.

We report on the fabrication of micro-axicons made of glass by laser-assisted wet etching (LAE) and laser polishing. The employed technique, relying on a direct-writing process using a femtosecond laser, allows revealing high fidelity profiles when the exposed glass samples are etched in a heated potassium hydroxide (KOH) solution. The remaining surface roughness is then decreased by carbon dioxide (CO2) laser polishing. Such polishing is limited to the superficial layer of the component so that the tip is only slightly rounded, with a radius of curvature of nearly 200 µm. It is then shown with 500 µm-diameter axicons that a quasi-Bessel beam is generated closely after the tip and features a 5.3 µm diameter maintained over a propagation distance of almost 3.5 mm.

The Experimental Verification of Direct-Write Silver Conductive Grid and ARIMA Time Series Analysis for Crack Propagation.

The paper presents experimental verification of customized resistive crack propagation sensors as an alternative method for other common structural health monitoring (SHM) techniques. Most of these are sensitive to changes in the sensor network configuration and a baseline dataset must be collected for the analysis of the structure condition. Sensors investigated within the paper are manufactured by the direct-write process with electrically conductive, silver-microparticle-filled paint to prepare a tailored measuring grid on an epoxy or polyurethane coating as a driving/insulating layer. This method is designed to enhance the functionality and usability compared to commercially available crack gauges. By using paint with conductive metal particles, the shape of the sensor measuring grid can be more easily adapted to the structure, while, in the previous approach, only a few grid-fixed sensors are available. A fatigue test on the compact tension (CT) specimen is presented and discussed to evaluate the ability of the developed sensors to detect and monitor fatigue cracks. Additionally, the ARIMA time series algorithm is developed both for monitoring and predicting crack growth, based on the acquired data. The proposed sensors’ verification reveal their good performance to detect and monitor fatigue fractures with a relatively low measurement error and ARIMA estimated crack length compared with the crack opening displacement (COD) gauge.

Electrochemical Sensing of Neonicotinoids Using Laser-Induced Graphene.

Neonicotinoids are the fastest-growing insecticide accounting for over 25% of the global pesticide market and are capable of controlling a range of pests that damage croplands, home yards/gardens, and golf course greens. However, widespread use has led to nontarget organism decline in pollinators, insects, and birds, while chronic, sublethal effects on humans are still largely unknown. Therefore, there is a need to understand how prevalent neonicotinoids are in the environment as there are currently no commercially available field-deployable sensors capable of measuring neonicotinoid concentrations in surface waters. Herein, we report the first example of a laser-induced graphene (LIG) platform that utilizes electrochemical sensing for neonicotinoid detection. These graphene-based sensors are created through a scalable direct-write laser fabrication process that converts polyimide into LIG, which eliminates the need for chemical synthesis of graphene, ink formulation, masks, stencils, pattern rolls, and postprint annealing commonly associated with other printed graphene sensors. The LIG electrodes were capable of monitoring four major neonicotinoids (CLO, IMD, TMX, and DNT) with low detection limits (CLO, 823 nM; IMD, 384 nM; TMX, 338 nM; and DNT, 682 nM) and a rapid response time (∼10 s) using square-wave voltammetry without chemical/biological functionalization. Interference testing exhibited negligible responses from widely used pesticides including the broad-leaf insecticides parathion, paraoxon, and fipronil, as well as systemic herbicides glyphosate (roundup), atrazine, dicamba, and 2,4-dichlorophenoxyacetic acid. These scalable, graphene-based sensors have the potential for wide-scale mapping of neonicotinoids in watersheds and potential use in numerous electrochemical sensor devices.

Characterization of the Functional Properties of Polycaprolactone Bone Scaffolds Fabricated Using Pneumatic Micro-Extrusion

Abstract Pneumatic micro-extrusion (PME) is a direct-write additive manufacturing process, which has emerged as a robust, high-resolution method for the fabrication of a broad spectrum of biological tissues and organs. PME allows for noncontact multimaterial deposition of functional inks for tissue engineering applications. In spite of the advantages and engendered potential applications, the PME process is inherently complex, governed not only by complex physical phenomena but also by material–process interactions. Consequently, investigation of the influence of PME process parameters as well as the underlying physical phenomena behind material transport and deposition in PME would be inevitably a need. The overarching goal of this research work is to fabricate biocompatible, porous bone tissue scaffolds for the treatment of osseous fractures, defects, and diseases. In pursuit of this goal, the objectives of the work are: (i) to investigate the influence of seven consequential scaffold design factors and PME process parameters on the mechanical properties of fabricated bone tissue scaffolds and (ii) to explore the underlying dynamics behind material transport in the PME process, using a three-dimensional computational fluid dynamics (CFD) model. To investigate the effects of the design and process parameters, a series of experiments were designed and conducted. Layer height was identified as the most significant factor in this study. An increase in the layer height led to less overlap between subsequent layers, which allowed for more shrinkage and ultimately a reduction in scaffold diameter. In addition, print speed appeared as an influential factor in this study. An increase in the print speed resulted in a decline in linear mass density and thus in the extent of fusion between subsequent deposited layers. Besides, it was observed that there was a strong correlation between deposition mass and compression modulus. Overall, the results of this study pave the way for future investigation of PME-deposited polycaprolactone (PCL) scaffolds with optimal functional and medical properties for incorporation of stem cells toward the treatment of osseous fractures and defects.