Optimal Ink Research Articles

Development of printable bacterial nanocellulose bioinks for bioprinting applications

This research has developed a printable bioink formulated with bacterial nanocellulose (BNC) and sodium alginate. The bioink composition offers a unique combination of printability, mechanical strength, and high BNC content. A low-cost 3D bioprinting platform, with in-situ crosslinking capabilities, was designed and fabricated to facilitate the creation of complex constructs. Rheological analysis revealed shear-thinning behaviour, a critical property for efficient 3D bioprinting. The printed constructs exhibited exceptional resolution, printability, and self-supporting features at BNC loadings up to 72 wt.% in dried structures. The optimal ink composition was found, containing 7.8 wt.% BNC and 3 wt.% sodium alginate. These findings highlight the potential for this BNC-alginate bioink for bio-fabrication applications demanding both printability and high biomaterial content.Graphical

Optimization of Reactive Ink Formulation for Controlled Additive Manufacturing of Copolymer Membrane Functionalization.

Multifunctional, nanostructured membranes hold immense promise for overcoming permeability-selectivity trade-offs and enhancing membrane durability in challenging molecule separations. Following the fabrication of copolymer membranes, additive manufacturing technologies can introduce reactive inks onto substrates to modify pore wall chemistries. However, large-scale implementation is hindered by a lack of systematic optimization. This study addresses this challenge by elucidating the membrane functionalization mechanisms and optimal manufacturing conditions using a copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) "click" reaction. Leveraging a data science toolkit (e.g., nonlinear regression, uncertainty quantification, identifiability analyses, model selection, and design of experiments), we developed two mathematical models: (1) algebraic equations to predict equilibrium concentrations after preparing reactive inks by mixing copper sulfate, ascorbic acid (AA), and an alkynyl-terminated reactant; and (2) reaction-diffusion partial differential equations (PDEs) to describe the functionalization process. The ink preparation chemistry with side reactions was validated through pH and UV-vis measurements, while the diffusion and kinetic parameters in the PDE model were calibrated using time-series conversion of the azide moieties inferred from Fourier-transform infrared spectroscopy. This modeling framework avoids redundant experimental efforts and offers a functionalization protocol for scaling up designs. Ink optimization problems were proposed to reduce the use of expensive and environmentally insulting ink materials, i.e., Cu(II), while ensuring the desired chemical distributions. With optimal ink formulation Cu(II)/AA/alkyne = 1:1:2 identified, we uncovered trade-offs between Cu(II) usage and functionalization time; for example, in continuous roll-to-roll manufacturing with a conserved functionalization bath setup, our optimal operational conditions to achieve ≥90% functionalization enable at least a 20% reduction in total copper investment compared to previous experimental results. The data science-enabled ink optimization framework is extendable for on-demand multifunctional membranes in numerous future applications such as metal recovery from wastewater and brine.

Towards Photocrosslinkable Lyotropic Blends of Organosolv Lignin and Hydroxypropyl Cellulose for 3D Printing by Direct Ink Writing.

Polymer blends containing up to 70% organosolv lignin content and lyotropic cellulose derivatives have been established as "lignin inks" for direct ink writing of fully biobased 3D parts. However, a fast-crosslinking mechanism is needed to improve throughput and design space. In this paper, UV-photocrosslinkable organosolv lignin/hydroxypropyl cellulose inks are formulated through doping with common photocrosslinkers. The most potent photocrosslinkers for neat hydroxypropyl cellulose, lignin and their blends are determined through a series of DOEs. Hydroxypropyl cellulose is significantly more amenable to photocrosslinking than organosolv lignin. The optimal photocrosslinkable ink formulations are printable and exhibit up to 70% gel content, although thermal post-curing remains essential. Chemical, thermal, and mechanical investigations of the photocrosslinked 3D parts evidence efficient crosslinking of HPC through its hydroxyl groups, while lignin appears internally plasticized and/or degraded during inefficient photocrosslinking. Despite this, photocrosslinkable inks exhibit improved tensile properties, shape flexibility, and fidelity. The heterogeneous crosslinking and residual creep highlight the need to further activate lignin for homogeneous photocrosslinking in order to fully exploit the potential of lignin inks in DIW.

Solvent-free Acrylate/BCB drop-on-demand (DOD) inkjet dielectric ink for 3D printing

Currently, drop-on-demand (DOD) inkjet-printed dielectric inks used to produce next-generation high-frequency electronic devices fail to meet the requirements of circuit board substrates in terms of thermal, mechanical, and dielectric properties. To address this gap, we synthesized a low-viscosity acrylate monomer featuring an intrinsically low dielectric structure (hexafluorobisphenol A) and multiple cross-linking sites, and a benzocyclobutene monomer with low curing shrinkage and excellent thermomechanical properties. By mixing these monomers with other reactive diluents and verifying them through theoretical calculations and printing, we have developed a range of dielectric inks suitable for inkjet 3D printing. To enhance the overall performance of the cured inks, a sequential UV/thermal curing strategy was employed to form interpenetrating networks (IPNs). The optimal ink formulations met the requirements of circuit board substrates, demonstrating excellent heat resistance (Td5% = 334 °C, Tg = 181 °C), mechanical properties (tensile strength = 85 MPa, elongation at break = 13.5 %), and dielectric properties (Dk = 2.526 and Df = 0.113 at 15 GHz). These formulations meet the performance requirements for circuit board substrates and lead the industry in mechanical and dielectric properties. Furthermore, the feasibility of inkjet printing heterogeneous integrated electronic devices was verified, showing excellent print resolution and continuity of the conductive network. This work provides new insights into the future development of dielectric materials for inkjet 3D printing.

Effect of high-boiling point solvents on inkjet printing of catalyst layers for proton exchange membrane fuel cells

Inkjet printing (IJP) is considered as a promising and flexible method for low cost and drop-on-demand pattern formation in proton exchange membrane (PEM) fuel cells. Although significant advancements have been made in inkjet-printed electrodes, identifying the optimal ink formulations remains challenging due to severe restrictions on the ink properties. Generally, the catalyst layer prepared by IJP using low boiling point solvents (water/alcohol) show reasonable electrochemical performance. However, low boiling point solvents lead to clogging of the printhead nozzle due to fast evaporation of the solvent in the inkjet print head. In this study, high boiling point solvents (propylene glycol (PG) or ethylene glycol (EG)) were used as additives to reduce nozzle clogging and improve printing efficiency. In this context, the relationship between catalyst ink properties, CL microstructure, and electrochemical performance of the MEA is investigated. Results show that printing time is significantly reduced with adding high boiling point solvents in the ink (66.67 % reduction of time with 30 wt% PG). However, an increased amount of additive is detrimental in terms of electrochemical performance due to formation of larger agglomerates, lower porosity of the catalyst layer, and reduced ECSA. The results of this work can be used to develop a strategy to adjust the trade-off between printing time and electrochemical performance of electrodes prepared using IJP.

Dissipative Particle Dynamics of Nano-Alumina Agglomeration in UV-Curable Inks.

Ultraviolet (UV) ink is a primary type of ink used in additive manufacturing with 3D inkjet printing. However, ink aggregation presents a challenge in nano-inkjet printing, affecting the stability and quality of the printing fluid and potentially leading to the clogging of nanometer-sized nozzles. This paper utilizes a Dissipative Particle Dynamics (DPD) simulation to investigate the aggregation behavior of alumina in a blend of 1,6-Hexanediol diacrylate (HDDA) and Trimethylolpropane triacrylate (TMPTA). By analyzing the effects of solid content, polymer component ratios, and dispersant concentration on alumina aggregation, the optimal ink formulation was identified. Compared to traditional experimental methods, DPD simulations not only reduce experimental costs and time but also reveal particle aggregation mechanisms that are difficult to explore through experimental methods, providing a crucial theoretical basis for optimizing ink formulations. This study demonstrates that alumina ceramic ink achieves optimal performance with a solid content of 20%, an HDDA-to-TMPTA ratio of 4:1, and 9% oleic acid as a dispersant.

(Invited) Integrated Approach of Modeling, X-Ray and Electrochemical Characterization to Understand Low Iridium Loading Catalyst Layers Integration and Durability

Proton exchange membrane water electrolyzers (PEMWEs) will share more than 40 % of green hydrogen production market by 2050. Currently, iridium catalyst in the anode catalyst layers amounts to 1 mg/cm2. To enable low-loading (0.4 mg/cm2) or ultra-low loading (<0.1 mg/cm2) designs we need to better understand both integration of catalysts into catalyst layer and their durability. At low loadings catalyst layer in-plane electric conductivity can be compromised, requiring either supported catalyst layers or the need for microporous layer (MPL) design. For durability study it is necessary to understand the pathway of surface of IrOx transformation during accelerated stress tests (ASTs) and identify optimal beginning of life (BOL) structure for long-duration deployment. In this work we evaluate the structure and performance of amorphous and crystalline IrOx catalysts at the BOL and EOL to build structure-to-property relations. Rheology studies are conducted to identify the optimal ink recipe for each of the catalysts for blade coating deposition method. We use operando x-ray diffraction technique to evaluate catalyst structure evolution during AST cycling. X-ray computed tomography (CT) was used to evaluate catalyst layer – MPL interfaces under various operando conditions. The morphological information was integrated into the two-dimensional model to better understand interfaces for the low-loading PEMWEs.

A wearable flexible electrochemical biosensor with CuNi-MOF@rGO modification for simultaneous detection of uric acid and dopamine in sweat

BackgroundIn health assessment and personalized medical services, accurate detection of biological markers such as dopamine (DA) and uric acid (UA) in sweat is crucial for providing valuable physiological information. However, there are challenges in detecting sweat biomarkers due to their low concentrations, variations in sweat yield among individuals, and the need for efficient sweat collection. ResultsWe synthesized CuNi-MOF@rGO as a high-activity electrocatalyst and investigated its feasibility and electrochemical mechanism for simultaneously detecting low-concentration biomarkers UA and DA. Interaction between the non-coordinating carboxylate group and the sample produces effective separation signals for DA and UA. The wearable biomimetic biosensor has a wide linear range of 1–500 μM, with a detection limit of 9.41 μM and sensitivity of 0.019 μA μM−1 cm−2 for DA, and 10–1000 μM, with a detection limit of 9.09 μM and sensitivity of 0.026 μA μM−1 cm−2 for UA. Thus, our sensor performs excellently in detecting low-concentration biomarkers. To improve sweat collection, we designed a microfluidic-controlled device with hydrophilic modification in the microchannel. Experimental results show optimal ink flow at 2% concentration. Overall, we developed an innovative and highly active electrocatalyst, successfully enabling simultaneous detection of low-concentration biomarkers UA and DA. SignificanceThis study provides a strategy for sweat analysis and health monitoring. Moreover, the sensor also showed good performance in detecting real sweat samples. This study has shown great potential in future advances in sweat analysis and health monitoring.

Preparation and characterization of starch-based binders for binder jetting.

In recent years, binder jetting technology has made significant advances across industries, expanding the range of material options to meet diverse needs. Commonly used binders may leave residues during the sintering process, affecting surface quality and performance, and some may contain harmful substances. Therefore, there is a high demand for binders that are environmentally friendly and easy to remove. This study proposes to use sodium alginate and polyvinylpyrrolidone as additives to prepare starch-based inks that are both environmentally friendly and safe. The effects of additive composition, starch content, dispersant content, and dispersant ratio on the viscosity and stability of starch-based inks were studied. Through performance testing, the particle size, surface tension, rheological properties, and printability of inks with different components were demonstrated. The optimal ink formulation consists of 1 wt% starch and 0.3 wt% additives (30 wt% sodium alginate and 70 wt% PVP). The viscosity reaches 23 mPa s and the stability is excellent. The surface tension of the ink is 69.5 mN m-1, which is slightly higher than the surface tension requirements for the printhead. This article provides a new process route for binder jetting technology and lays the foundation for its application in green and environmental protection.

Voidless metal lines sintered with intense pulsed light and their applications as transparent metal-mesh electrodes

An optimal metal ink that can be sintered without void generation by intense pulsed light (IPL) was systematically formulated. This ink is a hybrid ink in which silver (Ag) nanoparticle ink and copper (Cu) precursor ink are mixed in an optimal weight ratio, and the reduced Cu fills the space between Ag nanoparticles during the sintering process. The optimization process of the ink encompasses the shape and size of nanoparticles, the type of solvent, and the mixing ratio of the Cu precursor ink. This optimized ink was filled in the engraved trenches and IPL-sintered to fabricate flexible transparent metal-mesh electrodes. If there are voids in the metal lines after IPL sintering, the metal lines may shrink under condition of high temperature and high humidity for a long time, which may deteriorate the stability of the metal electrode. It was confirmed that the hybrid ink optimized in this study hardly shrinks due to no voids inside the metal lines after IPL sintering and has excellent environmental and mechanical stability. The metal-mesh electrode produced using this ink showed superior properties compared to other competing transparent electrodes such as indium-tin oxide or Ag nanowires when used for transparent heaters and electromagnetic shielding applications.

Understanding ink design and printing dynamics of extrusion-based 3D printing: Defect-free dense piezoelectric ceramics

The formulation and trade-off between a control rheological ink and its printability are significant challenges for designing a defect-free extrusion-based 3D printed feedstock of piezoelectric materials. In this study, we explore different strategies, including ink design, printing dynamics, and densification behaviors, to successfully manufacture a dense 3D-printed lead zirconate titanate (PZT) ceramic using material extrusion. First, PZT inks of different solid volumes (50–55 vol%) were designed using an aqueous binder system, and their rheological behaviors were evaluated. Second, the printing dynamics, that is, extrusion pressure, nozzle size, and printing speed, were optimized for printing an optimal ink without defects. Finally, the printed specimens were sintered at 1100–1300 °C in a lead-rich atmosphere. The specimens achieved a relative density of ∼97.59 %, which is close to conventional die-pressed PZT density ∼ 97.79 %, and exhibited similar piezoelectric behavior. This high-density 3D-printed PZT with tailorable design feasibility has potential applications in sensors, actuators, and energy harvesters.

Modeling a Dynamic Printability Window on Polysaccharide Blend Inks for Extrusion Bioprinting.

Extrusion-based bioprinting is one of the most widespread technologies due to its affordability, wide range of processable materials, and ease of use. However, the formulation of new inks for this technique is based on time-consuming trial-and-error processes to establish the optimal ink composition and printing parameters. Here, a dynamic printability window was modeled for the assessment of the printability of polysaccharide blend inks of alginate and hyaluronic acid with the intent to build a versatile predictive tool to speed up the testing procedures. The model considers both the rheological properties of the blends (viscosity, shear thinning behavior, and viscoelasticity) and their printability (in terms of extrudability and the ability of forming a well-defined filament and detailed geometries). By imposing some conditions on the model equations, it was possible to define empirical bands in which the printability is ensured. The predictive capability of the built model was successfully verified on an untested blend of alginate and hyaluronic acid chosen to simultaneously optimize the printability index and minimize the size of the deposited filament.

A hybrid multi-objective optimization of functional ink composition for aerosol jet 3D printing via mixture design and response surface methodology

The limited electrical performance of microelectronic devices caused by low inter-particle connectivity and inferior printing quality is still the greatest hurdle to overcome for Aerosol jet printing (AJP) technology. Despite the incorporation of carbon nanotubes (CNTs) and specified solvents into functional inks can improve inter-particle connectivity and ink printability respectively, it is still challenging to consider multiple conflicting properties in mixture design simultaneously. This research proposes a novel hybrid multi-objective optimization method to determine the optimal functional ink composition to achieve low electrical resistivity and high printed line quality. In the proposed approach, silver ink, CNTs ink and ethanol are blended according to mixture design, and two response surface models (ReSMs) are developed based on the Analysis of Variance. Then a desirability function method is employed to identify a 2D optimal operating material window to balance the conflicting responses. Following that, the conflicting objectives are optimized in a more robust manner in the 3D mixture design space through the integration of a non-dominated sorting genetic algorithm III (NSGA-III) with the developed ReSMs and the corresponding statistical uncertainty. Experiments are conducted to validate the effectiveness of the proposed approach, which extends the methodology of designing materials with multi-component and multi-property in AJP technology.

Development of Uniform Polydimethylsiloxane Arrays through Inkjet Printing

The inkjet printing method is a promising method to deposit polymer and functional nanoparticles at the microscale. It can be applied in the fabrication of multicolor polymer light emitting diodes (polyLEDs), polymer base electronics, multicolor color conversion layers, and quantum dot light emitting diodes (QLEDs). One of the main challenges is to print high-resolution polymer dots from dilute polymer solution. In addition, the quality of printed multicolor polyLEDs, QLEDs and multicolor color conversion layers is currently limited by non-uniformity of the printed dots. In this paper, polydimethylsiloxane (PDMS) is selected as the functional polymer, due to its high transparency, good reflective index value, inflammable and flexible properties. The optimal ink to form a uniform PDMS dot array is presented in this paper. Both the solvent and PDMS were tuned to form the uniform PDMS dot array. The uniform PDMS dot array was printed with a diameter of around 50 µm, and the array of closely spaced green quantum dots (QDs) mixed with PDMS ink was also printed on the substrate uniformly. While the green QD-PDMS film was printed at a resolution of 1693 dpi, the uniformity was evaluated using the photoluminescence (PL) spectrum and color coordinate value.

Study of color reproduction in pigment digital printing

In pigment ink digital printing, the color difference between the original and the printed product can be visible due to the varied color gamut of devices and the impact of fabric characteristics on the color-presenting effect. To improve the color reproduction accuracy of digital printing, two methods, the full-process optimized (FPO) method and the neural network aided original adjustment (NNAOA) method, were proposed to reduce printing color difference during the color-transferring process. The FPO method, based on accounting for the fabric type and the characteristics of the printing machine, was proposed by optimizing the printing parameter settings in the color management process. Here, the optimal single-channel ink limit was determined and dot gain was compensated using the inverse function correction method, and then the International Color Consortium profile was generated. After using the FPO method, the color difference between the printed sample and the original was reduced from 20.8 to 5.254 compared with performing the original color management. To reduce the reliance on personal experience in color adjustment, the NNAOA method was proposed and implemented for 20 color patches with the largest color differences in the FPO method, allowing the color of the originals to be adjusted with the establishment of the general regression neural network model. This method predicted the input CMYK values of the original using measured L*a*b* values for the printed sample. The results showed that the color difference of the 20 color patches was reduced from 8.54 to 6.60. Consequently, both methods offered distinguishable improvements in the color reproduction effect.

Large-Scale Ultra-Robust MoS2 Patterns Directly Synthesized on Polymer Substrate for Flexible Sensing Electronics.

Synthesis of large-area patterned MoS2 is considered the principle base for realizing high-performance MoS2 -based flexible electronic devices. Patterning and transferring MoS2 films to target flexible substrates, however, require conventional multi-step photolithography patterning and transferring process, despite tremendous progress in the facilitation of practical applications. Herein, an approach to directly synthesize large-scale MoS2 patterns that combines inkjet printing and thermal annealing is reported. An optimal precursor ink is prepared that can deposit arbitrary patterns on polyimide films. By introducing a gas atmosphere of argon/hydrogen (Ar/H2 ), thermal treatment at 350°C enables an in situ decomposition and crystallization in the patterned precursors and, consequently, results in the formation of MoS2 . Without complicated processes, patterned MoS2 is obtained directly on polymer substrate, exhibiting superior mechanical flexibility and durability (≈2% variation in resistance over 10,000 bending cycles), as well as excellent chemical stability, which is attributed to the generated continuous and thin microstructures, as well as their strong adhesion with the substrate. As a step further, this approach is employed to manufacture various flexible sensing devices that are insensitive to body motions and moisture, including temperature sensors and biopotential sensing systems for real-time, continuously monitoring skin temperature, electrocardiography, and electromyography signals.

Co-axial printing of convoluted proximal tubule for kidney disease modeling

Despite the increasing incidence of kidney-related diseases, we are still far from understanding the underlying mechanisms of these diseases and their progression. This lack of understanding is partly because of a poor replication of the diseases in vitro, limited to planar culture. Advancing towards three-dimensional models, hereby we propose coaxial printing to obtain microfibers containing a helical hollow microchannel. These recapitulate the architecture of the proximal tubule (PT), an important nephron segment often affected in kidney disorders. A stable gelatin/alginate-based ink was formulated to allow printability while maintaining structural properties. Fine-tuning of the composition, printing temperature and extrusion rate allowed for optimal ink viscosity that led to coiling of the microfiber’s inner channel. The printed microfibers exhibited prolonged structural stability (42 days) and cytocompatibility in culture. Healthy conditionally immortalized PT epithelial cells and a knockout cell model for cystinosis (CTNS -/-) were seeded to mimic two genotypes of PT. Upon culturing for 14 days, engineered PT showed homogenous cytoskeleton organization as indicated by staining for filamentous actin, barrier-formation and polarization with apical marker α-tubulin and basolateral marker Na+/K+-ATPase. Cell viability was slightly decreased upon prolonged culturing for 14 days, which was more pronounced in CTNS -/- microfibers. Finally, CTNS -/- cells showed reduced apical transport activity in the microfibers compared to healthy PT epithelial cells when looking at breast cancer resistance protein and multidrug resistance-associated protein 4. Engineered PT incorporated in a custom-designed microfluidic chip allowed to assess leak-tightness of the epithelium, which appeared less tight in CTNS -/- PT compared to healthy PT, in agreement with its in vivo phenotype. While we are still on the verge of patient-oriented medicine, this system holds great promise for further research in establishing advanced in vitro disease models.

Optimization of the sintering thermal treatment and the ceramic ink used in direct ink writing of α-Al2O3: Characterization and catalytic application

Unlike other engineered ceramic products, alumina (Al2O3) displays interesting mechanical and physical properties, which makes it an ideal candidate for a wide range of uses in different fields and in particular for catalytic applications. However, the manufacturing of ceramic components has still a major drawback in production of highly complex three-dimensional (3D) shapes, microfeatures or structures with tailored porosity. Direct Ink Writing (DIW), also known as robocasting, is a material extrusion Additive Manufacturing technology and is one of such versatile methods with unique flexibility in material and geometry. In this work, α-Al2O3 ceramic materials were designed and produced by DIW to determine the most suitable sintering treatment and ceramic ink composition to design new components for catalytic applications. Several thermal treatments varying sintering temperature and time were tested previously to the preparation of inks with different ceramic loadings, up to 75 wt%. A systematic study of the DIW specimens sintered at the optimal sintering temperature – time combination, in terms of microstructure (density and porosity) and mechanical properties (hardness and indentation fracture toughness), was performed to determine the optimize ceramic loading. Finally, finite element modeling and catalytic experiments conducted for the optimal ceramic ink showed that 3D printed parts with a rectilinear infill pattern and 40% infill density favored catalytic performance.

A metallosupramolecular polymer deposited via inkjet printing for fast-switching pixelated electrochromic devices

We investigated the process of patterning an Fe(II)-MEPE film on an ITO-coated PET film using a commercially available inkjet printing device and developed an optimal ink formulation consisting of water/ethanol/ethylene glycol for inkjet printing.

Development of a free-form piezo-resistive pressure sensor using advanced printing methods

The use of printed pressure sensors is becoming a popular alternative to conventionally made silicon-based pressure sensors in niche applications, ranging from automotive and aerospace to the biomedical industry. In this paper, a piezo-resistive pressure sensor is produced onto a free-form Polyamide (PA) substrate, by combining aerosol-based jet printing and screen-printing techniques. The result is a first study towards the implementation of printed piezo-resistive pressure sensors for biomedical applications.The piezo-resistive pressure sensor consists of silver interconnects which are printed onto the substrate and are connected through a layer of piezo-resistive ink. The sensor is printed onto a customized additive manufactured PA substrate, which is produced using Selective Laser Sintering (SLS). First, the interconnects are printed using silver ink, CUR AG-001, from Agfa N.V. Belgium, which is deposited using an aerosol-based jet printer, the Nebula 5X-100s, developed at the Advanced Manufacturing Laboratory (AML) of the KU Leuven. Next, the sensitive elements are printed using a piezo-resistive material, Carbon EMS CI 2050, which is applied across the sensor interconnects using an adapted screen-printing technique. An optimal ink composition for the two-component piezo-resistive ink was found at 40wt% CI-2050LR: 60wt% CI-2050HR, which are respectively low and high resistivity ink components. Both silver ink and printed piezo-resistive elements were sintered in an oven at 150°C for 240 and 45 min, respectively. Finally, a coupling for the sensor readout was provided using a silver conductive epoxy adhesive, which cures at room temperature, so that electrical connections could be attached to the sensor contact pads.Validation of the piezo-resistive pressure sensors was conducted using fingertip actuation, which showed a typical piezo-resistive behavior where the resistance of the sensor decreases in function of the applied pressure, and a maximal decrease in electrical resistance of 30.5Ω has been measured. These results show the feasibility of employing advanced printing techniques to produce piezo-resistive sensing devices onto free-form and additive manufactured substrates, for biomedical applications.