Composite Ink Research Articles

Hexaammineruthenium (II)/(III) and Hexacyanoferrate (II)/(III) Redox-Probes for Graphene-Based Inkjet-Printed Electrodes: Benchmarking with Gold and Glassy Carbon Screen Printed Electrodes

Gold and glassy carbon (GC) screen printed electrodes (SPEs) are being widely used for chemical and chemical/biological sensing. Number of commercial sources are available to use gold and GC SPEs off-the-shelf sensors for electrochemical characterization. Cyclic voltammetry (CV), while provides fundamentals of electrode processes pertaining to the charge transfer phenomena, is often obscured by the stability of electrodes due to unfavorable interactions between the redox probes and electrode materials even though gold and GC are excellent choices. Hexaammineruthenium (II)/(III) and Hexacyanoferrat (II)/(III) are the two most extensively used redox probes among several others. However, for example, the later one causes corrosion problems in case of gold electrodes leading to damage in electrode surface modification (such as thiol modification), and thus, providing unreliable results. Therefore, a need to understand the electrode-redox probe interaction and the underlying electrochemical properties is essential.Graphene inkjet-printed electrodes (G-IPEs) have emerged as promising electrochemical sensors that could minimize the cost, ease manufacturing, bring environmental remediation and have potential to revolutionize fabrication of mechanically flexible and bendable electrochemical sensors with number of practical applications such as environmental sensing and biomedical sensing. In addition, due to its exceptionally promising physical properties, graphene will provide multifunctional sensor systems such as electrochemical and strain-based sensors. However, a careful study and one-to-one comparison of G-IPEs with gold and GC SPEs in fundamental performance in terms of their CVs using Hexaammineruthenium (II)/(III) and Hexacyanoferrat (II)/(III) are not available, making it difficult to assess the suitability of use of G-IPEs. In this work, we present a scalable manufacturing of graphene-based inks (pure graphene ink as well as composite inks of graphene with carbon nanotubes, graphene aerosol gels, hexagonal boron nitride), and their additive manufacturing to produce electrochemical electrodes. Detailed cyclic voltammetry study to compare the graphene-based IPEs with gold and GC SPEs and benchmark their electrochemical performance in terms of the charge transfer properties as well as chemical/mechanical stabilities will be presented. Such a study will be greatly beneficial to select the redox probe of choice for graphene-based printed electrochemical sensors.Keywords: Graphene-based inks, Inkjet Printing, Redox-Probes, Cyclic Voltammetry, Charge-Transfer*Corresponding author: srdas@ksu.edu

Manufacturing a soft actuator/sensor integrated structure via multi-material direct writing processes technology

This paper describes a new actuator/sensor integrated structure fabricated via direct writing technique in which ionic polymer–metal composites (IPMCs) are used for actuators, and carbon nanotubes (CNTs) doped in polydimethylsiloxane (PDMS) were used for the sensors. First, we developed Nafion inks and CNT-PDMS composite inks with rheological properties tailored for direct writing. CNT-PDMS composite inks with a 4 wt% CNT concentration were three-dimensionally-printed in a Nafion structure to create an IPMC actuator structure; the CNT-PDMS composite was fabricated into a large array of strain sensors that were able to measure the bending deformation of the IPMC. A CNT-PDMS scaffold structure was semi-embedded in the Nafion structure via direct writing using CNT-PDMS composite inks with a 7 wt% CNT concentration to sense the external pressure and temperature of the IPMC actuator. Palladium and gold electrodes were applied on the surfaces of the Nafion/CNT-PDMS structure through an electroless plating and electroplating process, which enabled the three-dimensionally-printed Nafion/CNT-PDMS integrated structure to form an IPMC/CNT-PDMS integrated structure that could be actuated by voltage signals. The actuating/sensing integrated performances were tested to determine their feasibility and for using direct writing to create a smart structure that had both actuator and sensor functions and that could be used in a wide range of fields, such as micro-robotics, biomedicine, and medical science.

Programmable multistability for 3D printed reinforced multifunctional composites with reversible shape change

4D printing empowers 3D printed structures made of hydrogels, liquid crystals or shape memory polymers, with reversible morphing capabilities in response to an external stimulus. To apply reversible shape-change to stiff lightweight materials such as microfiber reinforced polymers, we developed a composite ink that can be printed using direct-ink-writing (DIW), and that exhibits multistability around its glass transition temperature. After curing at room temperature, the flat print thermally morphs into a predefined shape upon heating at an actuation temperature and cooling down. The sample can then reversibly snap between multiple stable shapes when heated above its glass transition temperature thanks to pre-stress-induced multistability. The key that allows thermal morphing and pre-stress multistability is the microstructuring of the 3D printed composites by shear-induced alignment of reinforcing microfibers. This alignment leads to local anisotropy in thermomechanical properties and the build-up of pre-stresses. Furthermore, the ink composition can be tuned to generate shape-dependant reversible functional properties, such as electrical conductivity. Based on finite element modelling and experimental results, the method proposed here can be used for variety of compositions and designs, for applications where stiffness, reconfigurability and shape-dependent functionalities can be exploited.

Silver flake/polyaniline composite ink for electrohydrodynamic printing of flexible heaters

Silver flake/polyaniline composite ink for electrohydrodynamic printing of flexible heaters

Recent advances in 3D printing technologies for wearable (bio)sensors

Wearable (bio)sensors driven through emerging three-dimensional (3D) printing technologies are currently considered the next-generation tools for various healthcare applications due to their exciting characteristics such as high stretchability, super flexibility, low cost, ultra-thinness, and lightweight. In this context, 3D printing, an emerging advanced additive manufacturing technology has revolutionized the concept of free form construction and end-user customization owing to its multifarious peculiarities that involve ease of operation, on-demand and rapid fabrication, precise and controlled deposition, as well as versatility with various soft functional materials. The customized functional structures with controllable geometry and design can be autonomously printed on the desired surfaces using the 3D printing technologies excluding the prerequisite amenities of microfabrication technologies. To accomplish this, both academics and industry experts have worked persistently to fabricate smaller, faster, and more efficient wearable devices using readily available 3D printing technologies. The contribution of 3D printing technologies in developing novel 3D structures for wearable applications using printable soft and functional materials is highlighted in this article. Moreover, the process of 3D printing along with major techniques, namely vat photopolymerization, material jetting, and material extrusion are summarized. Besides this, a number of 3D printed (bio)sensing platforms such as glucose sensors, lactate sensors, sweat sensors, strain sensors, tactile sensors, wearable oximeters, smart bandages, artificial skin, tattoo sensors, electroencephalography (EEG), electrocardiography (ECG) sensors, etc., are discussed in terms of design specifications and fabrication strategies of devices obtained via 3D printing techniques. Pictorial representation of 3D printed wearable (bio)sensors indicating numerous 3D printing materials (composite filaments, elastomers, functional inks, and hydrogels); techniques (vat photopolymerization, material jetting, and material extrusion), and different applications (glucose biosensing, strain sensing, tactile sensing, sweat sensing, ECG/EEG sensing, etc.) • Explores the recent advances in 3D printing technologies for wearable (bio)sensors. • Soft materials compatible with 3D printing technologies are discussed. • Process of 3D printing and techniques namely vat photopolymerization, material jetting, material extrusion are highlighted. • Potential 3D printed wearable (bio)sensors are reviewed in terms of design specifications and fabrication strategies.

Additive manufacturing of conductive and high-strength epoxy-nanoclay-carbon nanotube composites

Additive Manufacturing has increased our ability to fabricate complex shapes and multi-material structures. Epoxy is excellent as the base for structural composite materials. Furthermore, carbon nanotube (CNT) is an outstanding filler due to its unique properties and functionalities. Here, conductive epoxy-nanoclay-CNT nanocomposite structures were fabricated by direct-write 3D printing. In this process, 3D-printable composite inks were synthesized by incorporation of nanoclay and different concentrations of CNTs – 0.25, 0.5, and 1 vol%, 0.43, 0.86, and 1.7 wt% – in epoxy. CNTs were found to significantly improve the electrical and mechanical properties. Rheological characterization of the inks revealed a shear-thinning behavior for all the nanocomposite inks and an increase in the complex viscosity, storage, and loss moduli with the incorporation of CNTs. The CNT concentration of 0.5 vol% was found to be the optimum condition for enhancement of mechanical properties; an average increase of 61%, 59%, and 31% was measured for flexural strength, flexural modulus, and tensile strength, respectively, compared to the 3D printed epoxy-nanoclay nanocomposite structures. The electrical conductivity of 2.4 × 10 −8 and 2.2 × 10 −6 S/cm was measured for the nanocomposites containing 0.5 and 1 vol% CNTs, respectively. Multi-scale characterization of the morphology revealed partial alignment of CNTs in the direction of printing, CNT pull-out and breakage at the fracture surfaces, and nano-scale interactions of the constituents, all of which contribute to the superiority of the nanocomposite with CNTs. The findings show the promise of this ink material and printing method for various applications such as aerospace structures and electronics.

Superelastic and flexible 3D printed waterborne polyurethane/cellulose nanofibrils structures

Waterborne polyurethane (WPU) is a type of environmental-friendly aqueous suspension that has been widely used in varied applications. However, it remains a challenge to use WPU for three-dimensional structures by additive manufacturing due to its unsatisfied rheological properties. In this study, we developed an in-situ synthesis method to modify WPU (WPUCNF) by using cellulose nanofibrils (CNF) in order to enhance its printability. The addition of CNF during emulsification reduced the WPU nanoparticles size as well as increased the suspension viscosity. To further improve the printability, additional CNFs were added as rheological modifiers. After dewatering the suspension, WPUCNF/CNF composite inks showed excellent printability, as illustrated by the printed structures of various shapes such as honeycomb, woodpile, or human ear. For these samples, heights over 10 mm could be printed with good shape fidelity at the ink concentration of as low as 2.8–7.4%, significantly lower than previously reported WPU ink for 3D printing (~20–30%). The 3D printed structure can absorb 17–37% of water due to the presence of hygroscopic salt CaCl2, and demonstrated high flexibility and withstood over 20 compressive cycles. This versatile WPUCNF/CNF ink can be adapted for designing hierarchical porous 3D structures with broad emerging applications in the biomaterials field.

Triple mode and multi-purpose flexible sensor fabrication based on carbon black and thermoplastic polyurethane composite with propolis

This study reports the design, fabrication, and application of strain and tactile sensors molded with Carbon Black (CB)/Thermoplastic Polyurethane (TPU) composite ink into flexible-soft 3D printed TPU substrate. In sensor design, three different geometries, namely Straight Line/Zigzag/Wavy, are produced to investigate the effect on the sensing behavior. 3D printed molds are preferred due to their low cost and simplicity in fabrication. Additionally, propolis, which is antibacterial and has therapeutic properties for wound and fungal diseases, is added to the composite material to benefit its healing and protective properties. The sensing properties of propolis are investigated for its use as mixing and coating materials. According to electrical characterization, the fabricated structures can be used in triple mode as piezoresistive, piezocapacitive, and piezoinductive sensors. The gauge factor (GF) values for sensors using different ratios of materials varies between 3 and 10. Given conventional metallic strain gauges whose GF is typically around 2, the obtained GF values are remarkable. The distribution of the applied stress on the sensor surface is given by simulations. In addition, the characteristic changes of the samples according to the frequency are examined, and equivalent circuits are provided.

Engineering Natural Pollen Grains as Multifunctional 3D Printing Materials

AbstractThe development of multifunctional 3D printing materials from sustainable natural resources is a high priority in additive manufacturing. Using an eco‐friendly method to transform hard pollen grains into stimulus‐responsive microgel particles, we engineered a pollen‐derived microgel suspension that can serve as a functional reinforcement for composite hydrogel inks and as a supporting matrix for versatile freeform 3D printing systems. The pollen microgel particles enabled the printing of composite inks and improved the mechanical and physiological stabilities of alginate and hyaluronic acid hydrogel scaffolds for 3D cell culture applications. Moreover, the particles endowed the inks with stimulus‐responsive controlled release properties. The suitability of the pollen microgel suspension as a supporting matrix for freeform 3D printing of alginate and silicone rubber inks was demonstrated and optimized by tuning the rheological properties of the microgel. Compared with other classes of natural materials, pollen grains have several compelling features, including natural abundance, renewability, affordability, processing ease, monodispersity, and tunable rheological features, which make them attractive candidates to engineer advanced materials for 3D printing applications.

Wide range and highly linear signal processed systematic humidity sensor array using Methylene Blue and Graphene composite

This paper proposes a signal processed systematic 3 × 3 humidity sensor array with all range and highly linear humidity response based on different particles size composite inks and different interspaces of interdigital electrodes (IDEs). The fabricated sensors are patterned through a commercial inkjet printer and the composite of Methylene Blue and Graphene with three different particle sizes of bulk Graphene Flakes (BGF), Graphene Flakes (GF), and Graphene Quantum Dots (GQD), which are employed as an active layer using spin coating technique on three types of IDEs with different interspaces of 300, 200, and 100 µm. All range linear function (0–100% RH) is achieved by applying the linear combination method of nine sensors in the signal processing field, where weights for linear combination are required, which are estimated by the least square solution. The humidity sensing array shows a fast response time (Tres) of 0.2 s and recovery time (Trec) of 0.4 s. From the results, the proposed humidity sensor array opens a new gateway for a wide range of humidity sensing applications with a linear function.

Deep Eutectic Solvent Induced Porous Conductive Composite for Fully Printed Piezoresistive Pressure Sensor

AbstractManufacturing flexible pressure sensor using scalable and straightforward process is highly desired for next‐generation wearable electronics and intelligent robots. Here, a fully printed pressure sensor is proposed using a newly developed porous conductive composite. The composite ink is prepared by simply mixing the polydimethylsiloxane (PDMS), carbon black (CB), and a deep eutectic solvent (DES). The DES induces phase segregation between the PDMS and CB and serves as a liquid template to form the porous structure during the annealing process. This spontaneously formed conductive architecture that has not been previously reported in a printable ink endows superior electrical–mechanical performance to the composites, and significantly simplifies the device fabrication of flexible pressure sensors. The developed pressure sensors exhibit comprehensive performance capabilities appropriate for the wearable device, human–machine interfaces, and robot tactile.

Formulation of new screen printable PANI and PANI/Graphite based inks: Printing and characterization of flexible thermoelectric generators

The dearth of information about the fabrication of flexible polyaniline and graphite-based microporous and low-cost thermoelectric generators using screen printing for low-temperature applications has motivated us to undertake this research work. Polyaniline and graphite composite inks were formulated using cellulose acetate as resin and diacetone alcohol as the solvent. In this work, we have studied the influence of ink ingredients on the thermoelectric properties of composite inks. Diacetone alcohol improved the electrical conductivity of polyaniline by 7.9 times. The carrier concentrations and carrier mobility of composite ink were enhanced by 2.8 times. Simultaneously, cellulose acetate increased resistivity and carrier mobility of polyaniline by 13 and 44 times, respectively. Graphite improved the crystallinity but reduced carrier mobility, carrier concentration, and bandgap of the composite inks. Screen-printed porous ink film structure reduced the thermal conductivity of PANI ink by 11 times at 333 K. The maximum Seebeck coefficient and power output exhibited by the fabricated thermoelectric generator were 244.34 μV/K and 4.31 nW, respectively at 77 K. Present work explored fabrication and characterization of low cost, flexible polyaniline and graphite composite ink-based thermoelectric generator with improved Seebeck coefficient and power output for low-level heat energy conversion.

Reversible polymer-gel transition for ultra-stretchable chip-integrated circuits through self-soldering and self-coating and self-healing

Integration of solid-state microchips into soft-matter, and stretchable printed electronics has been the biggest challenge against their scalable fabrication. We introduce, Pol-Gel, a simple technique for self-soldering, self-encapsulation, and self-healing, that allows low cost, scalable, and rapid fabrication of hybrid microchip-integrated ultra-stretchable circuits. After digitally printing the circuit, and placing the microchips, we trigger a Polymer-Gel transition in physically cross-linked block copolymers substrate, and silver liquid metal composite ink, by exposing the circuits to the solvent vapor. Once in the gel state, microchips penetrate to the ink and the substrate (Self-Soldering), and the ink penetrates to the substrate (Self-encapsulation). Maximum strain tolerance of ~1200% for printed stretchable traces, and >500% for chip-integrated soft circuits is achieved, which is 5x higher than the previous works. We demonstrate condensed soft-matter patches and e-textiles with integrated sensors, processors, and wireless communication, and repairing of a fully cut circuits through Pol-Gel.

3D Printing of Microgel Scaffolds with Tunable Void Fraction to Promote Cell Infiltration.

Granular, microgel-based materials have garnered interest as promising tissue engineering scaffolds due to their inherent porosity, which can promote cell infiltration. Adapting these materials for 3D bioprinting, while maintaining sufficient void space to enable cell migration, can be challenging, since the rheological properties that determine printability are strongly influenced by microgel packing and void fraction. In this work, a strategy is proposed to decouple printability and void fraction by blending UV-crosslinkable gelatin methacryloyl (GelMA) microgels with sacrificial gelatin microgels to form composite inks. It is observed that inks with an apparent viscosity greater than ≈100 Pa s (corresponding to microgel concentrations ≥5 wt%) have rheological properties that enable extrusion-based printing of multilayered structures in air. By altering the ratio of GelMA to sacrificial gelatin microgels, while holding total concentration constant at 6 wt%, a family of GelMA:gelatin microgel inks is created that allows for tuning of void fraction from 0.20 to 0.57. Furthermore, human umbilical vein endothelial cells (HUVEC) seeded onto printed constructs are observed to migrate into granular inks in a void fraction-dependent manner. Thus, the family of microgel inks holds promise for use in 3D printing and tissue engineering applications that rely upon cell infiltration.

3D-Printable Hierarchical Nanogel-GelMA Composite Hydrogel System.

The strength of the extracellular matrix (ECM) is that it is hierarchical in terms of matrix built-up, matrix density and fiber structure, which allows for hormones, cytokines, and other small biomolecules to be stored within its network. The ECM-like hydrogels that are currently used do not possess this ability, and long-term storage, along with the need for free diffusion of small molecules, are generally incompatible requirements. Nanogels are able to fulfill the additional requirements upon successful integration. Herein, a stable hierarchical nanogel–gelatin methacryloyl (GelMA) composite hydrogel system is provided by covalently embedding nanogels inside the micropore network of GelMA hydrogel to allow a controlled local functionality that is not found in a homogenous GelMA hydrogel. Nanogels have emerged as a powerful tool in nanomedicine and are highly versatile, due to their simplicity of chemical control and biological compatibility. In this study, an N-isopropylacrylamide-based nanogel with primary amine groups on the surface was modified with methacryloyl groups to obtain a photo-cross-linking ability similar to GelMA. The nanogel-GelMA composite hydrogel was formed by mixing the GelMA and the photo-initiator within the nanogel solution through UV irradiation. The morphology of the composite hydrogel was observed by scanning electron microscopy, which clearly showed the nanogel wrapped within the GelMA network and covering the surface of the pore wall. A release experiment was conducted to prove covalent bonding and the stability of the nanogel inside the GelMA hydrogel. In addition, 3D printability studies showed that the nanogel-GelMA composite ink is printable. Therefore, the suggested stable hierarchical nanogel-GelMA composite hydrogel system has great potential to achieve the in situ delivery and controllable release of bioactive molecules in 3D cell culture systems.

All Screen‐Printed, Polymer‐Nanowire Based Foldable Electronics for mm‐Wave Applications

AbstractWith the surge in devices for Internet of Things (IoT) applications, there is great interest in flexible electronics to be mass manufactured at lower costs. Screen‐printing is well‐known for mass manufacturing, however, this method has mostly focused on printing metallic patterns. Rare efforts have been devoted to print substrates for high frequency (mm‐wave) electronics, which requires low dielectric loss to ensure a decent system efficiency. This paper presents a novel screen‐printable composite ink comprising of acrylonitrile‐butadiene‐styrene and ceramic particles, through which, dielectric substrates with various thicknesses (down to few microns), lateral dimensions, and relative permittivities can be printed. A low dielectric loss of 0.0063 at 28 GHz (fifth generation (5G) communication band) makes the substrates suitable for mm‐wave electronics. A custom silver nanowires based screen‐printable ink is utilized for metallic printing to provide high conductivity (3.4 × 106 S m‐1) and stable electrical response under bent or folded conditions. As a proof of concept for fully printed mm‐wave electronics, a flexible quasi‐Yagi antenna operating at 5G band (26.5–29.5 GHz) is demonstrated that exhibits decent performance in flat as well as bent conditions, confirming the suitability of the material system and printing processes for mass production of IoT and wearable electronics.

Structure-Processing-Property Relationships of 3D Printed Porous Polymeric Materials.

Imparting porosity to 3D printed polymeric materials is an attractive option for producing lightweight, flexible, customizable objects such as sensors and garments. Although methods currently exist to introduce pores into 3D printed objects, little work has explored the structure-processing-property relationships of these materials. In this study, photopolymer/sacrificial paraffin filler composite inks were produced and printed by a direct ink writing (DIW) technique that leveraged paraffin particles as sacrificial viscosity modifiers in a matrix of commercial elastomer photocurable resin. After printing, paraffin was dissolved by immersion of the cured part in an organic solvent at elevated temperature, leaving behind a porous matrix. Rheometry experiments demonstrated that composites with between 40 and 70 wt % paraffin particles were able to be successfully 3D printed; thus, the porosity of printed objects can be varied from 43 to 73 vol %. Scanning electron microscopy images demonstrated that closed-cell porous structures formed at low porosity values, whereas open-cell structures formed at and above approximately 53 vol % porosity. Tensile tests revealed a decrease in elastic modulus as the porosity of the material was increased. These tests were simulated using finite element analysis (FEA), and it was found that the Neo-Hookean model was appropriate to represent the 3D printed porous material at lower and higher void fractions within a 75% strain, and the Ogden model also gave good predictions of porous material performance. The transition between closed- and open-cell behaviors occurred at 52.4 vol % porosity in the cubic representative volume elements used for FEA, which agreed with experimental findings that this transition occurred at approximately 53 vol % porosity. This work demonstrates that the tandem use of rheometry, FEA, and DIW enables the design of complex, tailorable 3D printed porous structures with desired mechanical performance.

4D Aliphatic photopolymer polycarbonates as direct ink writing of biodegradable, conductive graphite‐composite materials

AbstractPhotopolymers are one of the fastest growing 3D printing material classes, despite the lack of diversity in performance or composition. A simple addition of graphite nanopowder is explored for tailoring aliphatic polycarbonate performance and enhancing advanced material properties, targeting the mechanical performance of the norbornene‐containing poly(norbornene trimethyl carbonate) using poly(trimethyl propane allyl ether carbonate) for thermoset formation using thiol‐ene photochemistry. A 30 wt% graphite‐composite photopolymer ink displays shear thinning behavior suitable for pneumatic direct ink write (DIW) printing, along with enhanced elastic modulus (15–542 MPa) and an increase in ultimate strength (2–22 MPa for the composite ink) without significant variation in the glass transition temperature. The composites are further demonstrated as 4D materials including shape memory, conductivity, and recyclability, with this work serving as a guide for designing DIW inks from photopolymer resins.

Engineered Electrode Structure for High‐Performance 3D‐Printed All‐Solid‐State Flexible Microsupercapacitors

A method to prepare graphene-carbon black composite ink for 3D printing electrodes of microsupercapacitors is developed by Lei Li and co-workers, article number 2100357. The carbon black (cartoon child) works as pillar to tailor graphene sheets space in electrode structure, relieving graphene aggregation issue. The tailored-structure of electrode facilitates electron (pupple ball) and electrolyte ion (red ball) transport, boosting the electrochemical performance of microsupercapacitors.

Low content and low-temperature cured silver nanoparticles/silver ion composite ink for flexible electronic applications with robust mechanical performance

In this study, a novel and cost-effective silver nanoparticles (AgNPs)/silver ion composite ink was prepared and used for printing conductive pattern on flexible substrate. Multi-sized AgNPs and silver organic ions contained in the composite ink acted as the conductive fillers. Relationship between the morphology, elemental composition, heating characteristics, and electrical resistivity of the ink and the printed pattern were intensively investigated. The tape test was carried out to rank the adhesion strength of the printed pattern on the substrate. The mechanical stability of the pattern was measured by a cyclic bending test. Analysis of the results demonstrated that the composition of the neck in neighbouring particles severely affects the inter-particle junction resistance. The AgNPs and silver ions in the composite ink mutually promoted each other to significantly reduce the curing temperature. The printed pattern achieved satisfying electrical resistivity of 6.88*10−5 Ω·cm with silver content of as low as 4.68 wt% in the composite ink.