A hydrogel pen for electrochemical reaction and its applications for 3D printing

Three-Dimensional Printing Strategies for Enhanced Hydrogel Applications

This study explores the dynamic field of 3D-printed hydrogels, emphasizing advancements and challenges in customization, fabrication, and functionalization for applications in biomedical engineering, soft robotics, and tissue engineering. It delves into the significance of tailored biomedical scaffolds for tissue regeneration, the enhancement in bioinks for realistic tissue replication, and the development of bioinspired actuators. Additionally, this paper addresses fabrication issues in soft robotics, aiming to mimic biological structures through high-resolution, multimaterial printing. In tissue engineering, it highlights efforts to create environments conducive to cell migration and functional tissue development. This research also extends to drug delivery systems, focusing on controlled release and biocompatibility, and examines the integration of hydrogels with electronic components for bioelectronic applications. The interdisciplinary nature of these efforts highlights a commitment to overcoming material limitations and optimizing fabrication techniques to realize the full potential of 3D-printed hydrogels in improving health and well-being.

Variable Nanoelectrode at the Air/Water Interface by Hydrogel-Integrated Atomic Force Microscopy Electrochemical Platform

A nanoelectrode with a controllable area was developed using commercial atomic force microscopy and a hydrogel. Although tremendous advantages of small electrodes from micrometer scale down to nanometer scale have been previously reported for a wide range of applications, precise and high-throughput fabrication remains an obstacle. In this work, the set-point feedback current in a modified scanning ionic conductance microscopy system controlled the formation of electrodes with a nanometer-sized area by contact between the boron-doped diamond (BDD) tip and the agarose hydrogel. The modulation of the electroactive area of the BDD-coated nanoelectrode in the hydrogel was successively investigated by the finite element method and cyclic voltammetry with the use of a redox-contained hydrogel. Moreover, this nanoelectrode enables the simultaneous imaging of both the topography and electrochemical activity of a polymeric microparticle embedded in a hydrogel.

Long-Range Electrification of an Air/Electrolyte Interface and Probing Potential of Zero Charge by Conductive Amplitude-Modulated Atomic Force Microscopy

The structure of an electrical double layer (EDL) at the interface of electrode/electrolyte or air/electrode/electrolyte is a fundamental aspect, however not fully understood. The potential of zero charge (PZC) is one of the clues to dictate the EDL, where the excess charge on the electrode surface is zero. Here, a nanoscale configuration of immersion method was proposed by integrating an electrochemical system into conductive atomic force spectroscopy under the amplitude modulation (AM) mode and agarose gel as the solid-liquid electrolyte. The PZC of boron-doped diamond was determined to be at 0.2 V (vs Ag/AgCl). By AM spectroscopy, the capacitive force shows remote electrification without direct electrode/electrolyte contact, which is dependent on the population of ions at the air/electrolyte interface. The surface potential by alignment of water is also evaluated. Prospectively, our study could benefit applications such as PZC measurement and non-electrode electrochemical processes at the air/electrolyte interface.

Electrodeposition of Silver Nanoparticles on Indium-Doped Tin Oxide Using Hydrogel Electrolyte for Hydrogen Peroxide Sensing

Nanoparticles are used in various fields, including fuel cells, energy conversion devices, and sensors, because of their large surface area and excellent catalytic properties. Although various methods of synthesizing nanoparticles are available, the most popular is the solution-phase reduction of metal ions. Electrodeposition is a method of reducing metal ions in solution and is widely used because of its various advantages. In this study, Ag nanoparticles with a narrow size distribution were evenly dispersed on the surface of an electrode by applying electrodeposition in an agarose hydrogel medium instead of in solution, confirming the feasibility of Ag deposition in agarose hydrogel, even at a lower reduction potential than that in solution. These results are attributed to the electrolyte effect owing to the hydrophilic backbone of the agarose hydrogel and the gel effect, which reduces unexpected convection. H₂O₂ was detected by using the Ag nanoparticles synthesized in agarose hydrogel, and the limit of detection for H₂O₂ was found to be 4.82 µM, with a dynamic range of 1-500 µM. The nanoparticle synthesis platform proposed in this study is expected to be actively used for the synthesis of other metal/nonmetal nanoparticles.

Electrochemical Polymer Pen Lithography

The development of a massively parallel lithographic technique called electrochemical polymer pen lithography is reported. Pyramidal pen arrays, consisting of more than 10 000 hydrogel pens loaded with metal salts, are integrated into a three-electrode cell and used to locally reduce ions at each pen tip. This system enables high-throughput patterning of a variety of metallic inks (e.g., Ni²⁺ , Pt²⁺ , Ag⁺ ) on the nanometer to micrometer length scale. By incorporating a z-direction piezo actuator, the extension length and dwell time can be used to precisely define feature dimensions (210 to 10 µm in width, and up to 900 nm in height, thus far). Furthermore, by controlling the potential and precursor concentrations, more than one element can be simultaneously deposited, creating a new tool for the synthesis of alloy features, such as NiCo, which are relevant for catalysis. Importantly, this methodology enables fine control over feature size and composition in a single pattern, which may make it ultimately useful for rapid, high-throughput combinatorial screening of metallic features.

Do-It-Yourself Pyramidal Mold for Nanotechnology

The handcrafted fabrication of a pyramidal mold on a silicon wafer for nanopatterning was investigated. This process started with the manual delivery of an aqueous glycerol solution onto the SiO₂/Si wafer using a micropipette and subsequent drying to form a hemisphere whose diameter is in the range of hundreds of micrometers. A coating of polystyrene (PS) onto this wafer generates a circular hole caused by dewetting. Subsequently, anisotropic wet-etching with the PS film as a mask produces a pyramidal trench, whose apex approaches hundreds of nanometers. Various elastomeric materials were casted into this pyramidal mold. A pyramidal tip mounted on a simple micropositioner was used for electrochemistry and patterning of a protein. First, an agarose hydrogel was cast with a hydrogel pen for the electrochemical reaction (HYPER). The redox reaction at the HYPER-electrode interface demonstrated the characteristics of an ultramicroelectrode or bulk electrode based on the contact area. Second, the pyramidal polydimethylsiloxane served as a polymer pen for the contact printing of silane on a glass substrate. After the successive immobilization of biotin and avidin with fluorescence labeling, the resulting fluorescence image demonstrated the successful patterning of the protein. This new process for the creation of a pyramidal mold, referred to as a "do-it-yourself" process, offers advantages to nonspecialists in nanotechnology compared to conventional lithography, specifically simplicity, rapidity, and low cost.

A wet-chemistry-based hydrogel sensing platform for 2D imaging of pressure, chemicals and temperature

Human skin can perceive pressure, chemical compounds and temperature with a high spatial resolution via specific receptors. Inspired by human skin, we present a wet-chemistry based hydrogel sensing platform for 2D imaging sensitive to specific external stimuli, e.g., pressure, chemicals and temperature. This platform is composed of a hydrogel pyramidal array on a single electrode. Each pyramid serves as a spatially separated reservoir for chemical reactions, enabling independent pixels for sensing without individual electrodes. Depending on the electrochemiluminescence (ECL) electrolyte for specific stimuli, our platform possesses 2D imaging capabilities with high sensitivity for pH and temperature in addition to pressure via the deformation of the viscoelastic hydrogel. This work represents an important step toward the application of sensitive chemical reactions for various external stimuli to biocompatible electronic skins based on hydrogels without addressing circuit for each pixel.

Highly Stretchable Conductors Based on Expanded Graphite Macroconfined in Tubular Rubber

Highly stretchable and durable conductors are significant to the development of wearable devices, robots, human-machine interfaces, and other artificial intelligence products. Although many respectable methods have been reported, it is still a challenge to fabricate stretchable conductors with a large elastic limit, high conductivity, and excellent reliability in rapid, effective, and economic ways. Herein, a facile method is offered to fabricate high-performance stretchable tubular conductors (TCs) based on a macroconfined structure of expanded graphite (EG) in rubber tubing by simply physical packing. The maximum original electrical conductivity of TCs reached a high value of 160.6 S/cm. Meanwhile, TCs showed more insensitive response of conductivity to increasing tensile strain compared to the TCs encapsulated with liquid metal or ionic liquid. The conductivity and effective stretchability of TCs can be adjusted by varying the packing density of EG. A low gauge factor below 3 was reached even under 400% stretching for TCs with a packing density of 1.233 g/cm³. The excellent resilience and good stability of conductivity of TCs during dynamic stretching-releasing cycles are attributed to the stable and rapid reconstruction of the percolation network of EG particles. The combination of high conductivity, tunable stretchability, and good reliability renders potential applications to TCs, such as highly stretchable interconnects or strain sensors, in human motion detection.

3D Printing of Ultratough Polyion Complex Hydrogels

Polyion complex (PIC) hydrogels have been proposed as promising engineered soft materials due to their high toughness and good processability. In this work, we reported manufacturing of complex structures with tough PIC hydrogels based on three-dimensional (3D) printing technology. The strategy relies on the distinct strength of ionic bonding in PIC hydrogels at different stages of printing. In concentrated saline solution, PIC forms viscous solution, which can be directly extruded out of a nozzle into water, where dialyzing out of salt and counterions results in sol-gel transition to form tough physical PIC gel with intricate structures. The printability of PIC solutions was systematically investigated by adjusting the PIC material formula and printing parameters in which proper viscosity and gelation rate were found to be key factors for successful 3D printing. Uniaxial tensile tests were performed to printed single fibers and multilayer grids, both exhibiting distinct yet controllable strength and toughness. More complex 3D structures with negative Poisson's ratio, gradient grid, and material anisotropy were constructed as well, demonstrating the flexible printability of PIC hydrogels. The methodology and capability here provide a versatile platform to fabricate complex structures with tough PIC hydrogels, which should broaden the use of such materials in applications such as biomedical devices and artificial tissues.

Electrochemical Hydrogel Lithography of Calcium-Alginate Hydrogels for Cell Culture

Here we propose a novel electrochemical lithography methodology for fabricating calcium-alginate hydrogels having controlled shapes. We separated the chambers for Ca²⁺ production and gel formation with alginate with a semipermeable membrane. Ca²⁺ formed in the production chamber permeated through the membrane to fabricate a gel structure on the membrane in the gel formation chamber. When the calcium-alginate hydrogels were modified with collagen, HepG2 cells proliferated on the hydrogels. These results show that electrochemical hydrogel lithography is useful for cell culture.

Template-Free 3D Microprinting of Metals Using a Force-Controlled Nanopipette for Layer-by-Layer Electrodeposition

A novel 3D printing method for voxel-by-voxel metal printing is presented. Hollow atomic force microscopy (AFM) cantilevers are used to locally supply metal ions in an electrochemical cell, enabling a localized electroplating reaction. By exploiting the deflection feedback of these probes, electrochemical 3D metal printing is, for the first time, demonstrated in a layer-by-layer fashion, enabling the fabrication of arbitrary-shaped geometries.

A Strain-Insensitive Stretchable Electronic Conductor: PEDOT:PSS/Acrylamide Organogels

Organogel-based stretchable electronic conductors exhibit electrical conduction even under a large stretching deformation of 300% without electrochemical reactions at DC voltages. The resistance change with stretching is almost strain-insensitive up to 50% strain and it remains at each deformation up to 1000 fatigue cycle. The polymeric conductive paths of

PEDOT

PSS are well preserved during the mechanical deformation.

Electrodeposition-based 3D Printing of Metallic Microarchitectures with Controlled Internal Structures

3D printing of metallic microarchitectures with controlled internal structures is realized at room temperature in ambient air conditions by the manipulation of metal ion concentration and pulsed electric potentials in the electrolyte meniscus during the meniscus-guided electrodeposition. Precise control of the printing nozzle enables the drawing of complex 3D microarchitectures with well-defined geometries and positions.