Science:Soft Or Hard! Gallium-copper Composite 3D Printing Ink

Today, electronic products have made significant progress in material selection to meet the diverse needs of all aspects of society, whether rigid or flexible materials. Rigid materials are mainly used in portable and user-friendly handheld interfaces such as smartphones and tablets. In contrast, flexible materials have great potential for wearable applications due to their inherent flexibility and ability to seamlessly adapt to the contours of the human body. However, one challenge remains: the fixed mechanical rigidity of modern electronics limits their wide application. Rigid electronics struggle to fit into our skin or organs, while flexible electronics lack enough stiffness to effectively carry loads. Recently, a potential solution has emerged in the form of "transformational electronic systems (TES)" that can switch between rigid and flexible. By simply adjusting stiffness and scalability, TES shows unlimited potential for improving the adaptability, convenience and versatility of electronics for a wide range of applications. Mechanical transformation electronic systems (TES) built using gallium have emerged as an innovative class of electronic products due to their ability to switch between rigid and flexible states, thus expanding the diversity of electronic products. However, gallium's high surface tension and low viscosity pose manufacturing challenges, limiting TES's high-resolution patterning.

To address this challenge, the team of Steve Park and Jae-Woong Jeong from the Korea Advanced Institute of Science and Technology (KAIST) introduced an adjustable hardness gallium-copper composite ink that is capable of direct ink writing to print complex TES circuits with high-resolution (about 50 microns) patterns, high electrical conductivity, and bi-directional soft-hard conversion. These features make it possible to design transformational bioelectronics with a level of complexity similar to traditional printed circuit boards. These TES remain rigid at room temperature for easy handling, but soften at body temperature and adapt to curved tissue surfaces, adapting to dynamic tissue deformation. The proposed direct ink writing printing inks make the manufacture of TES simple and diverse, opening up new possibilities in the fields of wearables, implantable devices, consumer electronics and robotics. The work is entitled "Body temperature softening electronic ink for additive manufacturing of transformative bioelectronics via direct. writing "was published in the February 28, 2024 issue of the top international journal Science Advances.

1. Innovative research content

In this study, we propose a one-step preparable, adjustable hardness gallium-copper (Ga-Cu) composite e-ink designed for nozzle-based DIW printing of high resolution transformational electronic system (TES) circuit boards, which can be used as both an electronic layer and a mechanical transition frame. Previous studies have attempted to incorporate copper with a high copper content (18-52% by weight of copper) into gallium in order to achieve rapid hardening (within 60 minutes at 60 ° C) of composite inks into solid intermetallic compounds (CuGa2) for high-resolution printing. However, this ink lacks the hardness adjustability that is critical to TES manufacturing and tends to clog nozzles during nozzle based direct writing printing. To overcome this problem, this research developed a gallium-copper composite ink with an optimized low copper content that does not solidify through intermetallic compound formation even after months of storage, while achieving excellent microscale printing uniformity. In this study, the wettability, viscosity and surface tension of the ink were adjusted by systematically studying the copper content in the gallium matrix to ensure high resolution printing. The printed gallium-copper composite ink forms a pattern with outstanding characteristics: (i) high hardness adjustability (hardness adjustment ratio of 990 for devices 150μm thick), and (ii) high conductivity (3.69×106 S m−1 at copper content of 5.0 wt %; About 8% better than pure gallium), and (iii) high resolution (about 50μm) patterning capability. The manufactured TES device demonstrates a temperature-dependent phase transition effect, enabling critical bidirectional hardness regulation. This study demonstrates these features through two devices: (i) an ultra-thin epidermal photoplethysmography (PPG) device for pulse sensing, and (ii) a carefully designed wireless photoelectronic device. These transition devices remain rigid at room temperature for easy handling, but seamlessly adapt to the wrinkles and curves of the skin at body temperature. They also highlight high resolution pattern making and precise control of width and thickness. The combined benefits of this one-step preparable, adjustable hardness ink and fast, high-resolution direct writing printing method simplify the construction of TES and bring new opportunities to the field.

[Introduction to Additive Manufacturing of multifunctional TES with adjustable hardness of Gallium-copper composite ink for direct writing and printing]

Figure 1 illustrates the key advantages that a one-step preparable gallium-copper composite ink for direct writing printing offers in TES. Gallium-copper composite inks are prepared by a single-step, solvent-free ultrasonic treatment of liquid gallium and copper fillers (spherical diameters of 10 to 25μm), where the weight percentage of copper fillers is optimal (5.0 wt %). Copper fillers act as rheological modifiers by increasing the viscosity of the ink and reducing the surface tension of gallium, enabling high-resolution pattern production. These fillers also improve heat conduction due to their high thermal conductivity (320.72 W m−1 K−1) and act as nucleating agents to promote rapid phase transitions between solid and liquid states. The ease of preparation of this ink gives it a significant advantage over traditional methods. Unlike traditional methods, which often require cumbersome pre-treatment steps (such as acid treatment and vacuum drying), or post-treatment steps (such as roasting), our gallium-copper composite ink does not require harsh treatment. Gallium-copper composite inks are highly printable, adjustable hardness, and easy to process, making them ideal building blocks for high-resolution printing functional TES.

Direct writing printing with this ink provides TES with a variety of flexible production capabilities. First, it uses additive manufacturing technology to achieve excellent printability on a variety of substrates, These include very high adhesive force (VHB) Scotch tape (3M), paper, tape, foam tape (3M), poly [Styryl-B -(ethylene-co-butene) -B-styrene] (SEBS), polydimethylsiloxane (PDMS), polyimide (PI) film, and Scotch tape, as shown in Figure 1A (a). This versatility, combined with the absence of harsh post-processing, makes direct write printing ideal for general-purpose applications. In addition, the preset print nozzle movement allows custom devices of various sizes to be made (Figure 1A, b), while the ink's shapiness at room temperature enables it to print highly conductive free-floating bridge structures without mechanical support (Figure 1A, c). The ink's superior electrical conductivity allows mechanically adjustable circuits to be integrated directly into electronic components without the need for additional circuit layers. Finally, by adjusting process parameters such as print speed, extrusion rate, and nozzle size, the line width/thickness can be easily and precisely controlled to achieve a minimum pattern resolution of 50μm (Figure 1A, d).

Figure 1 Overview of direct writing printing of GAN copper composite inks with adjustable hardness for additive manufacturing of multifunctional transformational electronic systems (TES)

[Systematic study of gallium copper composite ink with adjustable hardness]

The study's variable hardness Ga-Cu composite ink was prepared by dispersing 5.0 wt % copper filler (10 to 25μm) into liquid gallium using a cutting-edge ultrasound instrument (Figure 2A, 2 on the left). This study evaluated various metal filler candidates, including silver (Ag), nickel (Ni), and copper (Cu), and copper was the best choice for the e-ink composite for this study due to its high electrical conductivity, excellent thermal conductivity (320.72 W m−1 K−1), and low cost. During ultrasonic processing, the shear force and cavitation effect caused by the vibration of the probe break down the gallium oxide and disperse the copper filler in it. In addition, the heat generated by the vibrating probe keeps the gallium liquid, further promoting full mixing with the copper filler (Figure 2A, right). Therefore, ultrasonic treatment time significantly affects the dispersion uniformity of copper filler. Figure 2B compares the energy dispersive X-ray spectroscopy (EDS) images of gallium-copper composites after 1 min and 5 min ultrasonic treatment. Although 1 minute of ultrasonic treatment results in non-uniform dispersion and local aggregation of copper fillers, resulting in low print resolution or nozzle blockage (Figure 2B, lower left), a long enough ultrasonic treatment time (5 minutes or more) can achieve uniform dispersion of copper fillers (Figure 2B, lower right).

FIG. 2. Chemical, rheological and thermal characteristics of the Ga-Cu composite ink in a convertible mode are characterized

[Systematic study of printing conditions and electro-mechanical characteristics of printing TES]

Systematic study of printing conditions for rheological properties of inks not only improves printing resolution, but also introduces diversity to printable designs. The 3D printer customized for this study prints gallium-copper composite inks through four main steps: (i) initial contact, (ii) direct printing, (iii) rapid departure, and (iv) release (Figure 3A). A detailed view of the direct print and leave phases is graphically shown in Figure 3B. Direct Writing printing Through additive manufacturing operations, depositing the material layer by layer, has successfully printed gallium-copper composite inks on a variety of substrates, including VHB tape, SEBS, glass, and PDMS (Figure 3C). By adjusting the printing parameters, the width, thickness and vertical height of the print pattern can be adjusted to achieve accurate control of the print output. The vertical height of the printed structure depends on the speed at which the nozzle leaves. A slow departure (10 mm/s) forms a tall vertical interconnect (about 3 cm), and a fast departure (25 mm/s) produces a short vertical column (about 2 mm; Figure 3A). When vertical interconnection is not required, further increase the departure speed (> 30 mm/s) will cause the line to break immediately because the nozzle is separated from the base. In this 3D printer configuration, the thickness of the pattern can also be precisely adjusted by changing various parameters such as print speed (Figure 3D), pipe diameter, extrusion rate, extrusion pressure (Figure 3E) and nozzle size. To avoid adverse printing problems caused by Rayleigh instability, such as broken drops or bead chain patterns on the substrate, this study determined the optimal range of printing conditions, including print speed and extrusion pressure (Figure 3F).

FIG. 3 A systematic study of the printing conditions and the electro-mechanical properties of the printed gallium-copper composite ink

[Example of a transformative ultra-thin epidermal Pulse Wave (PPG) sensor]

The gallium-copper composite ink in this study has several unique characteristics, such as easy-to-adjust hardness, multifunctional printability, and high electrical conductivity, making it ideal for manufacturing bioelectronic devices adapted to tissue deformation. To illustrate its potential, a transformative epidermal pulse wave (PPG) sensor was developed using e-ink from this study (Figure 4A). The epidermal Electronic System (EES) is an ultra-thin, skin-compatible and stretchable electronic device capable of keenly sensing physiological signals under dynamic skin deformation. While EES maximizes user comfort by allowing for imperceptible wearing, its ultra-thin and skin-like properties often cause the device to fold before being applied to the skin. This requires the use of a temporary rigid carrier base to properly transfer it to the target location, which introduces cumbersome steps to the manufacture and implementation of EES. In order to solve this problem, this study designed an independent ultra-thin device (20 microns in thickness) that can not only be easily handled without additional reinforcing platforms (such as PVA layers, silk layers, etc.), but can also seamlessly adapt to wrinkles and deformations on the skin through rapid rigid-to-soft mode conversion, simplifying the overall manufacturing and use. The study's gallium-copper composite ink with high conductivity and micron-scale printable ability further enables high-resolution printing of multilayer circuits for transformational PPG devices, including 13 surface mount devices (SMDS) and 2 vertical interconnect channels (VIAs) (Figure 4B).

By precisely controlling the width and thickness of the electronic trace during the DIW printing process, the gallium-copper composite ink in this study can be used to manufacture highly complex and transformative electronic circuit devices. To demonstrate this concept, this study designs a transformative wireless optoelectronic device with high-resolution electronic traces that have different widths and thicknesses when connecting different circuit components. The wireless optoelectronic device in this study consists of three main parts: (i) a circular coil antenna (inner diameter 27 mm, outer diameter 36 mm, with six turns on a single layer) for wirelessly receiving power; (ii) A DC voltage quadrupling circuit, containing five pairs of Schottky diodes and capacitors, amplifying the received radio frequency (RF) signal by a factor of four and then converting it into a stable charging integrated lithium polymer (LiPo) battery (GMB-300910, PowerStream Technology; 12 mah) DC power supply; (iii) an integrated Bluetooth Low Power system chip (BLE SoC; RFD77101, RF Digital Corporation's wireless communication circuit for wireless control devices (Figure 5A). As shown in Figure 5B, the device is printed with gallium-copper composite ink in the form of a two-layer circuit, each layer connected to each other VIA 15 vertical interconnect channels (VIA), including 17 surface-mount devices (seven capacitors, two resistors, five Schottky diodes, an LED, a voltage regulator, and a BLE SoC) and a LiPo battery. Thanks to this design, the wireless optoelectronic device in this study is capable of wirelessly charging via inductive coupling and uses the generated power to drive integrated leds via BLE control to provide light stimulation for potential applications such as optogenetics and light therapy.

In addition, gallium-copper composites exhibit a bidirectional transition between rigid and soft modes, providing the required rigidity to hold the shape until the target position is reached, and providing the flexibility required to adapt to the curved surface (Figure 5C). By using soft and stretchable VHB tape (3M; With a shear modulus of 0.6 MPa) as the base, the device in soft mode exhibits excellent stability, withstanding 206% uniaxial strain (Figure 5D). Biaxial stretching of the device emphasizes the strong adhesion of the printed electronic circuit to the substrate while maintaining its high-resolution pattern. This versatile feature is very advantageous for implantable and wearable applications as it allows for adjustable stiffness, ease of handling and seamless integration with soft tissue. For example, the conceptual wireless transformative photoelectronic devices shown here can be used as implantable devices for optogenetics and phototherapy with minimal stress on surrounding tissues.

Figure 5. Transformational wireless optoelectronic devices with different line widths and high circuit densities using complex circuit designs

2. Summary and outlook

Given the properties and applications demonstrated, this study expects that gallium-copper composite inks combined with DIW printing technology could open up new possibilities for the next generation of transformational wearable, implantable, ingestible electronics, and a variety of other applications. This innovation will overcome the limitations of contemporary electronic products, which often have immutable mechanical properties, either completely soft or completely rigid. One challenge that needs to be addressed is that the gallium-copper complex may harden into a brittle intermetallic compound phase, although this was not observed for at least 8 weeks in our study. In this regard, future research and development should focus on ensuring the long-term stability of gallium-copper composite inks to improve the long-term reliability of manufactured transformational electronics.