Researchers have developed a novel carbon nanotube-based ink for 3D printing, creating highly stretchable and sensitive sensors that promise to revolutionize wearable health-monitoring devices. A team at the Seoul National University of Science and Technology engineered a nanocomposite material that overcomes long-standing challenges in fabricating flexible electronics, paving the way for a new generation of smart devices that can be precisely customized to the human body. This breakthrough enables the creation of complex, high-resolution sensors that can be seamlessly integrated into items like insoles to monitor gait and pressure, offering real-time physiological data.
The core of this advancement lies in solving a fundamental problem with carbon nanotubes (CNTs), which are materials known for their exceptional strength and electrical conductivity. For years, the potential of CNTs in flexible electronics has been hampered by their tendency to clump together, or agglomerate. This clumping creates an uneven dispersion within the polymer base, leading to inconsistent performance and structural weaknesses. Conventional manufacturing methods have struggled to control the distribution and shape of these nanotubes, forcing a trade-off between a sensor’s stretchability and its electrical sensitivity. Achieving both high conductivity and flexibility remained a persistent challenge, limiting the complexity and reliability of wearable sensors.
A Novel 3D Printing Approach
The research team, led by Professor Keun Park and Associate Professor Soonjae Pyo, focused on a sophisticated 3D printing technique known as vat photopolymerization (VPP). VPP works by selectively curing a liquid resin with light, layer by layer, to build intricate three-dimensional objects with high precision. The primary challenge was to create a CNT-infused ink that would not interfere with this delicate curing process. The researchers developed a new nanocomposite by dispersing multi-walled carbon nanotubes (MWCNTs) into a photopolymer resin made of aliphatic urethane diacrylate (AUD).
To ensure the nanotubes were distributed evenly, the team used powerful ultrasonic waves to break up the agglomerated CNT bundles within the resin. This critical step resulted in a homogeneous mixture that was optimized for VPP 3D printing. By preventing the nanotubes from clumping, the process minimizes light scattering during printing, which in turn allows for greater accuracy and the fabrication of complex geometries. This meticulous preparation of the ink was essential to achieving consistent electrical pathways and mechanical strength throughout the final printed sensor.
Unprecedented Performance Metrics
The newly developed material exhibits a combination of properties that surpasses previously reported nanocomposites. The team tested various formulations, finding that an ink with a 0.9 weight percent of carbon nanotubes offered the ideal balance of conductivity and elasticity. Sensors printed with this optimal composite could be stretched up to 223% of their original length before breaking, a remarkable level of flexibility for a conductive material. Even while being stretched, the material maintained a reliable electrical conductivity of 1.64 × 10⁻³ S/m.
Beyond its mechanical and electrical performance, the technique also achieves a significant level of structural detail. The VPP process, using the optimized CNT ink, reached a printing resolution of 0.6 mm. This high fidelity allows for the creation of intricate internal structures that are vital for sensor performance. The researchers used this capability to print sensors with a complex, lattice-like geometry known as a triply periodic minimal surface (TPMS). These structures are known for being lightweight yet robust, and their design makes them highly sensitive to mechanical pressure and deformation.
Real-World Application in Smart Insoles
To demonstrate the practical utility of their work, the researchers integrated these advanced sensors into a wearable device. They fabricated a smart insole platform designed to monitor the distribution of pressure across the bottom of a person’s foot in real time. The TPMS-based piezoresistive sensors were embedded into the flexible insole, where they could accurately detect subtle changes corresponding to different movements and postures, such as walking or standing.
The successful demonstration showed the technology’s potential for applications in personalized healthcare and athletics, where precise tracking of gait and balance can provide valuable insights. The ability to create custom-fit sensors that conform to an individual’s body highlights a key advantage of using 3D printing for manufacturing. The smart insole serves as a powerful proof of concept for how these materials can be used to create high-performance, wearable health-monitoring systems.
Future of Flexible and Wearable Electronics
The implications of this research, published in the journal Composite Structures, extend far beyond smart footwear. The ability to 3D print durable, highly stretchable conductors opens the door for innovation in several fields, including soft robotics, flexible electronics, and smart textiles. These materials could be used to create electronic skins that can sense touch and pressure, or clothing with integrated sensors that monitor vital signs without cumbersome wires. The technology fundamentally changes the approach to designing and manufacturing devices that need to be both electronically functional and mechanically compliant.
The research team plans to continue refining the material’s performance in practical environments and explore a wider range of applications. According to Professor Pyo, the smart insole is just the beginning. “The developed smart-insole device demonstrates the potential of our CNT nanocomposites for 3D printing the next generation of highly stretchable and conductive materials,” he stated. “We believe these materials will be indispensable for wearable health monitors, flexible electronics and smart textiles.” With further development, this technology could become a cornerstone of personalized medicine and interconnected wearable systems.