3D Printing for Wearables, Energy Storage, and Practical E-Textiles Using Silk Fibroin and Carbon Nanotube Inks

In the recently published ‘The Road to Practical E-Textiles is Smooth as Silk,’ the research of Zhang et al. outlines a new method of 3D printing that could be the catalyst for creating energy harvesting fabrics in E-textiles, leading to better performance in electronics and wearables.

As consumer electronics—and those that can be worn—become more progressive and more available, researchers continue to strive for advances in digital technology and performance for components like sensors—much of which is propelled by miniaturization processes also. Power, energy, and batteries are always a challenge, however, as any of us know just from trying to keep a cell phone up and running daily. The researchers point out that many wearables today are required to be bulky because they must encompass a battery. Biocompatibility can be an issue too now as consumers desire to wear so many different novel and innovative devices.

Structure and Morphology of the as-obtained CNTs@SF Core-Sheath Fibers on Textile (A) Scanning electron microscopy (SEM) image of top-view of a core-sheath fiber on textile. (B) Optical image of top-view of a core-sheath fiber on textile. (C) SEM image of cross-section of a core-sheath fiber on textile. (D) SEM image of cross-section of a fiber showing the CNT core. (from ‘Printable Smart Pattern for Multifunctional Energy-Management E-Textile‘)

Energy harvesting is a new concept to many, described by the authors as ‘a compelling complementary solution to onboard batteries.’ Energy can, in fact, be harvested from ambient light or kinetic energy made by the wearer—and then stored in devices like capacitors. This is where piezoelectric materials and triboelectric generators enter the picture; however, challenges remain in terms of structural design and production, and so much so that the researchers are concerned that much of this new technology could remain ‘mere lab-scale curiosities.’ And Zhang and the team of researchers search for solutions, they have created a 3D printing triboelectric generator composed of a silk fibroin (SF) sheath and an electrically conductive core of carbon nanotubes (CNT).

“The resulting CNT@SF fibers can be arranged into large-area grids (>80 cm²), which can achieve experimental power densities as high as 18 mW/m². SF@CNT fibers can therefore be potentially integrated as energy harvesting fabrics within E-textiles,” state the researchers. “3D printed CNT@SF fibers for use in triboelectric generators could address long-standing materials and manufacturing challenges through several important innovations.”

In manufacturing silk fibroin and carbon nanotube inks, the research team 3D printed SF and CNT into fibers which could feasibly be used to make complex networks. This process can also be used to integrate triboelectric fibers with existing fabrics.

“Silk fibroin inks can be combined with highly concentrated CNT inks using coaxial spinnerets to create CNT@SF fibers with a core-shell geometry. Both SF and CNT inks are shear thinning, which enables efficient extrusion into free-standing fibers,” state the researchers.

Printing of Core-Sheath Fiber-Based Patterns on Fabrics for Energy-Management Smart Textile (from ‘Printable Smart Pattern for Multifunctional Energy-Management E-Textile‘)

In using biocompatible commodity materials, the researchers foresee greater options for the textiles industry, especially since CNT@SF coaxial fibers are non-toxic in terms of wearables to be attached to human skin. SF can also be used, and is ‘ideally suited,’ due to its ability to form a triboelectric pair with poly(ethylene terephthalate) (PET).

“The performance of CNT@SF/PET-ITO triboelectric pairs is notable, conclude the researchers. “These devices can generate areal power densities up to 18 mW/m² with open-circuit voltages of 10–80 V. The high-voltage (>10 V) low-current behavior (1–10 μA) of CNT@SF/PET-ITO triboelectric generators can charge capacitors with capacities of ∼5 μF within 5 min, assuming reasonable velocities of 13 cm/s.

“These devices exhibit comparable power densities compared to other previously fabricated silk-based triboelectric generators, which report power densities of 0.194 mW/cm²and 4.3 mW/m². However, it should be noted that Kim et al. combined silk with aluminum-backed polyimide, which has a different electron affinity than PET in the triboelectric series.”

3D printing is often associated with wearables, energy storage devices and integrated electronics. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

Printing Inks and Their Rheological Properties (A) Photographs of silk cocoons and the obtained SF ink. (B) Optical image showing the SF microfibrils in the SF ink. (C) Photograph showing the highly injectable SF ink. (D) Photographs of CNT powder and the CNT ink. (E) TEM image showing the good dispersion of CNTs. (F) TEM image of a multiwall carbon nanotube wrapped by polymer on its outer wall; the inset is a zoomed-in image. (G) Apparent viscosity as a function of shear rate of the CNT and SF inks. (H) Storage (G′) and loss (G″) modulus as a function of shear stress of the CNT and SF inks. (I) Photograph of a free-standing CNTs@SF core-sheath fiber after being extruded, showing good spinnability of both inks. (From ‘Printable Smart Pattern for Multifunctional Energy-Management E-Textile‘)