Power/Performance Bits: June 23

Printing 3D circuits from wood

Researchers at Chalmers created three-dimensional objects made entirely of cellulose for the first time with the help of a 3D-bioprinter. They also added carbon nanotubes to create electrically conductive material.

The difficulty using cellulose derived from wood in additive manufacturing is that cellulose does not melt when heated. Therefore, the 3D printers and processes designed for printing plastics and metals cannot be used for materials like cellulose. The researchers solved this problem by mixing cellulose nanofibrils in a hydrogel consisting of 95-99 percent water. The gel could then in turn be dispensed with high fidelity into the researchers’ 3D bioprinter, which was earlier used to produce scaffolds for growing cells, where the end application is patient-specific implants.

Furthermore, the cellulose gel was mixed with carbon nanotubes to create electrically conductive ink after drying. Carbon nanotubes conduct electricity, and another project aims at developing carbon nanotubes using wood.

The tiny chair made of cellulose is a demonstrational object printed by the 3D bioprinter at Chalmers University of Technology. (Source: Peter Widing)

Using the two gels together, one conductive and one non-conductive, and controlling the drying process, the researchers produced three-dimensional circuits, where the resolution increased significantly upon drying. The two gels provide a basis for the possible development of a wide range of products made by cellulose with in-built electric currents.

“Potential applications range from sensors integrated with packaging, to textiles that convert body heat to electricity, and wound dressings that can communicate with healthcare workers,” says Paul Gatenholm, professor of Biopolymer Technology at Chalmers. “Our research group now moves on with the next challenge, to use all wood biopolymers, besides cellulose.”

Stretching batteries with kirigami

Origami, the centuries-old Japanese paper-folding art, has inspired recent designs for flexible energy-storage technology. But energy-storage device architecture based on origami patterns has so far been able to yield batteries that can change only from simple folded to unfolded positions. They can flex, but not actually stretch.

An Arizona State University research team overcame the limitation by using a variation, called kirigami, as a design template for batteries that can be stretched to more than 150 percent of their original size and still maintain full functionality.

An ASU research team has used a variation of origami, called kirigami, as a design template for batteries that can be stretched to more than 150 percent of their original size and still maintain full functionality. (Source: Jessica Hochreiter/ASU)

The team developed kirigami-based lithium-ion batteries using a combination of folds and cuts to create patterns that enable a significant increase in stretchability.

“This type of battery could potentially be used to replace the bulky and rigid batteries that are limiting the development of compact wearable electronic devices,” said Hanqing Jiang, an associate professor at ASU.

The kirigami-based prototype battery was sewn into an elastic wristband that was attached to a smart watch. The battery fully powered the watch and its functions – including playing video – as the band was being stretched.

An accordion conductor

Researchers from North Carolina State University have created stretchable, transparent conductors with a “nano-accordion” design. The conductors could be used in a wide variety of applications, such as flexible electronics, stretchable displays or wearable sensors.

The researchers begin by creating a three-dimensional polymer template on a silicon substrate. The template is shaped like a series of identical, evenly spaced rectangles. The template is coated with a layer of aluminum-doped zinc oxide, which is the conducting material, and an elastic polymer is applied to the zinc oxide. The researchers then flip the whole thing over and remove the silicon and the template.

Researchers from North Carolina State University have created stretchable, transparent conductors. The conductors could be used in a wide variety of applications, such as flexible electronics, stretchable displays, or wearable sensors. (Source: Abhijeet Bagal)

What’s left behind is a series of symmetrical, zinc oxide ridges on an elastic substrate. Because both zinc oxide and the polymer are clear, the structure is transparent. And it is stretchable because the ridges of zinc oxide allow the structure to expand and contract, like the bellows of an accordion.

The structure can be stretched repeatedly without breaking. And while there is some loss of conductivity the first time the nano-accordion is stretched, additional stretching does not affect conductivity.

The researchers are also experimenting with the technique using other conductive materials to determine their usefulness in creating non-transparent, elastic conductors.