MIT: A New Fiber Ink With Electronics Embedded Inside

Innovations in printed electronic devices have led to the development of alternative approaches and exciting applications for 3D printing, focusing on both printing methods and ink materials. It is a growing and evolving field which promises lower costs as well as opportunities for innovative functionality which tends to be more expensive with conventional electronics. Just imagine being able to 3D print an entire model airplane wing, using filaments that contain both light emitters and light detectors embedded in the material, so that it could continuously detect any microscopic cracks as they begin to form.

This is what MIT researchers have been working on, developing a new method that uses standard 3D printers to produce functioning devices with the electronics already embedded inside. The devices are made of fibers containing multiple interconnected materials, which can light up, sense their surroundings, store energy, and more.

According to the new 3D printing method described in a paper by MIT research assistant Gabriel Loke, professors John Joannopoulos, Yoel Fink, Rodger Yuan, Michael Rein, Tural Khudiyev and Yash Jain, the design and fabrication of functional systems shaped in 3D form factors can enable applications in diverse areas such as photonics, sensing, energy storage, and electronics. However, printing different material classes to create electronic devices was, up to now, a complex fabrication challenge in itself, because different print methods were developed specifically for various material classes. The researchers’ alternative approach claims to surpass some of these traditional hurdles.

Gabriel Loke from MIT

“While the filaments used in the model (airplane) wing contained eight different materials, in principle, they could contain even more. Until this work, a printer capable of depositing metals, semiconductors, and polymers in a single platform still did not exist, because printing each of these materials requires different hardware and techniques,” said Loke.

The system makes use of conventional 3D printers outfitted with a special nozzle and a new kind of filament to replace the usual single-material polymer filament, which typically gets fully melted before it’s extruded from the printer’s nozzle. The researchers’ new filament has a complex internal structure made up of different materials arranged in a precise configuration and is surrounded by polymer cladding on the outside.

According to MIT, in the newly reformated printer, the nozzle operates at a lower temperature and pulls the filament through faster than conventional printers do so that only its outer layer gets partially molten. The interior stays cool and solid, with its embedded electronic functions unaffected. In this way, the surface is melted just enough to make it adhere solidly to adjacent filaments during the printing process, to produce a sturdy 3D structure.

The printer used by the team was a RoVa3D multi-nozzle printer, a standard type of 3D printer known as a fused deposition modeling (FDM, material extrusion) printer, which can already be found in many labs, offices, and even homes. While the software used to generate and read the g-code for printing were Slic3r and Prometheus. The hot end was made up of a stainless steel tube that can range up to 2 mm in length and is heated by nichrome wire. Then high-temperature insulation tape is inserted between the hot end and the region above the hot end.

Loke suggested that “this method is up to three times faster than any other current approach to fabricating 3D devices, and as with all 3D printers, it offers much more flexibility regarding the kinds of forms that can be produced than typical manufacturing methods do. Unique to 3D printing, this approach is able to construct devices of any freeform shapes, which are not achievable by any other methods thus far.”

Just a few days ago, MIT’s Computer Science and Artificial Intelligence Laboratory team revealed another ink for use in their 3D printing endeavors, a reprogrammable ink that changes colors upon exposure to UV and other light sources. Known as the PhotoChromeleon system, it is made up of numerous dyes sprayed onto an object’s surface, allowing it to change color as needed. MIT is certainly progressing quickly with functionally engineered creations that keep advancing 3D printing technology even further.

Filaments with embedded circuitry show a lot of promise, they can be used to print complex shapes for biomedical and robotic devices, since the method uses thermally drawn fibers that contain a variety of different materials embedded within them. It is a process that co-author Fink, who is a professor of materials science at MIT as well as of electrical engineering and computer science, along with his collaborators have been perfecting for two decades.

Designable structured multimaterial filament inks for three-dimensional printed functional systems

The internal components of the filament include metal wires that serve as conductors; semiconductors that can be used to control active functions, and polymer insulators to prevent wires from contacting each other.

They have created an array of fibers that have electronic components within them, making the fibers able to carry out a variety of functions. For example, for communications applications, flashing lights can transmit data that is then picked up by other fibers containing light sensors. This approach has for the first time produced fibers, and fabrics woven from them, that have these functions built-in.

Now, this new process makes this whole family of fibers available as the raw material for producing functional 3D devices that can sense, communicate, or store energy.

According to MIT, to make the fibers themselves, the different materials are initially assembled into a larger-scale version called a preform, which is then heated and drawn in a furnace to produce a very narrow fiber that contains all those materials, in their same exact relative positions but greatly reduced in size.

The authors of the research paper claim that in this work, they established a fast, multiscale approach to print a diverse set of designable multi-material filament-based inks to create complex 3D hierarchical functional systems that bridge micron-scale device resolution to centimeter-scale object size. In particular, unlike current composite inks, which have limited control over spatial localization of constituent materials, they assure to have demonstrated structured filaments that combine different interchangeable material classes with controlled interfaces while internal materials can be microstructurally shaped into different topologies to enable varying ink functionalities.

To illustrate the utility of their capability to form complex freeform 3D devices, the researchers combined the functionalities of both light-emitting and light-detecting within a single printed structure by printing them into an airplane wing, and capable of detecting a structural defect at any point within the wing. The printed airplane wing is seen to have light-emitters at the top and bottom layers, and, light-detectors in the bulk of the wing. As the light-emitters are operated, the photodetectors generate a photocurrent, with its magnitude corresponding to the length of the printed photodetectors filament. Upon the occurrence of a structural defect to the printed structure, the length is cut short, reducing the photocurrent. They can then detect the localized position of damage within the structure. In the paper, the authors describe how this capability is highly applicable for objects that are prone to collisions and high mechanical stresses such as 3D printed drones, prosthetics and parts of mobile vehicles, in which the lack of ability to uncover internal structural defects can lead to major failures.

Moreover, one of the main advantages of the approach is the printing speed, which for this model structure wing took just 24 minutes, much faster than other 3D device fabrication methods. Also, the innovative team noted that the fabrication of such a highly-complex device wing shape can only be achieved through their volumetric freeform device-print approach because it allows for full shape customizability.

This ability to precisely customize each device is essential, and the method could potentially be developed further to produce a variety of different kinds of devices. Including biomedical, where matching the device to the patient’s own body can be important; such as in prosthetic limbs, using all the electronics to monitor and control the limb embedded in place. Additionally, this method might be a perfect fit to print materials for biomedical implants that would provide a scaffolding for the growth of new cells to replace a damaged organ, and include within it sensors to monitor the progress of that growth.

Yoel Fink from MIT

Over the years, the group has developed a wide array of fibers containing different materials and functionalities. Inspired by cucumber plants which use their tightly-coiled tendrils to pull the plants upwards, MIT researchers, including Fink, recently developed a new fiber-based system that could be used as artificial muscles for robots, prosthetic limbs, or other mechanical and biomedical applications. And just last year, they created a type of soft hardware that you can wear, that is, a cloth that has electronic devices built right into it. Fink even went on to create a smart thread that can send messages and change color.

MIT researchers’ idea to 3D print the multimaterial fiber ink in a printed model airplane wing was beyond original. When it comes to the aerospace field, a creation like this could make a big difference.

[Images: Felice Frankel and MIT]