Advances in DLP Printing Pave the Way for Futuristic Applications

Nicholas Fang, a professor of mechanical engineering at MIT, became fascinated with micro structures during his graduate study at UCLA, after reading a 2001 article published in Nature about scientists who used two laser beams to sculpt a tiny resin figure of a bull – about the size of a red blood cell – with a resolution of 120 nanometers. He has been studying micro structures and laser-based direct writing ever since.

Breaking the Diffraction Barrier

Using state-of-the-art lenses, Fang was able to create features about half a micrometer in size with blue LED light, which has a wavelength of about 430 nanometers. But that wasn’t enough. He wanted to print things like tiny biochemical robots even smaller than the bull that had captured his imagination – agents that could deliver medications targeted to individual cells.

Like other scientists for over a century, Fang was stymied by the diffraction barrier – the limit of optical resolution caused by the scattering of light waves beyond a certain length as they pass through an aperture, regardless of lens quality. However, after several years of research, Fang and a group of colleagues found a way to overcome the diffraction barrier, enabling them to print – at least in the lab – objects with features a hundred times smaller than a human hair.

The ability to print tiny features opens up a new world of possibilities for 3D printing. In addition to cellular-level robots, scientists could potentially grow tissue cultures, create photonic crystals that can bend light across a desired path, develop acoustic metamaterials with intricate geometries, produce more capacious electronic storage, or build optical circuits that operate faster than computers.

Using Multiple Materials in DLP Printing

But before they can begin to develop these applications, scientists must first find a way to overcome the limits of current DLP printing. One of them is the large vats of resin the printers typically use.

“In order to form one layer, we were using less than one percent of the material in the vat. It was a mismatch we observed immediately,” Fang says.

Instead of using a vat, Fang developed a set of automated microfluidic dispensing process in which small volumes of resin are injected on top of the build platform. A pump and valves control the flow, similar to an irrigation system.

Using smaller volumes allows scientists to overcome the second major obstacle DLP printing poses: working with a single polymer.

“Once you are using a small amount of fluid, you can change materials,” Fang says. By regulating the amount of time the pump stays open, he ensures that each substance is properly cured before the next one is injected.

Saving Time and Money with Single Droplet Printing

Another innovative process Fang developed is printing complex structures with a single droplet of resin at a time.

Printing always involves a certain amount of wasted material. While that’s never a good thing, if your material is plastic, it’s not a catastrophe. But if you’re working with biocompatible material, which may contain growth factor hormones or other delicate substances destined to be injected into living cells, costs increase astronomically. “One droplet can cost more than $100,” Fang says.

Single droplet printing reduces waste. It also minimizes the time a substance is exposed to the environment. “The longer something stays in a vat, the greater the chance of contamination,” Fang says. Even without contamination, simply being exposed to air can cause a material’s properties to change, making quality control difficult.

In an attempt to minimize adhesion of the droplet to the build platform surface, Fang and his colleagues tested three different types of surfaces. The one that worked best was impregnated with oil, which kept the droplet moving as it generated the tiny features of a design.

Single droplet printing proved to be revolutionary not only in saving material, but in saving time and increasing throughput.

“For every layer we print, there is a settling time for the liquid to flow over and become flat. By switching to single droplet printing, we cut printing time by close to 60 percent,” Fang says.

From Lab to Market

Already, Boston Micro Fabrication – a manufacturer of microstereolithography 3D-printers – has been spun out of Fang’s research. Early adopters of this type of technology include the electronic and biomedical industries. The technology is also promising for the production of tissue cultures and biomaterials, but entering this market, which is more complicated and highly regulated, will take longer.

According to Prof. Fang, a whole ecosystem has to come together to mature bioprinting technologies and bring them to market. Developments in biomaterials and hardware are needed.

In particular, when it comes to DLP technology, higher light intensities and larger build areas would help bring bioprinting closer to commercialization.

“More light intensity will enable exotic polymer systems that use a very small fraction of photo cross-linkers,” Fang says.

Photo cross-linkers – or photoinitiators – are the chemical components triggering the photopolymerization reaction when exposed to light. The quantity of photo cross-linker needed to complete the photopolymerization for successful 3D-printing depends on the intensity of the light source. When light intensity increases, smaller fractions of photo crosslinkers can be used. This is especially critical for bioprinting, because photo cross-linkers can lead to intermediary reactions and byproducts that are harmful to cells. Using a DLP projector with higher light intensity can overcome the formation of these byproducts and open up for printing living materials containing cells.

Larger build areas would enable users to print microstructures in quantity, thereby reducing the cost of production. In order to achieve larger build areas, DLP projectors will need more pixels than most currently have. Alternatively, it may be possible to use several projectors in conjunction, Fang says.

Another innovation Fang would like to see is printers that use near-infrared light waves, which would enable the development of monitoring systems for living cells. Broadening the spectrum to include more light wavelengths could also enable machine vision, which would allow scientists to accurately measure the contour and height of structures being printed and minimize bending and pulling in sensitive junctures.

Some of these capabilities already exist. In 2020, In-Vision launched Helios, the most powerful UV DLP projector on the market, with up to 12 Watt of output power. The company’s 4K light engine Phoenix was designed to allow several projectors to be stacked next to each other to cover a larger build area.

The further DLP printing advances, the closer researchers become to making Fang’s futuristic-sounding applications real.

“As we learn to control light and waves better, we can unlock many new capabilities,” Fang said. “Regenerative medicine, tissue engineering, the manufacturing of stem cells, cell therapy – the possibilities are very exciting.”