Advancing Soft Tissue Regeneration with DLP 3D Bioprinting

Researchers in the Kaplan Lab at Tufts University recently developed a proof-of-concept 3D-bioprinter that leverages two advantages of DLP technology for the development of on-demand soft tissue regeneration: Speed and cell viability. The advance has the potential to transform the way physicians currently treat cartilage and other soft tissue injuries. The long-term goal is to be able to offer on-demand replacement of damaged tissues based on the patient's own cells and needs, greatly reducing the risk of tissue rejection and failure.

The Kaplan Lab has become a pioneer in the use of silk-based biomaterials for tissue engineering and regenerative medicine. Having already devised a technique for 3D printing bone via paste extrusion, post-doctoral researcher Vincent Fitzpatrick aimed to expand his work into techniques for developing soft tissues, such as cartilage, based on silk substrate material.

The Challenge: Build a Faster Printer that Maintains Maximum Cell Viability

The challenge, however, would be to first develop a 3D printer capable of producing soft structures without causing extensive cell damage. Due to their inherent process and the types of input materials required, traditional extrusion-based printing technologies aren’t capable of creating the types of soft-cell structures Vincent’s research required. And, because of the laser curing technique used in SLA printers, producing a model of virtually any reasonable size would be very time-consuming as the extremely small laser beam must individually cure each tiny segment of the model.

Complicating matters, cell viability was a major concern. Previous experiments by a Korean research team that compared extrusion technology to DLP 3D-printing technology showed that more than half of the cells would die in the extrusion process, and give off signals to neighboring cells detrimental to functional material integration with the surrounding tissues. The DLP technology, however, improved the cell viability by 80-90%.

Building a custom DLP printer from scratch emerged as the only realistic solution. Serendipitously, Vincent met two key collaborators almost simultaneously: Riley Patten, a very bright undergrad student with expertise in 3D printing technology, and Karine Blandel, senior manager of technology and strategic cooperations in the Boston office of In-Vision, an Austrian-based manufacturer of advanced DLP light engines for 3D printing.

“Because DLP technology cures an entire layer of the image simultaneously, it works much faster for producing larger models,” Riley explained. “And, because its speed and efficiency drastically reduce the amount of UV light exposure on the cells, it could substantially increase post-print cell viability. We’re talking from 40-60% cell viability with other methods to 80-90% with DLP. Not killing the cells is obviously a primary goal, so that makes DLP friendlier in every way.”

The Solution: Adjustable Intensity Light Engine

To make the project a reality, In-Vision provided the Kaplan lab with an Ikarus DLP projection module, a compact, lightweight and customizable LED-based optical module, along with the software and technical support to assist Riley in building the printer around the UV projector.

For this particular application, Ikarus provided an important key feature that would make the project possible: adjustable beam intensity.

“Most DLP printers use lenses to convert a conventional laser into a flat beam, which only provides a single intensity, so you’re locked into an exposure time. With Ikarus, we have very fine control to change the intensity of UV light that we apply to our material—anywhere from 1 microwatt per centimeter squared up to 13,000 microwatts per square centimeter,” Riley said. “By increasing the intensity, we can get faster polymerization, which means we can reduce the UV exposure time from say 30 seconds to 5 seconds. That ensures greater cell viability in the resulting matter.”

Ikarus’ 100-micron resolution was also a big advantage, allowing the Kaplan team to print a much higher resolution model out of the gate compared to other UV projectors of its type, while also offering the flexibility to make further adjustments to increase image resolution.

The Results: Technology that Brings Viable Print-on-Demand Tissues within Reach

Having built the printer, developed a silk-based resin that suits the unique application, and produced a viable biomaterial, the team is now in the process of analyzing the material’s mechanical properties and optimizing the process for various applications.

“In the short term, we need to first understand the properties of the constructs we’re creating, how cells interact with them, and then figure out how to push these cells toward the phenotype we’re interested in making—either cartilage or bone,” Vincent says. “We want to print a construct that has growth factors built in and use that as a template to grow whatever cells are needed.”

Vincent says the team is excited about the next steps, especially the potential to print material seeded with a single type of cell that will “know” whether it needs to be a bone cell or cartilage cell, allowing them to print both types of cells from a single material. This would solve the weak-link problem that’s common between interfacial tissues—where hard and soft meet.

“These interfaces are biologically and mechanically complex. They are usually a gradient of extracellular matrix proteins, cells and biological factors that mix and merge together, transitioning from bone to cartilage,” Vincent envisions. “The goal is to create that gradient—to start from one material, one cell type and one printer to print a ligament, for example, with cells that would intertwine the same way that they would naturally blend together in the human body.”