New 3D-Printed Liquid Crystal Elastomer Developed for Spine

Product CUDenver LCE Spine Device

Degenerative disc disease (DDD) has long been a painful reality for as many as 80% of American adults. The disease is the leading cause of low back pain and radiating leg pain, often surgically treated with spinal fusion. While fusion is the common treatment method for DDD, having a device that conforms to your anatomy is hard to accomplish. If it doesn’t fit or conform perfectly, you have increased areas of stress, which then cause the implant to sink into the bone and causes the spine to fuse incorrectly.

A new material, developed by a team led by CU Denver Associate Professor of Mechanical Engineering Chris Yakacki, Ph.D., could prove to be an improvement on current treatment techniques. Dr. Yakacki’s research team created a 3D-printed semi-crystalline liquid crystal elastomer (LCE) spinal fusion cage that more closely mimics the properties of biological tissue. They detailed the new material in a published set of studies that utilize LCEs for both spinal fusion techniques and potential disc replacement.

The Concept

Traditionally it has been difficult to develop a material that can expand or contract once it’s implanted into the body. The challenge has been creating a material that is both as soft as tissue and dissipative, meaning it absorbs energy.

“The body does this really well,” Dr. Yakacki said. “Your body has all of these tissues that are really soft and good at absorbing energy. Nature is the best inspiration. A lot of our tissues are anisotropic, or directionally independent. Typically, collagen is aligned, so if you pull on it in one direction, it’s really tough and strong. And then if you compress it in a different direction, it’s all of a sudden really soft and compliant.”

Dr. Yakacki’s team sought to emulate the body’s natural tissues, fabricating the anisotropic properties into the LCE material.

The Material

Liquid crystals are everywhere, Dr. Yakacki said. In biological systems, they are anywhere there are molecules that are free to move but ordered together. A recognizable non-biological example is a liquid crystal display (LCD), which uses liquid crystal molecules oriented in directions that either let light pass through or not.

“We’re taking those same molecules and incorporating them directly, chemically, into the backbone of elastomers,” he said. “You get the best of both worlds. The rubber makes things soft, the liquid crystals make things dissipative and oriented, in terms of matching biological tissues.”

Dr. Yakacki’s team at CU Denver patented a two-stage reaction. The first stage makes the LCE into a photo-curable, honey-like resin — a low-molecular-weight polymer. The second reaction relies on photo cross-linking to cure and harden the material.

“The advantage of doing photochemistry is that it’s relatively easy to shine light on things,” Dr. Yakacki said. “What makes it kind of exotic with 3D printing is that you can use a projector that can project an image of UV light to cure the polymer where you want it to, as you’re doing the printing process. Layer by layer, you adjust what light patterns are shining on it, and you can cure these really intricate 3D objects.”

The result is a good energy-absorbing device that can improve outcomes for patients with DDD. Joints are an ideal application as they are meant to take impact, they degenerate and wear down, Dr. Yakacki said.

“I’ve looked at fusions for a long time with our research, and I always think the holy grail is, can you restore the joint as best as possible to the way nature had it originally?” he said. “With fusion, you’re limiting the mobility and growing bone between two joints where there shouldn’t be bone. It’s supposed to be soft and flexible, and absorb energy.”

Modern disc repair and replacements and custom ligament replacements solve the motion issue, but without much shock absorption, he said. This often leads to jarring of the joint. While attempts have been made to create an intervertebral disc replacement that is soft in the form of nucleus replacements, the problem hasn’t been completely solved, according to Dr. Yakacki.

“Liquid crystal elastomers are relatively young; they’re still a new material,” he said. “They’re complex. In the last five years some big barriers have been taken down on how to make these materials. It’s exciting, because they’ve been stuck in academic labs for a long time, since the late ’70s, early ’80s.”

Product CUDenver LCE Spine Device

Digital light processing-printed LCE concept device of a spinal cage.

A Key Improvement on LCEs

In the past, it had been difficult to make anything out of LCE other than very small objects, but with the patented two-stage process developed by the CU Denver research team, they are at a point where they can create large devices.

“Previously, you might spend a week making something the size of your fingernail, which is cool for a paper and for studies, but not cool if you’re a manufacturer,” Dr. Yakacki said. “Now we’re at the point where we can 3D print them, we can make intricate devices, we can tailor the chemistry, we can do a lot with them in a way that was just not available even five years ago.”

The team’s patented two-stage reaction brought the process down from days to hours and is now scalable and more efficient.

As part of their research, they 3D printed a prototype interbody device and were able to show that LCEs were 12 times more rate dependent than traditional elastomers and effective at absorbing energy, similar to biological tissues.

Their research shows that in the crystalline system, a 3D-printed prototype device is capable of withstanding 1 million cycles of physiologic compression with minimal creep. The team explored the effect of the LCE molecules on the rate of rigidity, material processability and mechanical properties, which informed the first bulk 3D-printed LCE demonstrated to date, according to the research paper.

“For us, this is interesting because if we can show it in one joint, then it would open the gateways to try it in other joints,” Dr. Yakacki said. “Orthopedics is typically conservative on adopting new materials. If you’re going to try something new, it had better be game changing or next generational; it shouldn’t be just incremental. I don’t think there’s anything out there that really would compete with liquid crystal elastomers.”

What’s Next?

Thus far, the CU Denver team has worked on the science of getting the material to work quickly and efficiently for practical manufacturing. The next step is toward making an actual device.

“We spent a lot of time figuring out how to 3D print the material and understand how we tweak the chemicals to make them behave a certain way,” Dr. Yakacki said. “But now it’s time to actually realize some devices and prove them out. I think that’s the exciting part now. We’ve done a lot of background work. Now it’s actually time to do something with it.”

The CU Denver research team has received funding to explore applications including spine and a metatarsophalangeal joint treatment for the big toe. The LCE is also being tested in safety equipment such as football helmets, thanks to its shock-absorbing properties.

“Overall, we want to partner with medical device manufacturers to use this material in implants and joint replacements,” Dr. Yakacki said. “My company, Impressio, recently received a National Science Foundation Small Business and Innovation Research (NSF SBIR) grant to develop an MTP (i.e. big toe) joint replacement in conjunction with MedShape Inc. I don’t think it’s crazy to think that you could see LCEs in a joint within three to five years.”

Photos Courtesy of CU Denver.

HT

Heather Tunstall is a BONEZONE Contributor.

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