Holding it Together

Using his background in mechanical engineering and his interest in biomedical devices, Drexel Engineering’s Sorin Siegler continues to innovate in the bioengineering arena.

Orthopedic screws are one of the most common pieces of hardware in medicine. Each year, more than seven million orthopedic surgeries are performed in the United States alone, and almost all involve the use of screws in one way or another, whether for pressing fractured pieces of bone together, affixing a plate to bone or providing an anchor for tendons or ligaments to be sutured back to bone.

Drexel’s Sorin Siegler, PhD, a biomechanics expert and professor of mechanical engineering and mechanics, is revolutionizing the field with a new material that can be used to make better, stronger screws that set more firmly in bone and can be customized for use in different types of bone.

Using screws in bone is not unlike using them around your house,” Siegler explained. “When you put the screw into a hard material like wood, the material pushes back against the screw, creating a shear force that holds the screw in. But when you place a screw in drywall, for example, the material is so soft that it crumbles and can’t push back, so when any force is applied to pull the screw out, it does so easily and takes part of the wall with it.”

This is exactly the problem with surgeries with dangers of poor fixation, including the 500,000 rotator cuff repairs done annually in the U.S., or in the case of patients with low bone density, such as the 14 million annual osteoporosis cases nationwide.

“There are very vulnerable populations for whom traditional orthopedic screws are not a good long-term solution,” Siegler said.

The solution for your home projects is a drywall anchor, which expands as you screw into it, improving fixation. Siegler’s material does something similar when placed into the body.

“We created a copolymer that swells when exposed to liquid,” Siegler said. “The material can be used to create screws of any type, and can be used for both bone fixation, where we attach something like a suture to bone with an anchor, and for bone integration, where screws are used to join two pieces of bone.”

The research, with support from the Coulter Foundation, which funds translational biomedical engineering research, has been quietly happening for nearly 15 years. Siegler had hoped to bring the product to market years ago, but the industry had turned its attention to bioabsorbable materials, which dissolve after some time and can be processed by the body, relieving the need for surgery to remove materials. The shift gave Siegler valuable time to test and perfect his material, leading to an important breakthrough.

“When we tested the material with rabbits, we found that if it was porous, bone would heal through it rather than around it,” he said. “This makes the bond stronger and longer lasting.”

The discovery combined the best of both worlds — the strength of a traditional screw with the lack of invasiveness provided by a bioabsorbable. Screws using Siegler’s swelling material require 20 percent more force to pull out, and the porosity means that, as bone grows back, the bond grows stronger. Siegler experimented with different ways of making the material porous, ultimately landing on a simple answer: salt.

The beauty of using salt is that, when exposed to the liquid polymer, it stays crystalized, but when placed in a water-based solution — just the kind of solution that causes our material to swell and fuse to bone more effectively — it dissolves,” Siegler explained. Even better is that when the salt dissolves, it leaves pores about 300 micrometers wide, which is just the right space for bone to grow into.

With the process dialed in, Siegler says that the lab can create the material to almost any specifications, changing up the porosity to fit any use. They can also mix different densities of the material, creating a solid fixation screw, a more porous anchor, or a combination of the two, with the softer material forming a sleeve around the harder.

Siegler filed a provisional patent for the material in December 2021. This will protect his work as he looks to commercialize the material, either by licensing it or forming a new company to manufacture devices made with it.

First, though, comes more extensive testing. Siegler is partnering with researchers at the University of Pennsylvania to test it on live sheep. They will implant different types of screws made with the material into the sheep’s bones and check back in four months to determine how well it performed.

“We have mechanical engineering experts on hand to be sure that the device worked as we expected it to and a biologist to check that the animals remain healthy,” Siegler explained. “The key will be for us to demonstrate that the material interacts with bone in the way that we expect, which would mean that it’s bio-compatible and would be ready to attract commercial partners or investors to start commercialization of different products using this new bone anchoring technology."

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