Elastic Bones
Being under the knife of a skilled surgeon is one reality of a serious bone shattering injury, requiring many special medical tools. What if, in future operations, these special tools were replaced with a 3D printer and some unique ink? This is what researchers at Northwestern University have predicted with the creation of what they call hyperelastic bone, a 3D printable scaffold made possible by the enormous advancements in 3D printing technology over the past decade. Allowing for applications in regenerative medicine. However, you may think to yourself “don’t broken bones regenerate themself?” Yes, this is true although in some cases where bones are too damaged or completely missing we need to replace the bone to allow for regeneration to happen. This is known as bone grafting and uses natural or synthetic bone, like hyperelastic bone, to replace the previous bone.
Hyperelastic bone is not the first synthetic bone scaffold to be created for grafting. Current methods include the use of ceramics made out of minerals similar to natural bone. However, there are a slurry of problem with these material. One is the difficulty in surgical implementations because of its stiffness. Making it difficult for surgeons to manipulate it into confined spaces. Other limitations include its high cost and often rejections by the immune system.
Another option that is preferred by many surgeons is using natural bone from the patient’s own body called autografts. This is preferred because of the natural scaffold it contains and other natural growth factors and cells. Also the new bone is from the patient’s own body, therefore no risk of immune rejection. But this requires another surgery to gravest this bone, creating longer recover times. Additionally, one person only has so much bone that is safe to relocate, putting a restraint on the size of bone being used.
What if there was a way to keep the natural ability of a scaffold to promote regeneration while at the same time be easily manipulated and produced?
Thankfully, researchers at Northwestern University in Illinois are working on a material to do just that. As mentioned above they call this material hyperelastic bone, which is made up of a naturally occurring mineral in bone called hydroxyapatite, a biodegradable polymer called polycaprolactone, and a solvent. Hydroxyapatite adds strength and allows stem cells to start proliferating to form bone. Polycaprolactone adds flexibility to the material and the solvent allows for the material to be a liquid but as it evaporates it allows the layers to stick to each other. Therefor this material is ideal for 3D printing technology. When in liquid form it is placed into ink cartridges and than printed into layers to form any shape or object imaginable. This allows for the construction of simple sheets to large and oddly shaped structures resembling bones in the body like the spinal bones.
What makes hyperelastic bone better than other materials is its user-friendly construct, which I’m sure surgeons will appreciate. Due to its 3D printability they can easily take an x-ray of the patients bone to be replaced, enter this data into a computer and press print, and within a day the object can be created. This created patient specific molds that are easily personalized. Another plus is the materials flexibility allowing the surgeon to manually manipulate the bone mold to squeeze into tight spots for a perfect fit, and bounce back to its original shape (figure 1). Its ease of use and production make this cheep and readily available, almost as easy as purchasing a regular printing cartridge at a stationary store.
Figure 1 – Elasticity of hyperelastic bone under the force of a hydraulic press. A) Initial shape of hyperelastic bone before compressed. B) Compressed hyperelastic bone. C) Reformation of the initial shape of hyperelastic bone after compression.
Like other previously created scaffolds hyperelastic bone works in a similar way to regenerate large portions of bone. It supposedly works by acting as a scaffold to lay down the foundation for stem cells migrate into. Once in place the stem cells proliferate and transform into new bone tissue within the new material. As these new tissues form the synthetic scaffold is integrated into the new bone and eventually breaks down to be recycled, as all normal bone tissue does.
To test their predictions these researchers preformed animal studies to test hyperelastic bones ability to support cell proliferation and integration into the animal’s natural bone. One interesting test done on a macaque monkey, which resembles humans more than using rats, used hyperelastic bone to replace a region of damaged skull. Only 4 weeks after the surgery the hyperelastic bone scaffold was filled with new tissue including blood vessels, which are needed to support new bone tissue formation, and some calcified bone (figure 2). Additionally, they did not notice any adverse effects to the monkey such as immune rejection or infection, which is very common in other synthetic scaffolds.
Figure 2 – Animal surgery using hyperelastic bone to replace damaged skull bone in macaque monkey. A) Replacement of damaged skull bone with hyperelastic bone. B) Results after a 4 week recovery post replacement surgery. C) Zoom in of the surgically replaced bone after the 4 week recovery.
As the title of their report suggests “Hyperelastic “bone”: A highly versatile, growth factor–free, osteoregenerative, scalable, and surgically friendly biomaterial”, they claim it is a growth factor-free biomaterial for regeneration. But, bone regenerates on its own without the help of added growth factors, as long as it has a scaffold, so that is an unnecessary claim. Additional to this, in one of the animal studies fusing the backbone of rats using hyperelastic bone, they added a growth factor with hyperelastic bone to further enhance bone formation. This is contradictory to their claims of growth factor-free material. However, results did show hyperelastic bone, on its own, inducing and supporting bone formation. They should have either left the growth factor-free claim out or not included this data.
There are roughly 4 million bone grafting surgeries annually in North America. Requiring large amounts of grafting material making this a multibillion-dollar industry. Its no surprise that enormous amounts of research are being done innovate this field, such as hyperelastic bone. However, you must stay skeptical about some claims until further support is uncovers. Such as the animal trial on the monkey that was only done on one monkey. A larger sample size is needed to support their clams in order to be implemented in humans. Although, I feel these results will be easy to obtain and will hopefully show the same promising results.
In the future, the reality of a bone shattering injury will be less drastic and a lot easier for the surgeon, who will now be alongside a skilled 3D printing specialist.