Is the bone implant of the future a hydrogel?

Bones broken in a (skiing) accident usually heal on their own. But if the break is too severe or a bone tumour needs to be removed, surgeons insert an implant that enables the bone to grow back together. 

Implants often consist of pieces of the patient¡¯s own bone, known as autografts, or metal or ceramic parts. A key drawback of many of today¡¯s implants is that they require a second surgery to harvest the tissue for the autografts. Additionally, metal implants tend to be too rigid and may loosen over time, compromising stability. 

Taking biology into account

What¡¯s even more significant is that bone is an incredibly complex organ with numerous tunnels and cavities. ¡°For proper healing, it is vital that biology is incorporated into the repair process,¡± says Xiao-Hua Qin, Professor of Biomaterials Engineering at ETH Zurich. A successful repair of this nature depends on various cell types that must first colonise the implant before forming new bone tissue.

This prompted the ETH researcher to adopt a new strategy: Qin, along with his team and ETH Professor Ralph M¨¹ller, has created a novel hydrogel suitable for future implants. This hydrogel, which is as soft as jelly, dissolves gradually in the body and could potentially be used for personalised bone implants. The study detailing this development has recently been published in the journal external page Advanced Materials.

Healing begins with soft material

The researcher explains that at the start of natural bone healing, the body initially employs a soft material. In the first days after a fracture, a haematoma or bruise forms that is permeable and facilitates the migration of reparative and immune cells and the delivery of nutrients. A fibrin network binds these cells together. This initial soft structure gradually transforms into hard, stiff bone.

The hydrogel is modelled on this natural bone healing process. It is made up of 97 percent water and 3 percent biocompatible polymer. To make it solidify, the researchers introduced two special molecules: one that links the polymer chains together and another that, when exposed to light, triggers the reaction.

Wanwan Qiu, Qin and M¨¹ller¡¯s former doctoral student, developed the connecting molecule specifically for this application. ¡°It enables rapid structuring of hydrogels in the sub-micrometre range,¡± she says. The polymer chains are linked as soon as laser pulses of a certain wavelength hit the hydrogel. The irradiated areas immediately become solid, while the non-irradiated parts can be washed out later. 

Jelly can be set at world-record speed 

In this way, the researchers can use the laser beam to print any shapes and structures into the hydrogel with very fine resolution and extreme precision. The structures can be as small as 500 nanometres. 

¡°Hydrogels resemble jelly, making them difficult to shape,¡± says ETH Professor Qin. ¡°With our newly developed connecting molecule, we can now not only structure the hydrogel in a stable and extremely fine manner but also produce it at high writing speeds of up to 400 millimetres per second. That¡¯s a new world record.¡± 

Structures in the nanometre range 

In their study, the researchers created complex, structured hydrogels that resemble real bone and feature a fine network of bone trabeculae. They used medical imaging as a template. 

Even healthy natural bone is criss-crossed by a fine network of channels that are only nanometres thick and filled with fluid. ¡°A piece of bone the size of a dice contains 74 kilometres of tunnels,¡± says Qin. By way of comparison, the longest railway tunnel in the world, the Gotthard Base Tunnel, measures 54 kilometres. 

Material is biocompatible 

So far, the researchers have tested the material only in a test tube. Results showed that bone-forming cells rapidly colonise the structured hydrogel and begin forming collagen, a vital component of bone. The tests also confirmed that the material is biocompatible and does not damage the bone-forming cells. The researchers have patented the base material and plan to make it available to the medical industry.  

The researcher¡¯s declared goal is for the hydrogel-based implant to one day be used in clinics to repair broken bones. However, more work is needed. Qin is preparing to conduct animal tests in collaboration with the AO Research Institute Davos. The team aims to determine whether their new bone repair material promotes the migration of bone-forming cells in living organisms and whether it restores bone strength over time.