Print, replace and regenerate

Materials World magazine
,
31 Mar 2017

Hannah Little looks at the role of 3D-printed bone scaffolds for the regeneration of critical-size defects.

Bone is an extraordinary naturally smart material that responds appropriately as and when required. In most cases, when a bone is broken its inbuilt natural response repairs itself. However, there are some cases when bone needs extra help. Fractures, tumour removal and bone-related diseases can often result in the formation of critical-sized bone defects. They become critical-sized when there is not enough bone-to-bone contact to allow mechanotransduction, a mechanical stimulus for bone to naturally reform. This requires the insertion of support material or ‘scaffold’ to assist the bone in regaining its function. Worldwide, the necessity for an appropriate solution for replacing bone in critical-sized defects is now considered a major clinical and socio-economic need.

Resorption is the dissolution of a material – in the case of implantable bioresorbable polymers, it is elimination through natural pathways in the body. The majority of implants for bone repair have previously focused on materials that have little or no resorption characteristics and are treated as permanent implants, such as metals and non-degradable polymers. 

Only recently has attention turned to creating resorbable implants using materials with clinically relevant degradation rates, closely matching the reformation rate of bone to allow efficient repair. Unfortunately, these highly resorbable materials are also renowned for their processing sensitivity and unpredictability. Currently, it is not fully understood how processing parameters affect subsequent clinical resorption rates and thus the product’s performance in vivo. There are examples of FDA-approved products being recalled due to this failure to accurately predict clinical performance, proving expensive for the manufacturer and distressing for patients.

Manufacturing a new solution

Using synthetic materials and additive manufacturing techniques, a team at the School of Mechanical and Aerospace Engineering at Queen’s University Belfast, UK, has developed a solution, addressing limitations in current procedures (auto and allografts) by improving the process. Scaffolds can be easily and efficiently produced as and when required. Surgical implantation can be improved by adaptating computed tomography images into representative patient-specific models. Architecture is precisely controlled for tailored mechanical and biocompatibility responses. In addition, the repair process has the potential to go full circle through selective development of a bioresorbable material, by providing correct mechanical support but also facilitating natural bone regeneration in a timely manner. This solution can aid the regeneration of bone in defects, which are otherwise unrepairable, while avoiding complications, revision surgeries and stress shielding associated with permanent implants. 

Scaffolds must have a porous structure like bone itself to allow the flow of cells and nutrients essential for bone regeneration. When creating a resorbable scaffold, it is also important to maintain controlled degradation through a uniform structure, presenting new challenges on how to manufacture a controlled porous scaffold. 

Additive manufacturing is expanding across all industries from food to fashion, but most exciting is its potential in healthcare. Through adapting additive manufacturing technologies for the creation of scaffolds, the team created highly controlled complex architectures with regular internal porous networks, which would be impossible to manufacture with existing techniques. The new method provides enhanced control over internal architecture and interconnectivity for guided bone growth, but also allows the freedom to manipulate external geometry to suit each patient’s bone defect. 

In this research, a RepRap Fused Deposition Modelling (FDM) printer was used. It works by feeding thermoplastic filament to a heated nozzle, which draws the desired shape over three axes. Several complex scaffold models were created using computer-aided design and printed using FDM. Parameters were adapted for the creation of scaffolds made from single strands of filament layered alternately on top of each other. Feed rate, print speed, nozzle size and temperature were found to be the key influential parameters. Once the technology's ability to manufacture the implant had been demonstrated, the next challenge was developing the material. Up until now, only commercially available filament had been used – a non-medical grade material that the human body would reject. The problem is that there are no printable biocompatible materials available with a suitable resorption rate. Overcoming this challenge requires the development and production of tailored compatible materials. 

Tailor-made

Poly(DL-lactide-co-glycolide) (PDLGA) is a tailorable bioresorbable copolymer – depending on its ratio of lactide to glycolide, its degradation profile can be adjusted within the range required. This material also has FDA approval as a biomaterial, meaning no toxic responses are elicited when implanted and it will exit the body through natural pathways as it resorbs. The elastic modul

us of the polymer is sufficiently close to bone, suggesting that it can stimulate mechanotransduction – the conversion of mechanical stimuli to electrochemical activity – from the surrounding bone, allowing regeneration while avoiding stress shielding. To further encourage bone reformation, bioactives can be added to the composition. Beta-Tricalcium Phosphate (ß-TCP) is a mineral close to the phase of bone. As it is released from the polymer matrix, it encourages new bone growth into the defect. Three compositions were produced – 100% PDLGA, PDLGA with 5% ß-TCP and PDLGA with 20% ß-TCP. The filament was manufactured using twin-screw extrusion. To minimise degradation, temperatures were reduced as much as possible, adding air rings where the extrudate exited the die to help dissipate heat and trickled nitrogen gas into the hopper to minimise hydrolytic degradation. A consistent and FDM-compatible filament was achieved, which successfully printed all bioresorbable polymer compositions. The architecture was analysed and found to satisfy the target scaffold requirements. When creating a resorbable implant the key factor must be material performance. It is essential to investigate how this material has degraded from processing and determine how it will perform once implanted. 

Realising resorption 

Exposure to thermal and hydrolytic environments during processing elicits degradation effects on the polymer, reducing the resorption profile when compared to the as-supplied material. The next step involved finding an appropriate technique to characterise. Differential scanning calorimetry failed to identify common degradation traits due to the amorphous nature of the polymer, whereas measurements of molecular weight showed distinct trends identifying degradation through the breaking of molecular chains. Comparing the processing stages, the team identified a 36% drop in molecular number from the as-supplied material to the extruded filament, and an overall 50% drop from the as-supplied to the FDM scaffold, highlighting how each of these thermal processes has a significant effect on material degradation. Where scaffolds are stored after production, simulating hospital ‘shelf-storage’, the sensitivity of the material to environmentally-induced degradation is also evident. For two months, printed scaffolds were stored in various conditions (desiccator, freezer, fridge, room, fridge in phosphate buffer solution (PBS) and room in PBS) then their molecular numbers were measured. There was up to a 15% drop in molecular number from the desiccator sample to the sample stored in the room in PBS, showing how sensitive this bioresorbable polymer is and in turn demonstrating the importance of having a full understanding of its degradation behaviour.  

Ongoing work

The most important aspect of degradation to understand is how the material will be affected once implanted. In vivo, the scaffold will be exposed to normal body temperature and fluids eliciting thermal and hydrolytic degradation. To simulate this, a series of in vitro degradation tests are underway to better understand the bioresorbable materials degradation behaviour – essential information when implanting the material into a patient. Samples are being tested for up to six months placed in PBS and stored in a 37oC oven. So far, pure PDLGA samples at time points zero, eight and 16 weeks have been tested. The difference between these three time points has been extensive.

At zero weeks, the scaffolds were clear in appearance, when compression tested a compression modulus of 55MPa was measured. However, at eight weeks wet mass measurements revealed a 118% increase in mass, and when dried for one week in a 43oC oven a 25% increase was still present showing water absorption. When compressed, the scaffolds fracture and recorded a modulus drop of 66% compared with the zero week samples. A colour change can also be noted indicating degradation. At 16 weeks, scaffolds were unrecognisable, the porous structure had disappeared with water saturation and mechanical properties became unmeasurable. 

A change in resorption rate affects how the material will perform in vivo by achieving timely bone repair by cellular response. A final study will consider how cell response is affected by the degradation of the scaffold. It is hoped that demonstrating the performance of clinically relevant resorbable materials as timely bone repair implants it will provoke future research in regenerative (opposed to replacement) solutions and lead to improved healthcare. Combining emerging technologies with advances in materials science helps to design a system that benefits not only the patient, but also the surgeons and hospital budgets. Patients can receive treatment quickly and efficiently with minimal surgeries and revisions. They also benefit from being treated with materials that will have been repeatedly tried and tested, to minimise risk and facilitate reformation of their own natural bone, preventing patient re-admittance. Surgeons benefit from using an implant that has been specifically designed, custom-printed for the defect with a reduction in required surgeries. This will lessen material waste and save time. In turn, costs to the hospital can be cut as patients can be treated more efficiently and pressures on operating theatres are reduced. 

Hannah Little is a PhD researcher in the School of Mechanical and Aerospace Engineering at Queen’s University Belfast, UK. She is a member of the Bioengineering Research Group and is in her final year, working towards the completion of her research. In 2016, Hannah represented Ireland in the Young Persons’ World Lecture Competition in Brazil