The adaptive foam reticulation (AFR) technique is a combination of the freeze casting and foam reticulation techniques used to produce scaffold structures.
Freeze casting is a technique whereby a slurry of the biomaterial is produced that is frozen. Upon freezing, ice crystals form that are sublimed as a vapour using controlled heating under pressure. This allows for the generation of microporous scaffolds, with lower temperatures required for larger pores. The nature of the freeze allows for directionality to the pores, enabling structures that are more similar to that of natural bone.
The main limitation with this is the achievable pore size. Incorporating different porogens into the slurry can generate larger pores, however these are still of micro size.
There are many natural and synthetic porous structures that have a range of porosities and pore sizes and from which it is possible to develop structures suited for tissue engineering applications. Foam reticulation aims to utilize such porous constructs as a template from which a scaffold is based. Replication techniques lead to the generation of macroporous scaffolds with a controllable pore size. They involve coating a sacrificial template with biomaterial slurry before sublimation of the template to leave the desired structure. Briefly, a slurry of the biomaterial is produced that is coated onto a sacrificial template, which is generally a polymer although can also be natural marine sponge. The coated template is then sublimed under a controlled firing regime. Controllability is achieved through the template and a variety of pore sizes can be obtained. It is also possible to realise a directional nature to the pores.
The main limitation with this technique is associated with the oxymoronic relationship between porosity and strength. It is not unusual to have porosities of 90%, leading to a reduction in the mechanical properties of the raw material. Coupled with the production of microcracks during template sublimation, low mechanical properties limit the use of foam reticulated scaffolds. Increasing the number of coatings of the slurry on the template increases the strength of the component, but reduces the pore size. Therefore a trade-off must be made to maximize strength and create pores of a suitable size. Sample strength can be improved by increasing the sintering temperature as this causes increased densification. However, this enhances shrinkage which must be considered for the production of appropriately sized pores.
The limitations of the foam reticulation technique need to be overcome to enable its widespread use in producing bioscaffolds. One possible way to achieve this is to generate microporous struts that enhance the biological performance and hence significantly increase the regrowth rate of host tissue by providing cell attachment sites. Microporosity can be achieved by the incorporation of a porogen within the slurry as with freeze casting. The porogen is sublimed either during freeze drying or at the polymer burnout stage to leave a macroporous structure of microporous struts. The technique follows that of foam reticulation with a freeze drying stage to generate micropores. Briefly, a template is coated with a biomaterial slurry containing a porogen. This is freeze dried to sublime the porogen, and the structure then subjected to a controlled sintering protocol to enable complete polymer burnout and densification of the scaffold. Combining the two techniques outlined above has a great potential for producing structures for both structural and non-structural augmentations. Freeze drying generates micropores, whilst sublimation of the template generates macropores.
This leads to structures with both macro and micro porosity, giving the structures enhanced biological activity in in-vitro tests. The microcracks generated during polymer sublimation should have less of an impact. Until now, only two materials, hydroxyapatite and Bioglass® have been used with this technique. Although these represent a great potential for future applications, implants for bone replacement and regeneration are not currently manufactured using these materials.
Advantages of AFR over foam reticulation and freeze casting
The main advantage of AFR lies in the production of micropores within the struts, which is expected to increase the biological activity of the sample by encouraging cells to adhere to it. It is also hypothesized that the mechanical properties will be increased as micropores should stop the generation of microcracks during polymer burn out by providing routes for the template to be removed from within the scaffold.
There is significant control over the structure of the final construct. Not only is macroporosity controlled by template choice, but the microporosity is also controllable as during freeze casting. Furthermore this overcomes the limitations of the freeze cast method associated with the maximum size of the construct and the small pore sizes, as small micropores are desired and the thickness of the sample to be freeze cast is that of the coating on the strut.