The use of 3d printed Scaffold Models in Medical and Biomedical Engineering Education. Abstract

Polymer scaffolds

Degradable polymer materials are a famous choice of material for tissue-engineered 3D printed scaffolds for three reasons. Particularly, polymers are easy to process in the shape of a 3-D scaffold with a pore morphology suitable for tissue engineering fields. Secondly, polymers can have high tensile properties and high toughness and the mechanical properties of polymers can be controlled very easily by changing the molecular weight (chain length) of the polymer. Thirdly, bioresorbable polymers have been used successfully as dissolving sutures for many years. Therefore, these degradable polymers, such as the polyesters of poly(lactic acid) (PLA), poly(glycolic acid) (PGA) and poly(lactic acid-co-glycolic acid) (PLGA) are used for scaffold applications because they have passed FDA regulations, and scaffolds made from these materialscan provide a quick route to a commercial and clinical product. The methods used to produce porous networks in these polymers are fibre bonding or weaving, solvent casting, particulate salt leaching, phase separation, gas foaming, freeze drying and extrusion.
To create an open pore structure, the polymer solution can be foamed. Blowing agents, gas injection, supercritical fluid gassing, and freeze–drying can all be used to accomplish this.
The polymers that can be used for supercritical fluid gassing must have an high amorphous fraction. Polymer granules are plasticised due to the use of a gas, such as nitrogen or carbon dioxide, at high pressures. The dissolution of the gas into the polymer matrix results in a reduction of the viscosity, which allows the processing of the amorphous bioresorbable polyesters in a temperature range of 30–40 °C. However, on average, only 10–30% of the pores are interconnected.

1. 
   Bioactive ceramic scaffolds
   

Ceramics are crystalline materials and therefore tend to have
high compressive strength and Young’s modulus but low toughness, i.e. they
are brittle materials. Alumina and synthetic HA are the ceramics that are
most commonly used in biomedical applications. Alumina is a bioinert ceramic,
so it is not ideal for use as a regenerative scaffold.
HA and TCP have been used successfully in the clinic as bone filler
materials in powder form. Synthetic HA has been used most regularly because
it has a similar composition, structure and Young’s modulus to bone mineral.
Tricalcium phosphate (β-TCP) is also similar to bone mineral in that they are
both calcium phosphate ceramics, but β-TCP is resorbable. The challenge is
to develop them into 3-D porous scaffolds, i.e. into a open pore structure,
with the properties listed in Section 1.1.
In a porous form, HA and β-TCP ceramics can be colonised by bone
tissue. A problem with introducing pores into a ceramic is that the compressive
strength of the material decreases dramatically. The strength of the scaffold
depends on the thickness and strength of the struts or pore walls. Generally
for brittle compression of a foam,
(1.1)

where σ_cr is the critical strength of the pore walls and ρr is the relative

density, where
(1.2)
where ρ_b is the bulk density of the scaffold and ρ_s is the skeletal (true)
density of the material.
Naturally occurring porous structures are being considered for fabricating HA scaffolds. A frequently used structure is coral. Hydrothermal and solvothermal methods are used to transform natural coral into HA after removal of the organic component by, for example, immersion in sodium Scaffolds for tissue engineering 209 hypochlorite. Pore size in typical coral formations is in the range 200 to 300 µm. The porosity is interconnected and the structure resembles that of trabecular bone. An HA scaffold does not satisfy the criteria of an ideal scaffold because
HA resorbs only very slowly, the dissolution products do not stimulate the
genes in the osteogenic cells and HA is only osteoconductive, it is not
osteoproductive and does not bond to soft tissue. HA is still a bone replacement
material rather than a regenerative material. It is possible, however, to modify
the HA composition to achieve gene activation.

2.