Material Properties

In this section, we talk about the material properties of collagen & elastin, and then introduce some of the additives that are commonly used in tissue engineering scaffolding to improve properties.

For tissue engineering, the desired materials depend on the application/type of tissue; there are many materials currently in use, but not all are appropriate for all types of tissue. For scaffolding specifically (the extracellular matrix), there are 5 key functions that ECM materials must be able to do:

"[provide] structural support and [a] physical environment for cells residing in that tissue to attach, grow, migrate and respond to signals"
"[give] the tissue its structural and therefore mechanical properties, such as rigidity and elasticity that is associated with the [specific] tissue functions."
"provide bioactive cues to the residing cells for regulation of their activities"
"act as reservoir of growth factors and potentiate [cells'] bioactivities."
"[provide] a degradable physical environment so as to allow... tissue dynamic processes, namely: morphogenesis, homeostasis and wound healing, respectively." (Chan & Leong, 2008)

Therefore, when evaluating the material properties of collagen and elastin, we wanted to find:

Subtypes & composition, so we could better understand what the molecules look like, what they're made of, and what type of polymers they are.
Properties relating to strength, so we could evaluate structural integrity (including tensile strength, fatigue strength, breaking properties)
Properties related to stretchiness, so we could evaluate the tissue's ability to move dynamically (including elongation, elastic modulus, and bulk modulus)
Properties related to chemical and thermal durability, so we could evaluate biocompatibility with use in various organs.

See below for quantitative & qualitative data:

Material Properties (Composition, Mechanical, Thermal, Chemical) of Collagen & Elastin

As can be seen in the above table, both collagen and elastin are elastomers, protein chains that contain glycine and proline, although they each also contain a few other amino acids. Collagen is far stronger than elastin, given its yield strength, elastic modulus, and fatigue strength. However, elastin is far stretchier: note the elongation. Both collagen and elastin are insoluble in water, explaining that they can hold water and therefore work well in organs; both collagen and elastin are slightly soluble in some acids and alkaline solutions, but also somewhat resistant to dissolving in acid, especially when the cells are young. Overall, the resilience of collagen and elastin to most environments explains why they are so common in so many body systems' scaffolding.

Additives for Scaffolding

Collagen is traditionally used as a way to re-create certain arteries as scaffolding, but often this scaffolding is too weak to function properly in vivo due to its poor strength (Dong & Lv, 2016). Therefore various additives can improve its material properties (as well as the material properties of other, non-collagen scaffolds (like PLLA, PLGA, elastin, etc., see “Synthetic Tissue Engineering”).

Elastin: Elastin is a very, very common additive. Found in most tissues in the body, elastin is commonly added to scaffolding to improve tissues’ material properties, especially for organs and connective tissues. Studies have shown that the collagen scaffolding increases in a more idealistic elastic solid mechanical behavior when mixed with the elastin; the composite material creates a membrane that is more stable for arteries in general (Berglund et al., 2004).

Elastin is not always the additive, however: elastin scaffolding may be used as a way to also be able to replicate arterial walls, due to its high elasticity and function as a base for other additives (Berglund et al., 2004). For elastin arterial scaffolding, roles might be reversed, with collagen as the additive; natural arteries have collagen and elastin (reference from skin section). Arteries are not the only place that elastin scaffolding is used. In fact, in bone defects, elastin scaffolding is now used to replicate the missing bone. For these scaffolds, hydroxyapatite (a bioceramic) and bone morphogenetic protein are commonly used as the

Above: Depiction of the two additives used in the bone marrow scaffolding (Muhoz et al., 2020).

additive - they are mixed into holes into the scaffolding. When testing the composite as a base for rat femurs to re-grow, there were no negative effects with the growth of bone or the bone marrow within. The created material also helped encourage growth with a greater speed which was evident when compared to the control group whose volume of bone was less than the experiment with the elastin and hydroxyapatite composite (Muhoz et al., 2020).

Above: Depiction of the different density of bone growth between different scaffolding used (Muhoz et al., 2020).

Above: 3D diagram of bone growing onto scaffolding with the base of the original bone at the bottom diagram of the scaffolding (Antman45000 2010).

Chitosan: Chitosan, a naturally occurring cationic biopolymer, is used as an additive, commonly in anionic polymer scaffolding. For most applications, chitosan acts like a filler; the cationic-anionic combination makes for optimal mechanical properties (Dong & Lv, 2016). Collagen-chitosan composite scaffolds have been shown to create a neuroprotective effect - essentially, the scaffold reduces the rate of neuron injury and death (Zhang et al., 2015); collagen/chitosan materials are dually promising in skin tissue engineering applications (Dong & Lv, 2016).

Silk fibroin: Collagen has also been combined with silk fibroin (a natural polymer and a very fascinating macromolecular material in and of itself) to increase cell affinity, adhesion, and mechanical properties; the combination of these two molecules shows promise for corneal tissue regeneration/replacement (Dong & Lv, 2016). Silk fibroin is therefore a filler.

PCL: poly (ε-caprolactone), or PCL, is a synthetic polymer that has been used in collagen scaffolds for great results in muscle tissue engineering. PCL/collagen scaffolds have shown low crystallinity, small crystal sizes, and high levels of uni-directional alignment, all great properties for muscular cell orientation and functioning (Dong & Lv, 2016).

PLA: polylactic acid, or PLA, is also a synthetic polymer, that is widely used in non-tissue-engineering applications. Collagen/PLA scaffolds have been shown to be porous and highly stiff… and PLLA(a type of PLA)/collagen scaffolds have improved cell proliferation in the injection stage over pure-collagen scaffolds by 256% (Dong & Lv, 2016). A lot of research is going on here.

Hydroxyapatite: Hydroxyapatite, also known as “HA” or Ca10(PO4)6(OH)2, is a bioceramic - a type of inorganic material, and a very, very common additive. HA is especially common in bone tissue engineering due to its osteoinductive properties, hydrophilicity, and ability to improve early cell capture for bone grafts (Dong & Lv, 2016).

More: Many, many more additives exist, largely categorized into natural polymers, synthetic polymers, bioceramics, and inorganics… we barely scratched the surface. A few more additives that may be of interest (this is by no means an exhaustive list):

Natural polymers: hyaluronic acid, alginate
Synthetic polymers: poly (ethylene glycol) (PEG), polyglycolide (PGA), poly (lactide-co-glycolide) (PLGA) and polyvinyl alcohol (PVA)
Inorganic/Hybrid: silicate, β-tricalcium phosphate (β-TCP, Ca3(PO4)2), carbon nanomaterials (carbon nanotube (CNT) and graphene) (Dong & Lv, 2016)

Next: Structure