Bio-inspired materials for 3D printing
Introduction to thermoplastic polymers
Thermoplastic polymers are being heavily researched for biomedical applications around the world. One reason thermoplastics are interesting is because they can be extruded in a modern Fused Filament Fabrication (FFF) 3D printer. The FFF type of 3D printing takes advantage of the temperature-dependent rheological properties of thermoplastic polymers. In other words, the plastic becomes a liquid (melts) at elevated temperatures while remaining solid at room temperature. Thermoplastics are distinguished from thermoset polymers which typically decompose at elevated temperatures and are typically made by combining two precusors or exposure to oxygen i.e. urethanes, epoxies and silicones. While there are lot of negative opinions regarding polymers, likely due to the negative impacts some polymers have on our ecosystem and body, we must alas respect that as mammals we use polymers in our cells everyday.
Cellulose – A sugar polymer, specifically glucose in a 1,6-Beta linkage. the 2,6-alpha linkage makes up Starch!
Collagen is a ubiquitous protein found outside of cells to connect them together. Collagen is what makes up our skin and can perform functions such as disease protection, tissue regeneration, filtration (kidneys).
Lignocellulose – trees use a polymer of lignin and cellulose (glucose molecules bonded together)
Gelatin – An amino-acid polymer
All types of collagen are triple helices. It is beasically what allows multicellular organisms like us to exist in the first place. The cytoskelleton, the materials that make up the structure INSIDE of the cell is also a polymer, Cellulose nanofibrils and nanocrystals are simply polymers of glucose. Actin filaments are polymers of the protein called actin. Collagen
triple helix of peptide chains
Spiral shape, overbech jet pattern
bundled short strands (300nm) form strong fibers
Exotic Filaments vs. Traditional filaments
There has been a lot of inquiry about using exotic materials in 3D printing. The traditional mainstays for 3D printers has been PLA (poly(lactide)), and ABS (Acrylonitrile, Butadiene, Styrene). ABS is made from petroleum and PLA is made from corn starch.
The following thermoplastics are what I consider interesting test subjects for exotic filament extrusion. All are commercially available and some are already used in hospitals, biomedical research, prosthesis, as cellular scaffolds, in tissue engineering, and as bioresorbable composites (1).
Poly(lactide) or Polylactic acid (PLA) – mp = 150 – 178 dC
PLA is a biodegradable polymer made from the condensation of lactic acid, usually derived from corn starch, sugar or other starchy substances. A traditional mainstay for a FFF 3D printer due to it’s low coefficient of thermal expansion leading to better bed adhesion and less warping. PLA is technically biodegradable, but I’ve had crushed PLA sitting outside for quite sometime in the dirt and it shows no sign of deterioration.
Polyhydroxybutyrate (PHB) – mp = 175 dC
PHB is biodegradable and even biosynthetically derived. PHB is used by microorganisms as energy storage. PHB is chemically very similar to PLA except it has one more carbon in it’s polymer chain (3). Biopol is a mixture of PHB with polyhydroxyvalerate (PHV) used for dissolvable sutures.
Poly(caprolactone) (PCL) – mp = 60 dC
PCL is a biodegradable, compostable, hydrophobic synthetic polymer which is completely bioresorbable, that is, it decomposes in the conditions of the human body(2). What does it decompose into? Likely 6-hydroxycaproic acid, also called 6-hydroxyhexanoic acid. PCL polymer is moldable at 60 degrees C and is marketed under the names, Thermomorph and PolyMorph” , „Plaast
Polyvinyl acetate (PVA) – mp = 112 dC
PVA is a biodegradable water-soluble synthetic polymer that is useful as support material in a 3D printer. It can be printed alongside PLA in a printer that supports dual extrusion and then washed away after printing is complete. The PVA polymer is more pliable than PLA or ABS, rubbery even, and is definitely more hygroscopic than most polymers for 3D printing (absorbs lots of water). PVA dissolves in warm water with agitation, like sonication, to form an emulsion. Like other acetates, PVA undergoes base hydrolysis to form Polyvinyl alcohol (The other PVA) and acetic acid. This emulsion is the main component of washable glue sticks. PVA is used in making polyurethanes. It can be extruded around 210 degrees C