Biomaterials
A bone fixation pin of crystalline poly-L-lactic acid (PLLA), made only of the L-isomer, takes several years to be fully resorbed in the body. Which is a correct way to make it degrade faster?
- ACopolymerize with the D-isomer to make amorphous poly-DL-lactic acid
- BRaise the crystallinity by drawing or heat treatment, increasing the crystalline regions that are hydrolyzed
- CIncrease the molecular weight to raise the number of ester bonds that hydrolysis can cleave
Show answer and explanation
Answer: A. Copolymerize with the D-isomer to make amorphous poly-DL-lactic acid
Hydrolysis proceeds first in the amorphous regions, which water penetrates easily, and is slow in the crystalline regions, where the chains are tightly packed. Poly-DL-lactic acid (PDLLA), copolymerized with the D-isomer, loses its stereoregularity and becomes amorphous, so it degrades faster. PLGA, copolymerized with glycolic acid, which has no methyl group and is more hydrophilic, also degrades faster. The degradation period of PLGA can be tuned from weeks to months by its composition, and it is also used as a carrier that releases drugs gradually. Raising the crystallinity makes it harder for water to enter, slowing degradation. Increasing the molecular weight raises the number of bonds that must be cut before strength is lost, so the loss of strength and the resorption are again slower.