Low-Temperature Green Synthesis and Assembly of Poly(Malate-co-Lactate) Gel-Based Microdroplets from Polylactate Plastic Straw Waste

Poly(malate-co-lactate) (PMALA) gels can carry molecular cargo, are immuno-negative and biodegradable in human plasma, and are also capable of passing the blood–brain barrier, making them plausible drug delivery vessels. However, the functional properties of PMALA of varying length and lactate-malate ratios remain poorly studied due to a lack of a cost-effective, sustainable synthetic method. As a proof of concept, here we showcase a low-temperature chemical method to synthesize PMALA oligomers that can assemble into gels and droplets derived from polylactate (PLA) plastic waste. Proteinase K followed by ethyl acetate extraction and evaporation first converts PLA plastic waste into free l-lactic acid (l-LA) monomers. Taking inspiration from prebiotic chemistry, we used a low-temperature (80 °C), catalyst-free, and/or tin catalyst-incorporating synthetic method to synthesize PMALA gels by simple aqueous dehydration of plastic-derived l-LA and supplemented commercially available l-malic acid (l-MA); subsequent rehydration of the PMALA gels resulted in spontaneous assembly of membraneless microdroplets. Chemical and microscopic analyses unequivocally validate that the droplets produced contained PMALA heteropolymers. This sustainable, cost-effective green method for PMALA droplet synthesis and assembly established herein combining techniques from biocatalysis, origins of life, and polymer chemistry may allow for the upcycling of PLA plastic waste to produce high-value biomedical materials used for drug delivery more sustainably in the future.