Man­u­fac­tur­ing a solu­tion to plan­et-clog­ging plastics

Most bioplastics are made from cel­lu­lose, a plant-based poly­sac­char­ide mater­i­al. The Wyss Insti­tute team developed its bioplastic from chitosan, a form of chitin, which is a power­ful play­er in the world of nat­ur­al poly­mers and the second most abund­ant organ­ic mater­i­al on Earth. Chitin is a long-chain poly­sac­char­ide that is respons­ible for the hardy shells of shrimps and oth­er crus­ta­ceans, armor-like insect cuticles, tough fungal cell walls — and flex­ible but­ter­fly wings. The major­ity of avail­able chitin in the world comes from dis­carded shrimp shells, and is either thrown away or used in fer­til­izers, cos­met­ics, or diet­ary sup­ple­ments, for example. How­ever, mater­i­al engin­eers have not been able to fab­ric­ate com­plex three-dimen­sion­al (3D) shapes using chitin-based mater­i­als — until now. The Wyss Insti­tute team, led by Postdoc­tor­al Fel­low Javi­er Fernan­dez, Ph.D., and Found­ing Dir­ect­or Don Ing­ber, M.D., Ph.D., developed a new way to pro­cess the mater­i­al so that it can be used to fab­ric­ate large, 3D objects with com­plex shapes using tra­di­tion­al cast­ing or injec­tion mold­ing man­u­fac­tur­ing tech­niques. What’s more, their chitosan bioplastic breaks down when returned to the envir­on­ment with­in about two weeks, and it releases rich nutri­ents that effi­ciently sup­port plant growth. ​“There is an urgent need in many indus­tries for sus­tain­able mater­i­als that can be mass pro­duced,” Ing­ber said. ​“Our scal­able man­u­fac­tur­ing meth­od shows that chitosan, which is read­ily avail­able and inex­pens­ive, can serve as a viable bioplastic that could poten­tially be used instead of con­ven­tion­al plastics for numer­ous indus­tri­al applic­a­tions.” The advance reflects the next iter­a­tion of a mater­i­al called Shrilk that rep­lic­ated the appear­ance and unique mater­i­al prop­er­ties of liv­ing insect cuticle, which the same team unveiled about two years ago in Advanced Mater­i­als. They called it Shrilk because it was com­posed of chitin from shrimp shells plus a pro­tein from silk. In this study, the team used the shrimp shells but ditched the silk in their quest to cre­ate an even cheap­er, easi­er-to-make chitin-based bioplastic primed for wide­spread man­u­fac­tur­ing. It turns out the small stuff really mattered, Fernan­dez said. After sub­ject­ing chitosan to a bat­tery of tests, he learned that the molecu­lar geo­metry of chitosan is very sens­it­ive to the meth­od used to for­mu­late it. The goal, there­fore, was to fab­ric­ate the chitosan in a way that pre­serves the integ­rity of its nat­ur­al molecu­lar struc­ture, thus main­tain­ing its strong mech­an­ic­al prop­er­ties. ​“Depend­ing on the fab­ric­a­tion meth­od, you either get a chitosan mater­i­al that is brittle and opaque, and there­fore not usable, or tough and trans­par­ent, which is what we were after,” said Fernan­dez, who recently won the Bay­er ​“Early Excel­lence in Sci­ence” Award for his achieve­ments in mater­i­als sci­ence and engin­eer­ing. After fully char­ac­ter­iz­ing in detail how factors like tem­per­at­ure and con­cen­tra­tion affect the mech­an­ic­al prop­er­ties of chitosan on a molecu­lar level, Fernan­dez and Ing­ber honed in on a meth­od that pro­duced a pli­able liquid crys­tal mater­i­al that was just right for use in large-scale man­u­fac­tur­ing meth­ods, such as cast­ing and injec­tion mold­ing. Sig­ni­fic­antly, they also found a way to com­bat the prob­lem of shrink­age whereby the chitosan poly­mer fails to main­tain its ori­gin­al shape after the injec­tion mold­ing pro­cess. Adding wood flour, a waste product from wood pro­cessing, did the trick. ​“You can make vir­tu­ally any 3D form with impress­ive pre­ci­sion from this type of chitosan,” said Fernan­dez, who mol­ded a series of chess pieces to illus­trate the point. The mater­i­al can also be mod­i­fied for use in water and also eas­ily dyed by chan­ging the acid­ity of the chitosan solu­tion. And the dyes can be col­lec­ted again and reused when the mater­i­al is recycled. This advance val­id­ates the poten­tial of using bioin­spired plastics for applic­a­tions that require large-scale man­u­fac­tur­ing, Fernan­dez explained. The next chal­lenge is for the team to con­tin­ue to refine their chitosan fab­ric­a­tion meth­ods so that they can take them out of the labor­at­ory, and move them into a com­mer­cial man­u­fac­tur­ing facil­ity with an indus­tri­al part­ner.

Ori­gin­al publication:

Javi­er G. Fernan­dez, Don­ald E. Ing­ber. Man­u­fac­tur­ing of Large-Scale Func­tion­al Objects Using Bio­de­grad­able Chitosan Bioplastic. Mac­ro­molec­u­lar Mater­i­als and Engin­eer­ing, 2014; DOI: 10.1002/mame.201300426 Source: Wyss Insti­tute for Bio­lo­gic­ally Inspired Engineering