Swim­ming with­out an en­gine and with muscels made of plastics

Re­searchers at ETH Zurich un­der the di­rec­tion of ETH Pro­fes­sor Kristina Shea and col­leagues at Cal­tech in Pasadena, Cal­i­for­nia, have de­vel­oped a new propul­sion con­cept for swim­ming ro­bots. The ro­bot ex­ploits tem­per­a­ture fluc­tu­a­tions in the wa­ter for propul­sion with­out the need for an en­gine, pro­pel­lant or power sup­ply.

As a proof-of-con­cept study, the re­searchers de­vel­oped a 7.5-cen­time­tre mini-sub­ma­rine equipped with pad­dles, which they fab­ri­cated en­tirely us­ing a multi-ma­te­r­ial 3D printer. The re­searchers re­port on this in the jour­nal PNAS [http://dx.doi.org/10.1073/pnas.1800386115]. 

The pad­dles are ac­tu­ated us­ing a bistable propul­sion el­e­ment trig­gered by two shape mem­ory poly­mer strips as pre­vi­ously de­vel­oped by Shea and her doc­toral stu­dent Tim Chen. De­signed to ex­pand in warm wa­ter, the poly­mer strips power the ro­bot by act­ing like “mus­cles”. If the wa­ter in which the mini-sub­ma­rine floats is heated, the ex­pan­sion of the “mus­cles” causes the bistable el­e­ment to quickly snap, trig­ger­ing a pad­dle stroke. The di­rec­tional mo­tion, force and tim­ing of the pad­dle strokes are pre­cisely de­fined by the ro­bot’s geom­e­try and ma­te­r­ial.

At pre­sent, each ac­tu­at­ing el­e­ment can ex­e­cute a sin­gle pad­dle stroke and must then be re­pro­grammed man­u­ally. How­ever, as the sci­en­tists point out, it is pos­si­ble to fab­ri­cate com­plex swim­ming ro­bots with mul­ti­ple ac­tu­a­tors. The sci­en­tists have al­ready made a mini-sub­ma­rine that can pad­dle for­ward with one stroke, re­lease its “cargo” (a coin) and then nav­i­gate back to the start­ing point with a sec­ond pad­dle stroke in the op­po­site di­rec­tion, all by sens­ing changes in tem­per­a­ture of the wa­ter. Vary­ing the geom­e­try of the poly­mer mus­cles al­lowed the sci­en­tists to de­fine the se­quence at which the pad­dle stroke is trig­gered: thin poly­mer strips heat up faster in warm wa­ter and there­fore re­spond faster than thicker ones.

A po­ten­tial de­vel­op­ment would be us­ing poly­mers that do not re­act to the wa­ter tem­per­a­ture, but to other en­vi­ron­men­tal fac­tors such as the acid­ity or salin­ity of the wa­ter.

“The main take­away from our work is that we have de­vel­oped a new and promis­ing means of propul­sion that is fully 3D printed, tune­able and works with­out an ex­ter­nal power source", says ETH Pro­fes­sor Shea. This could pos­si­bly be de­vel­oped fur­ther to cre­ate a low-power ves­sel for ex­plor­ing ocean depths.

• Chen T, Bilal OR, Shea K, Daraio C: Harnessing bistability for directional propulsion of soft, untethered robots, PNAS, 15. Mai 2018, doi: 10.1073/pnas.1800386115 [http://dx.doi.org/10.1073/pnas.1800386115]
• Chen T, Shea K: An Autonomous Programmable Actuator and Shape Reconfigurable Structures Using Bistability and Shape Memory Polymers, 3D Printing and Additive Manufacturing, 24. Mai 2018, doi: 10.1089/3dp.2017.0118 [http://dx.doi.org/10.1089/3dp.2017.0118]