New cel­lu­lose-based mater­i­al gives three sensors in one

The abil­ity to meas­ure pres­sure, tem­per­at­ure and humid­ity is import­ant in many applic­a­tions, such as mon­it­or­ing patients at home, robot­ics, elec­tron­ic skin, func­tion­al tex­tiles, sur­veil­lance and secur­ity, to name just a few. Research until now has tar­geted integ­rat­ing the dif­fer­ent sensors into the same cir­cuit, and this has presen­ted sev­er­al tech­nic­al chal­lenges, not least con­cern­ing the inter­face to the user. Sci­ent­ists in the Labor­at­ory of Organ­ic Elec­tron­ics at Linköping Uni­ver­sity have now, under the lead­er­ship of Pro­fess­or Xavi­er Crispin, suc­cess­fully com­bined all three meas­ure­ments into a single sensor. Aero­gel This has been made pos­sible by the devel­op­ment of an elast­ic aero­gel of poly­mers that con­ducts both ions and elec­trons, and sub­sequent exploit­a­tion of the ther­mo­elec­tric effect.A ther­mo­elec­tric mater­i­al is one in which elec­trons move from the cold side of the mater­i­al towards the warm side, and in this way cre­ate a voltage dif­fer­ence. When nan­ofibres of cel­lu­lose are mixed with the con­duct­ing poly­mer PEDOT:PSS in water and the mix­ture is freeze-dried under vacu­um, the res­ult­ing mater­i­al has the same struc­ture as a wash­ing sponge, an aero­gel. Adding a sub­stance known as poly­sil­ane causes the sponge to become elast­ic. Apply­ing an elec­tric­al poten­tial across the mater­i­al gives a lin­ear cur­rent increase, typ­ic­al of any res­ist­or. But when the mater­i­al is sub­ject to a pres­sure, its res­ist­ance falls and elec­trons flow more read­ily through it. Since the mater­i­al is ther­mo­elec­tric, it is also pos­sible to meas­ure tem­per­at­ure changes: the lar­ger the tem­per­at­ure dif­fer­ence between the warm and cold sides, the high­er the voltage developed. The humid­ity affects how rap­idly the ions move from the warm side to the cold one. If the humid­ity is zero, no ions are trans­por­ted. ​”What is new is that we can dis­tin­guish between the ther­mo­elec­tric response of the elec­trons (giv­ing the tem­per­at­ure gradi­ent) and that of the ions (giv­ing the humid­ity level) by fol­low­ing the elec­tric­al sig­nal versus time. That is because the two responses occur at dif­fer­ent speeds”, says Xavi­er Crispin, pro­fess­or in the Labor­at­ory of Organ­ic Elec­tron­ics and prin­cip­al author of the art­icle pub­lished in Advanced Sci­ence. ​“This means that we can meas­ure three para­met­ers with one mater­i­al, without the dif­fer­ent meas­ure­ments being coupled”, he says. A lot of pos­sible applic­a­tions Shaobo Han, doc­tor­al stu­dent, and Seni­or Lec­turer Simone Fabi­ano at the Labor­at­ory of Organ­ic Elec­tron­ics, have also found a way to sep­ar­ate the three sig­nals from each oth­er, such that each can be simply read indi­vidu­ally.
​“Our unique sensor also pre­pares the way for the inter­net of things, and brings lower com­plex­ity and lower pro­duc­tion costs. This is an advant­age not least in the secur­ity industry. A fur­ther pos­sible applic­a­tion is pla­cing sensors into pack­ages with sens­it­ive goods”, says Simone Fabi­ano. Fin­an­ci­ers of the research include the Knut and Alice Wal­len­berg Found­a­tion (the Tail of the Sun pro­ject), the Wal­len­berg Wood Sci­ence Cen­ter, the Vin­nova Digit­al Cel­lu­lose Cen­ter, and the government’s stra­tegic invest­ment into advanced func­tion­al mater­i­als at Linköping Uni­ver­sity, AFM. Source: Linköping Uni­ver­sity, News, 25.03.2019