4D printing: intelligent materials of the future?

3D print­ing tech­nol­o­gy, which has been around for near­ly 35 years, has played a major role in rein­vent­ing tra­di­tion­al man­u­fac­tur­ing mod­els. It has enabled the cre­ation of a mar­ket esti­mat­ed at €30 bil­lion euros with a growth rate of 20% per year. How­ev­er, when an inven­tion reach­es matu­ri­ty, there comes a time when a new tech­nol­o­gy arrives on the mar­ket that will replace it. 4D print­ing, where the fourth dimen­sion is time, rep­re­sents this break­through technology.

In a way, 4D print­ing rep­re­sents a func­tion­al form of 3D print­ing and allows the print­ing of dynam­ic objects that active­ly respond to exter­nal stim­uli. This pos­si­bil­i­ty of pro­gram­ming the mate­r­i­al so that arti­fi­cial objects can behave like intel­li­gent organ­isms opens new per­spec­tives for research along­side an infi­nite num­ber of poten­tial applications.

Para­dox­i­cal­ly, the fas­ci­nat­ing hypoth­e­sis of being able to pro­gram mat­ter has pre­vi­ous­ly been intro­duced in anoth­er sci­en­tif­ic field. In 1991, Tof­foli and Mar­go­lus, two com­put­er sci­en­tists from MIT, intro­duced the term “pro­gram­ma­ble mat­ter” to describe a set of com­pu­ta­tion­al nodes arranged in a cer­tain space, which can com­mu­ni­cate with each oth­er only via first neigh­bours¹.

This idea, by cross-fer­til­i­sa­tion, spread to oth­er dis­ci­plines, until in 2005 the DARPA (Defense Advanced Research Projects Agency) launched a mul­ti-year project with the enti­tled “Real­iz­ing Pro­gram­ma­ble Mat­ter”, focus­ing on mod­u­lar robot­ics, pro­gram­ming assem­blies and nano­ma­te­ri­als². Now, the sto­ry cross­es paths with that of intel­li­gent mate­ri­als; mean­ing mate­ri­als with prop­er­ties that can be acti­vat­ed or mod­i­fied by exter­nal stim­uli either phys­i­cal (elec­tric field, mag­net­ic field, light, tem­per­a­ture, vibra­tions), chem­i­cal (PH, pho­to­chem­istry) or bio­log­i­cal (glu­cose, enzymes, biomolecules).

Final­ly, in 2013, Sky­lar Tib­bits, founder of the Self-assem­bly lab at MIT, dur­ing his speech at a TedX con­fer­ence, pro­posed using smart mate­ri­als in 3D print­ing process­es to pro­duce pro­gram­ma­ble objects, and pro­posed the name “4D print­ing” for this new tech­nol­o­gy. The con­ver­gence of these three areas of research – 3D print­ing, pro­gram­ma­ble mate­ri­als and smart mate­ri­als – led to the 4D rev­o­lu­tion³.

Clear­ly, at the heart of this new tech­nol­o­gy are smart mate­ri­als. This is both the great­est asset and the biggest hur­dle to its devel­op­ment, as research in this area is still in its infan­cy and few smart, print­able mate­ri­als are cur­rent­ly avail­able (most­ly poly­mers). This is why part of the research is focused on the pos­si­bil­i­ty of extend­ing the set of print­able mate­ri­als to ceram­ic and metal­lic mate­ri­als, but also to bio­log­i­cal and com­pos­ite materials.

How­ev­er, the mate­r­i­al is not the only cri­te­ri­on to con­sid­er, it is also nec­es­sary to be able to design and cre­ate an object with a desired behav­iour. Hence, such oper­a­tions require work to cor­rect­ly com­bine mate­r­i­al, process­es, and func­tion­al­i­ties. As well as devel­op method­ol­o­gy based on the tri­ad of design-mod­el­ling-sim­u­la­tion so that the print­ed object responds in an appro­pri­ate way to exter­nal stimuli.

In par­al­lel with com­put­er sci­ence, if a “bit” is the basic unit of pro­gram­ming, the vox­el (a con­trac­tion of the words vol­ume and ele­ment) is the ele­men­tary vol­ume that stores the physical/chemical/biological infor­ma­tion of an active mate­r­i­al in 4D print­ing. Pro­gram­ming an object with print­ed behav­iour in 4D there­fore means mod­el­ling and sim­u­lat­ing the opti­mal dis­tri­b­u­tion of vox­els so that the appli­ca­tion of a stim­u­lus cor­re­sponds to a deter­min­is­tic effect. This com­plex prob­lem requires ad hoc solu­tions where the desired behav­iour is treat­ed as an input vari­able, while the action (the vox­el dis­tri­b­u­tion) is treat­ed as an out­put variable.

Final­ly, an object print­ed in 4D can be het­ero­ge­neous. That is to say: com­posed of one or more active mate­ri­als inter­spersed with pas­sive ele­ments. This requires the devel­op­ment of mul­ti-mate­r­i­al print­ers and spe­cif­ic codes to adapt them to the mate­ri­als used and the stim­uli introduced.

The pos­si­bil­i­ty of com­bin­ing com­plex geome­tries and evolv­ing behav­iours allows 4D print­ing to push the lim­its of object design and to rev­o­lu­tionise the world of man­u­fac­tur­ing as it is under­stood today. It will fos­ter the devel­op­ment of new tech­nolo­gies that are based, for exam­ple, on self-assem­bly, if the print­ed ele­ments can assem­ble them­selves autonomous­ly at a spe­cif­ic time and place with­out human inter­ven­tion. On self-adapt­abil­i­ty if the print­ed struc­tures can com­bine sens­ing and actu­a­tion with­in the same mate­r­i­al. Or on self-repair, if print­ed objects pos­sess the abil­i­ty to detect and repair defects (wear, man­u­fac­tur­ing) by them­selves reduc­ing the need for inva­sive procedures.

There are count­less pos­si­bil­i­ties. 4D print­ing is already a dri­ving force in flex­i­ble robot­ics for the fab­ri­ca­tion of ever small­er robots (mil­li-robots, micro-robots, nano-robots) capa­ble of work­ing in haz­ardous envi­ron­ments or mov­ing in con­fined envi­ron­ments, such as in the human body, to deliv­er a drug or to per­form micro-inva­sive oper­a­tions. In the field of bio­med­ical appli­ca­tions, stud­ies are under­way to be able to bio-print stents, organs, and intel­li­gent tis­sues. 4D print­ing will pro­mote the devel­op­ment of flex­i­ble and embed­ded elec­tron­ics as well as intel­li­gent sen­sors adapt­ed to the con­nect­ed city.

In the field of ener­gy, research is under­way to max­i­mize the effi­cien­cy of solar cells by inte­grat­ing microstruc­tures print­ed on flex­i­ble sub­strates. We can imag­ine con­sumer appli­ca­tions in the field of fash­ion and lifestyle, such as self-adapt­ing bio­mimet­ic tex­tiles or intel­li­gent self-fold­ing shoes. In archi­tec­ture, 4D print­ing will allow the devel­op­ment of a new approach focused on sus­tain­able devel­op­ment such as the Hygroskin project, which uses the hygro­met­ric prop­er­ties of wood to close and open a pavil­ion accord­ing to the humid­i­ty with­out any inter­ven­tion or exter­nal ener­gy. 4D print­ing also finds appli­ca­tions in research and cre­ation prac­tices in art and sci­ence around the notion of mat­ter with behav­iour to ques­tion the rela­tion­ship between the liv­ing world and the arti­fi­cial world.

To quote Bernard de Chartres “we are like dwarfs on the shoul­ders of giants”. We can already affirm that 4D print­ing has been added to the process of pro­found trans­for­ma­tion, of design and pro­duc­tion of indus­tri­al objects, ini­ti­at­ed by addi­tive man­u­fac­tur­ing. Although com­pared to the glob­al mar­ket for 3D tech­nol­o­gy (€30bn/year), the mar­ket for 4D print­ing is still mod­est (€30–50m/year), its dis­rup­tive char­ac­ter is obvi­ous. This is not sur­pris­ing because in the life cycle of a prod­uct, 4D print­ing is still in its infan­cy. For this rea­son, and beyond the tech­ni­cal-sci­en­tif­ic devel­op­ments, 4D print­ing still needs to find its eco­nom­ic mod­el and demon­strate the pos­si­bil­i­ty of indus­tri­al pro­duc­tion at an eco­nom­ic cost. Final­ly, in order for 4D print­ing to leave the research lab­o­ra­to­ries, this tech­nol­o­gy must nec­es­sar­i­ly go through the imple­men­ta­tion of a clear and ambi­tious roadmap and cre­ate in par­al­lel a “social desir­abil­i­ty”. This will also require the will­ing­ness of investors and indus­tri­al­ists to sup­port 4D print­ing and to push it towards eco­nom­ic maturity.