Magnet-controlled Transfer Printing Technique Based On Bio-inspired,Tunable Interfacial Adhesion

Flexible and stretchable electronics is an emerging technology that breaks the intrinsic rigid,brittle and planar constraints associated with conventional electronics.This resulting technology has enabled many promising applications such as flexible display,energy and medicine.Transfer Printing(TP)technique is the key to fabricating flexible and stretchable electronics.The most general form of the TP process utilizes a soft,elastomeric stamp to transfer inks from the fabrication substrate to the receiver substrate.It typically comprises two steps: 1)retrieval of inks from the fabrication substrate and 2)printing of inks onto the receiver substrate.In the retrieval process,the adhesion between the stamp and the inks should be stronger than that between the inks and the fabrication substrate.In the printing process,the adhesion between the stamp and the inks should be mediated to be weaker than that between the inks and the receiver substrate.Consequently,the key for a successful transfer printing is the switch of the stamp/ink adhesion between strong state and weak state.In the natural world,the aphid modulates its adhesion to a surface by changing the topography of its pads hence the contact area with the surface.Inspired by the aphid,we developed a novel magnet-controlled transfer printing technique based on tunable interfacial adhesion in this thesis.The bio-inspired,magnetically-controlled stamp features open reservoirs filled with magnetic particles and encapsulated by a thin surface membrane,which can be deformed in a controlled manner via the magnetic field,thus to tune the adhesion.The sufficient adhesion for retrieval relies on the flat surface membrane while the magnetic field is off.When the magnetic field is on,the particles are magnetized and push the thin surface membrane to bulge around the interface,thereby decreasing the contact with the ink,reducing the interfacial adhesion for printing.In this thesis,extensive experimental and theoretical analyses were conducted to characterize the adhesion,investigate the underlying mechanism,optimize the material and geometrical parameters,and demonstrate the magnet-controlled transfer printing technique.Firstly,we fabricated the magnet-controlled stamp using polydimethylsiloxane(PDMS)through multiple duplicate molding.We also constructed a pull-off test platform based on the tensile tester to characterize the adhesion of the stamp.Pull tests show that the magnet-controlled stamp offers continuous tunability,great switchability(the switchability,i.e.,the ratio between the maximum adhesion strength and the minimum adhesion strength,can reach infinity)and high repeatability(the adhesion strength remains constant during a 50-time cycle)as well as easy and fast detachment characteristics(? 100 ms for the detachment of a 4-inch silicon wafer).Under certain conditions,the adhesion strength can be reduced below zero,which enables the noncontact printing.Then,we established a two-dimensional mechanics model based on the energy method to investigate the stamp configuration evolution during the pulling process.The delamination of the stamp from the ink hence the theoretical relation between the interfacial adhesion strength and the magnetic pressure as well as the material properties and geometric parameters is derived and the analytical predictions agree well with experiments.The results show that the adhesion strength decreases linearly with the increase of the magnetic pressure.Under elevated magnetic pressure,the adhesion strength becomes negative,which indicates that the total force exerted on the ink by the stamp is a pressure when the delamination begins,thus the non-contact mode printing can be realized.From this,we obtained the zero-adhesion-strength criterion for non-contact printing.Further,the influences of material properties and geometrical parameters on the adhesion strength were fully investigated.These results provide the theoretical guidelines for the understanding of the underlying mechanism and the optimization of the magnetcontrolled transfer printing.Finally,we further demonstrated the application of this protocol in contact and noncontact transfer printing of silicon wafers in air and in a vacuum.Utilizing high-accuracy linear,tip/tilt and rotational stages,and the Lab VIEW controlling software,we constructed an automatic platform for transfer printing.With a localized magnetic field integrated in the automatic platform,selective and programmable transfer printing was also realized,which shows the great potential of this transfer printing method.The work in this thesis provides a novel method for transfer printing.This simple yet robust bio-inspired,magnet-controlled stamp with rapidly tunable and highly reversible adhesion strength is helpful to advance the application of the transfer printing technique and the development of flexible and stretchable electronics.