Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

Hongyu Luo , Chenglong Li , Chunqin Shi , Shung Nie , Jizhou Song , b , *

a Department of Engineering Mechanics, Soft Matter Research Center, and Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Hangzhou 310027, China

b State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou 310027, China

Keywords: Switchable dry adhesive Shape memory polymer Transfer printing

ABSTRACT Transfer printing based on switchable adhesive is essential for developing unconventional systems, in- cluding flexible electronics, stretchable electronics, and micro light-emitting diode (LED) displays.Here we report a design of switchable dry adhesive based on shape memory polymer (SMP) with hemispher- ical indenters, which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP.Experimental and numerical studies reveal the fundamental aspects of design, fabrication, and operation of the switchable dry adhesive.Demonstrations of this adhesive concept in transfer printing of flat objects (e.g., silicon wafers), three-dimensional (3D) objects (e.g., stainless steel balls), and rough objects (e.g., frosted glasses) in two-dimensional (2D) or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

Transfer printing has attracted much attention from both academia and industry due to its unique capability of heteroge- neous material integration in parallel or individual ways with ap- plications in developing existing and envisioned electronic systems such as flexible electronics [1–4] , stretchable electronics [5–8] , and micro light-emitting diode (LED) displays [ 9 , 10 ].The success of transfer printing critically replies on the ability of adhesion switch between strong for pick-up from the donor substrate and weak for printing to the receiver substrate.Compared to surface chem- istry assisted adhesives (e.g., water soluble tape, thermal releasable tape, etc.) without reversibility, switchable dry adhesives based on van der Waals force with high reversibility are more attractive for transfer printing.Kinetic approaches based on viscoelasticity [ 11 , 12 ] to realize the adhesion switch are valuable, but the ad- hesion switchability is often smaller than desired.Strategies based on structure designs, which are usually bio-inspired, by controlling the contact area [13–16] , the surface topography [17–19] or the air pressure [ 20 , 21 ] have provided a wide range of strong adhesion to weak adhesion and improved the efficiency of transfer print- ing significantly.Recently, shape memory polymers (SMPs) with properties of memorizing temporary shapes (low storage modulus) and fully recovering to their original shape (high storage modulus) [22–24] upon external stimuli have been used to develop switch- able adhesives, which provide simple but efficient ways for adhe- sion switch through micropillars [25] , microtips [26–29] or modu- lus change [30] .However, achieving suitably weak adhesion based on shape memory polymers during printing still remains a chal- lenge due to the difficult control of interfacial contact surface.

In this paper, we report an alternative design of switchable dry adhesive based on a thermal responsive shape memory poly- mer featuring surface hemispherical indenters.The switchable ad- hesive can be easily fabricated by the simple molding processes and offers an easy control on the interfacial contact by combin- ing the preloading effect and the thermal actuation of SMP, thus yielding a continuously tunable and reversible adhesion.Experi- mental and numerical studies are carried out to understand the underlying adhesion switch mechanism from the strong adhesion state to the weak adhesion state.We demonstrate this switch dry adhesive in transfer printing different objects, including flat sili- con wafers, three-dimensional (3D) stainless steel balls and frosted glasses, into various configurations in two-dimensional (2D) or 3D layouts to illustrate its robust capability of manipulation with great potential in deterministic assembly.

Figure 1 illustrates the transfer printing process enabled by the switchable dry adhesive.The adhesive is made of a thermal re- sponsive shape memory polymer with hemispherical indenters on the surface, whose deformation can be controlled by the preload- ing combined with the thermal actuation of SMP.Initially, the ad- hesive is moved over the donor substrate with inks (e.g., electronic components) prepared on it (Fig.1 a).After heating the adhesive above the glass transition temperature, press the heated adhesive in the low modulus state onto the target ink with the indenters partially flattened (Fig.1 b).The cooling of the adhesive to room temperature fixes the deformed indenters in the high modulus state and ensures a relatively large conformal contact between the adhesive and the ink (Fig.1 c), thus yielding a high interfacial ad- hesion for pick-up.The inked adhesive is then retracted from the donor substrate, and moved to the top of the receiver substrate (Fig.1 d).The reheating of the adhesive above the glass transition temperature restores the indenters to the original hemispherical shape with only point contacts to the ink, thus yielding a very low interfacial adhesion (Fig.1 e) to release the ink onto the receiver substrate (Fig.1 f).

The switchable dry adhesive can be prepared through a se- ries of molding processes, as illustrated in Fig.2 .A positive alu- minum alloy mold is firstly obtained through computer numeri- cal control (CNC) machining methods (Fig.2 , I).Pouring the liq- uid polydimethylsiloxane (PDMS, Dow Corning Sylgard 184) mix- ture with the monomer: cross-linking agent ratio of 10:1 into the aluminum alloy mold followed by degassing in a vacuum cham- ber for 30 min and curing in an oven at 75 °C for 4 h yields the negative PDMS mold (Fig.2 , II and III).After demolding, the PDMS mold is exposed to an UV/Ozone environment for 2 hours to form a non-stick oxide layer on the surface (Fig.2 , IV).We adopt a kind of epoxy shape memory polymer [24] whose glass transi- tion temperature is around 50 °C and fracture strain is over 75% with a high shape fixation ratio of over 99%.Pouring the liquid epoxy SMP with the monomer E44 (China Feicheng Deyuan Chem- ical Corp.) to the curing agent Jeffamine D230 (Sigma-Aldrich) ra- tio of 81:46 into the UV/Ozone treated positive PDMS mold fol- lowed by curing in an oven at 100 °C for 2 h (Fig.2 , V) and demolding (Fig.2 , VI) completes the preparation of the switch- able dry adhesive.It should be noted that the above fabrication can be easily scaled up or down by using an appropriate positive mold.

Figure 3 a shows an optical image of the switchable dry adhe- sive (45 mm ×45 mm ×8 mm) with 25 hemispherical indenters (diameter: 4 mm and spacing: 8 mm).Figure 3 b shows the ad- hesion on state with the indenters partially flattened by pressing the adhesive at a temperature larger than the glass transition tem- perature followed by cooling it to room temperature (smaller than the glass transition temperature).The partially flattened indenters increase the interfacial contact area and yields a relatively strong adhesion.Figure 3 c shows the adhesion offstate with the inden- ters fully recovering back to the hemispherical shape by heating the adhesive over the glass transition temperature.The fully recov- ered indenters reduce the interfacial contact area dramatically and yields a very weak adhesion.Figure 3 d shows the storage modulus and loss factor of epoxy SMP as functions of temperature from 30 °C to 130 °C.It is shown that the storage modulus varies from 700 MPa to 0.318 MPa and the glass transition temperature of SMP is around 50 °C.

Vertical pull tests are carried out under various preloads to measure the adhesion of the adhesive/glass interface.A typical force-displacement curve with the preload of 25 N is shown in Fig.3 e.At first, heat the adhesive to 130 °C for 10 min to ensure the adhesive to be fully heated.The adhesive then approaches to the glass substrate at a speed of 50μm/s until the preload reaches the preset value followed by cooling the adhesive to room temper- ature of 25 °C with the preload remaining unchanged.The retrac- tion of the adhesive at a speed of 500μm/s gives a large pull-off force, which corresponds to the strong adhesion under the adhe- sion on state.The same procedure without the cooling process can be applied to measure the pull-offforce (or the adhesion) under the adhesion offstate.A three-dimensional finite element model is established to assess the deformation of adhesive with the ad- hesive modeled as an elastic body (modulus: 0.318 MPa and Pois- son’s ratio: 0.38) and the glass substrate as an ideal rigid body.For the maximum preload of 25 N used in this paper, the maximum principal strain of the adhesive is 23.9% (Fig.3 f), which is far less than the fracture strain 75 % of SMP.

Figure 3 g presents the adhesions under the adhesion on and the adhesion offstates as functions of preload from 0.6 N to 25 N.The adhesion under the adhesion on state increases dramati- cally from 0.33 N to 14.21 N while that under the adhesion off state increases only slightly from 0.59 N to 0.96 N.The reason is that the shape fixing under the adhesion on state involves a large modulus increase, which contributes significantly to the adhesion increase.In addition to the preload, the temperature also affects the adhesion through the control of the interfacial contact area due to the temperature-dependent modulus (Fig.3 d).Figure 3 h shows the relative contact area of the real contact area to the projected area of hemispherical indenter as function of temperature from 50 °C to 130 °C under the preload of 25 N.The relative contact area increases as the increase of temperature due to the decrease of the storage modulus of SMP.Since the storage modulus of SMP changes very little after the temperature exceeds 100 °C, the rel- ative contact area reaches the maximum value of 53% and remains unchanged with the further increase of temperature.

The excellent adhesion regulation ability of the proposed dry adhesive could enable a high efficient transfer printing.Figure 4 a demonstrates the transfer printing of a 4-inch silicon wafer in a non-contact mode.At first, move the adhesive to the top of the wafer (Fig.4 a, I), then heat the adhesive over the glass transition temperature by a hot air gun (Fig.4 a, II).After the adhesive is fully heated, press the adhesive on the silicon wafer until the hemi- spherical indenters are partially flattened (Fig.4 a, III).Then retract the adhesive to pick up the wafer after it is cooled to room temper- ature with the partially flattened shape fixed (Fig.4 a, IV).Reheat the adhesive over the glass transition temperature to complete the printing of wafer with partially flattened indenters fully recover- ing back to the hemispherical shape (Fig.4 a, V and VI).The large modulus decrease of adhesive after heated over the glass transi- tion temperature enables its conformal contact not only to flat ob- jects but also to 3D objects, thus making the adhesive applicable to transfer print objects of different shapes.Figure 4 b schemati- cally shows the transfer printing process of a 3D ball.The ball can be easily embedded into the heated soft adhesive with a large con- formal contact (Fig.4 b, I) and locked by the cooled hard adhesive (Fig 4 b, II), which ensures an efficient pick up.The reheating of adhesive over the glass transition temperature recovers the shape of hemispherical indenters and pushes the ball downward to the donor substrate (Fig.4 b III).To demonstrate the unusual capabil- ity of manipulating 3D objects, 25 stainless steel balls with the diameter of 4 mm are transfer printed onto an acrylic substrate with 25 holes with the diameter of 3 mm (Fig.4 c).Besides the robust transfer printing in 2D layout, the proposed dry adhesive also works well for 3D deterministic assembly.Figure 4 d shows silicon slices (25 mm ×25 mm ×0.7 mm) stacked to a pyramid shape on a glass substrate enabled by the switchable dry adhesive.Figure 4 e shows four pieces of frosted glass (25 mm ×25 mm ×1 mm) leaning against a square glass substrate.The above examples demonstrate the excellent performance of the proposed switchable dry adhesive for manipulating objects with various shapes to 2D and 3D layouts.

In summary, we report a switchable dry adhesive based on shape memory polymers with hemispherical indenters, whose shape and modulus can be controlled by combining the preload- ing effect and the thermal actuation of SMP.Systematic experimen- tal and numerical studies show that the adhesive offers a continu- ously tunable and reversible adhesion due to the change of inter- facial contact area.Demonstrations of the adhesive in determinis- tic assembly of flat objects (e.g., silicon wafers), 3D objects (e.g., stainless steel balls), and rough objects (e.g., frosted glass) in 2D or 3D layouts illustrate the unusual manipulation capabilities with great potential in heterogeneous material integration applications to develop unconventional electronics such as flexible electronics, stretchable electronics, and micro LED displays.

Declaration of Competing Interest

The authors declare that they have no known competing finan- cial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the supports of the National Natural Science Foundation of China (Grant Nos.11872331 and U20A6001) and Zhejiang University K.P.Chao’s High Technology Development Foundation.