A Prototype of Thermally Actuated Microscale Scissorswith a Sharpened Blade

TANGE Manabu∗, KUSUDA Shu∗, TOBA Masaki∗,ABE Seiichiro†, HARAGUCHI Kohei‡, and YUSA Tomoaki§

∗ Department of Mechanical Engineering, Shibaura Institute of Technology, Toyosu, Koto, Tokyo 135–8548† Mitsubishi Materials, ‡ Japan Cashmachine, § Gojo Paper Mfg.

Abstract—This paper presents a prototype of ther-mally actuated microscale scissors. The scissors havea similar structure to micro-grippers except that thescissors tip has a sharpened blade. With this blade,the scissors aim to chop soft objects like biologicalsamples while micro-grippers are designed to handleor manipulate them. A movable beam with the bladeare hinged to a base structure while another beamare fixed to the base structure. A disk are hingedboth to the base structure and to the bottom of themovable beam. The disk expands when heated, andthe disk pushes and slants the movable beam to closethe scissors. The scissors were made of single layeredSU-8 and fabricated with an ordinal photolithographytechnique except for the blade tip. The blade tipwas covered with the additional mask to prevent thefirst exposure, and then it was exposed in inclinedUV light. The scissors were actuated by heating itsexpanding disk with a focused IR laser. Experimentalresults showed that higher laser output results inlarger scissors’ displacement. In the cutting trial of atofu film, the microscale scissors made a sharp crackon the surface of the tofu film.

1. Introduction

Micro-grippers and micro-tweezers consist of pairs ofmoving beams or arms for handling and grasping tinyobjects. Handling of microscale soft tissue is a key issueof bio manipulation application. There have been a lotof driving principles for micro-grippers, such as pneu-matic actuation [1], electro-thermal actuation [2], photo-thermal actuation [3], piezoelectric actuation [4], bi-morph structure [5], shape memory alloy [1], etc. Amongthese actuation methods, Elbuken et al. [3] employedphoto-thermal actuation for their micro-gripper. Theirmicro-gripper was driven by thermal expansion of a partof the structure heated with a diode laser. On the device,there was no electrical circuits or metallic parts proba-bly harmful for biological objects. On the other hand,cutting of the soft tissue is also important operationfor bioMEMS devices. Some advanced techniques have

been proposed such as microscale surgical knife of siliconnitride/metal [6] and microfluidic control [7].

This paper presents a prototype of thermally actuatedmicroscale scissors. A developed microscale scissors hasa similar structure to micro-gripper and sharpened bladeat a tip of the gripper. A sharpened blade on the tip ofa scissors’ beam was fabricated by inclined lithographymethod. A disk connected to the beam was heated bya IR laser and expanded thermally. The driving experi-ments was investigated to find the relation between laseroutput and scissors’ displacement. Experimental resultsshowed that higher laser output results in larger scissors’displacement and it took over 10 seconds to close thescissors. Cutting test of thin soft film made of tofu wasalso conducted. The closing blade made a crack on thesurface of the tofu film.

2. Experimental System

2.1Microscale Scissors

Fig. 1. Microscale scissors driven by thermal expansion of itsstructure; (1) a holding beam fixed to a base structure, (2) amovable beam with a blade, and (3) a expanding disk.

Figure 1 is a microscope photo of microscale scissorsillustrating its strucuture. The scissors were made ofsingle layered SU-8 100 μm thick. The structure of thescissors consists of three parts connected to each other:(1) a holding beam fixed to a base structure, (2) amovable beam with a blade, and (3) a expanding disk,

as numbered in Fig. 1. The movable beam with theblade is connected to the base structure with neckedhinge. The expanding disk is connected both to the basestructure and to the bottom of the movable beam. Thediameter of the disk is 2 mm. The bridges connecting thedisk and other structure are necked to limit heat escapefrom the disk to the base structure. The disk expandswhen heated, and the expansion of it pushes and slantsthe movable beam to close the scissors. The disk waspainted black with a colored adhesive paste (KE-3418)to increase heat absorption.

Fig. 2. Schematics of an inclined lithography method.

SU-8 is a UV curable material and an ordinal lithog-raphy process makes only two dimensional patten. Fab-rication of sharpened blades needs special lithographytechnique for three dimensional structure. Onishi et al.[8] developed a lithography method to make three dimen-sional structures by diffraction at edges of a photo maskpattern. There is some other method to make a slopeof SU-8, such as grayscale lithography [9] and inclinedlithography [10]. This study employed 2-step lithographyprocess: perpendicular lithography and inclined lithogra-phy. First, SU-8 spin-coated on a copper thin plate waspatterned by perpendicular UV light with a patterningmask and a covering mask. The blade tip was shieldedfrom UV light with the covering mask to prevent theexposure. And then the blade tip was exposed in inclinedUV light while the rest of the structure was coveredfrom UV as shown in Fig. 2. The incident UV angleis 60 degree. In the inclined lithography process, thescissors were immersed in silicon oil to reduce refractionof UV light at the surface of the SU-8 which blunts theblade. After the lithography process, the copper plateunder the scissors was removed with etching. Figure 3illustrates the shape of the blade sharpened with thisinclined lithography. Blade angle was 39.7 degree and itagrees with the estimated value calculated from refrac-tion index.

Fig. 3. Sharpened blade fabricated by inlined lithography method.

2.2Experimental Apparatus and Procedure

Fig. 4. Experimental system.

The scissors were actuated by heating its expandingdisk with a focused IR laser as shown in Fig. 4. Theexperimental system consists of an IR laser, an concavemirror, a microscale scissors supported with adjustabletable, and an observing camera. An IR laser diode (wavelength of 808 nm) was employed as a heat source. Pulsewidth modulation of the laser output can change theaverage power of the laser. The laser power was measuredby a power checker and estimated at 50 mW when aduty ratio of PWM was 10 percent. The laser poweris assumed to be proportional to the duty ratio. Theconcave mirror focused the laser to the expanding diskof the microscale scissors and the laser diameter wasfocused to 2 mm by adjusting the distance between themirror and the scissors. The movement of the scissorswere recorded by a CMOS camera with a microscope

lens behind an IR cut filter. An fiber light was employedto illuminate the scissors.

Driving tests of the microscale scissors were conductedwith various laser output to investigate the relationbetween heating power and remaining clearance. Heatingtime was fixed to 10 seconds and the laser power wasranged from 50 mW to 200 mW. After the each laserheating, the scissors was cooled by natural convection ofair for enough time (about 1 minute) to reach room tem-perature. The gap between the blade and the fixed beamwas measured from the images recorded by the CMOScamera. Cutting test were also examined to evaluate thesharpness of the blade. As a material of the test piece, athin tofu film was employed. the tofu film was made bymixing soy milk and bittern and supported by a U-shapemetal plate for handling. Thickness of the tofu film wasabout 120-140 μm. The film was inserted into the gap ofthe scissors and then the blade was actuated.

3. Results and Discussion

0 2 4 6 8 10 12 14 16 18 20Time, t [s]

−300

−250

−200

−150

−100

−50

0

Off

set

Dis

plac

emen

t,δ(t)−δ(

0)[μ

m]

50 mW100 mW150 mW200 mW

Fig. 5. Displacement of a tip of a movable beam.

Figure 5 shows displacements of the scissor’s tip ofa movable beam, δ(t), under the various laser power,Qlaser. Unfortunately, the scissor’s tip did not recoverits initial position after each heating as described later.In Fig. 5, each displacement is offset to zero at eachinitial position to extract the displacement only by eachheating. Negative value of the displacement indicatesthat the gap between two beams decreases and thescissors closes. Figure 5 concluded that the larger changeand the faster closing speed resulted from the larger laseroutput. Maximum displacement at 200 mW reached near250 μm. This scissors did not realize complete closing

because more than 200 mW of laser output resultedin the ablation of the disk. Smaller gap and/or largerdisplacement are required to achieve the complete closingof the scissors. Figure 5 shows that the slope of thedisplacement decreased as heating time went on. Thisillustrates the diminishing of the temperature rise of theheated disk due to the heat escape to the environment.Therefore, the displacement might be stopped when theheat input by the laser and the heat escape to theenvironment are balanced. This experiments, however,did not complete the movement during the heating andrevealed that it takes more than 10 seconds to havethermally steady state. Similarly, the recovering of thetip to its original position seems to require over 10seconds.

50 75 100 125 150 175 200Laser Power, Qlaser [mW]

340

360

380

400

420

440

460In

itia

lGap

,δ(0

)[μ

m]

Initial Gap

Fig. 6. Initial gap before each heating.

Figure 6 shows the initial gap between the blade andthe opposite beam before each heating, δ(0). The changeof the initial gap explains the irrecoverable deformationof the scissors’ structure due to each heating, whileapparent deformation of the movable beam was notobserved. This may be explained by plastic deformationof the heated disk. As SU-8 has its glass transitiontemperature and become plastic and deformable overthe temperature, plastic deformation of the disk resultedfrom compression due to thermal expansion and reactiveforce from the movable beam. After heating, the cooleddisk shrunk to less than original dimensions and it pulledand bent the movable beam to open the gap more thanbefore. Some refined structures for large displacementwith relatively low temperature, such as larger disk, arerequired to avoid plastic deformation and to obtain highrepeatability.

A driving test was conducted while a tofu film wasinserted into the gap for an evaluation of the cutting

A: Before cutting. B: During cutting. C: After cutting.

Fig. 7. Sequential snapshots of a cutting test of a Tofu film; (1)a holding beam, and (2) a movable beam.

ability, as sequentially shown in Fig. 7. As Fig. 7-Ashows, dashed lines emphasizes the initial position of(1) the holding beam and (2) the movable beam of thescissors. In the beginning, both beams did not contactthe tofu film. Then the moving blade cut into the tofu(Fig. 7-B). After heating, the blade departed from thetofu (Fig. 7-C). Fig. 7-C shows a clear crack on thesurface of the tofu encircled by a black dashed line.

4. Conclusion

In this paper, a prototype of microscale scissors hasbeen developed. The microscale scissors is a micro-gripper with a sharpened blade fabricated by inclinedlithography. A movable beam with the blade was bentby a thermally expanding disk heated with a IR laser.Experimental results showed that higher laser outputresulted in larger scissors’ displacement. Glass transitionand plastic deformation of the heated disk was not neg-ligible in this microscale scissors design. In the cuttingtrial of a tofu film, the microscale scissors made a sharpcrack on the surface of the tofu film.

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