Medical microrobots have been widely studied for biomedical applications, including targeted drug delivery, biopsy, hyperthermia radioactive therapy, scaffolding, in-vivo ablation, stenting, sensing, and marking. These applications can be performed by magnetically activated microrobots which provide accurate, minimally-invasive and localized operations. DEMRC has been striving to develop such medical microrobotic systems. The microrobotic system consists of microrobots, magnetic actuation, and imaging. The microrobots are fabricated by a novel method called photolithography [1, 2]. Fig.1[a] illustrates scaffold-type microrobots fabricated for transporting cell and drug. Fig.1[b] shows world’s first ciliary stroke motion microrobot pioneered by our research center.
Fig.1 A) Scaffold-type microrobots fabricated for transporting drug and cell , B) World’s first ciliary stroke motion microrobot developed at DEMRC (Scale bar: 100um) .
The magnetic field generating (MFG) system is developed for actuating the microrobot. The MFG generates a wide range of static and dynamic magnetic field. The versatile MFG system produces gradient field, rotating field, and stepping field. It, also generate the magnetic intensity from 20mT to 100mT. The other is two- and three-dimensional medical filming, remote robots for minimal exposure to radioactivity and more minimized pain of chronic arterial occlusion treatment.
Fig.2 A) Middle-type magnetic field generating system for microrobot control [3, 4]. B) X-ray and master-slave systems for imaging and remote control.
- Sangwon Kim, Famin Qiu, Samhwan Kim, Ali Ghanbari, Cheil Moon, Li Zhang, Bradley J. Nelson, and Hongsoo Choi, “Fabrication and Characterization of Magnetic Microrobots for Three-dimensional Cell Culture and Targeted Transportation,” Advanced Materials, Vol. 25, Issue 41, pp. 5863–5868, Nov. 2013. (Featured as a front cover page, Introduced at TVs and newspapers) (IF= 17.493 in 2014)
-  Sangwon Kim, Seungmin Lee, Jeonghun Lee, Li Zhang, Bradley J. Nelson, and Hongsoo Choi, “Fabrication and Manipulation of Ciliary Microrobots with Non-reciprocal Magnetic Actuation,” Scientific Reports, Vol. 6, No. 30713 (9pp), Jul. 2016. (Introduced at TVs and newspapers) (IF= 5.525 in 2014)
-  Ali Ghanbari, Pyung Hun Chang, Bradley J. Nelson, Hongsoo Choi, “Electromagnetic steering of a magnetic cylindrical microrobot using optical feedback closed-loop control,” International Journal of Optomechatronics, Vol. 8, Issue 2, pp. 129-145, May 2014. (IF= 0.478 in 2014)
-  Ali Ghanbari, Pyung H Chang, Bradley J Nelson and Hongsoo Choi, “Magnetic actuation of a cylindrical microrobot using time-delay-estimation closed-loop control: Modeling and experiments,” Smart Materials and Structures, Vol. 23, No. 3, 035013, Mar. 2014. (IF= 2.502 in 2014)
Ultrasonic transducer is the most important component for the development of an ultrasound system. Ultrasonic transducers are mainly divided into two groups by their fabrication method. One is bulk ceramic transducer, fabricated by mechanical dicing with diamond saw blades, and the other one is micromachined ultrasonic transducer (MUT) using micro electro mechanical system (MEMS) technologies. MUTs have been investigated as an alternative to conventional piezocomposite ultrasonic transducers, primarily due to the advantages that microelectromechanical systems provide. Miniaturized ultrasonic systems require ultrasonic transducers integrated with complementary metal-oxide-semiconductor circuits; hence, piezoelectric MUTs (pMUTs) have been developed as the most favorable solutions. We are developing the next generation ultrasonic transducer using advanced piezoelectric MEMS technologies[1-3]. The application we are aiming to is the neuronal stimulator using pMUT array. Using ultrasound, we can less invasively transfer energy into the body and stimulate the function of the brain without damages. Fig. 1 shows the fabricated annular array to stimulate the deep brain. Fig. 2 shows the cell stimulation system to stimulate neural cells (PC-12) spontaneously.
Figure 1. (Left) Optical image of segmented pMUT annular array and (Right) measured ultrasound intensity at its resonance frequency of 1.5 MHz .
Figure 2. Schematic view of the cell-stimulation system using the piezoelectric micromachined ultrasonic transducer (pMUT) and a Transwell. (Right) fluorescence images of PC12 cells with Ki-67 and 4’,6-diamidino-2-phenylindole (DAPI) for the control group and 5 min and 15% ultrasonic stimulation parameters .
The another application using pMUT is ultrasonic fingervein sensor for biometric security system. The conventional optical based fingervein sensors require a large amount of power and it consume a high cost due to complexity of the system. To overcome these issues, we are developing a new approaches using pMUT. pMUT has unique advantage over conventional ultrasonic transducers for fingervein recognition system such as low power requirement, small in size, and one chip fabrication techniques with CMOS circuitry. For the fingervein recognition, the frequency of ultrasound has to be high (<10 MHz) to enable to reach the high resolution fingervein images. We are developing the fabrication process for high frequency pMUT with CMOS bonding techniques. Fig. 3 shows the schematic view of finger vein recognition system using pMUT arrays. The pMUT is bonded with application specific integrated circuit (ASIC) chip for signal processing and readout.
Figure 3. schematic view of ultrasonic fingervein recognition system using pMUT arrays.
-  Joontaek Jung, Sangwon Kim, Wonjun Lee, and Hongsoo Choi, “Fabrication of a Two-Dimensional Piezoelectric Micro Machined Ultrasonic Transducer Array Using a Top-Crossover-to-Bottom Structure and Metal Bridge Connections,” Journal of Micromechanics and Microengineering, Vol. 23, No. 12, 125037, Dec. 2013. (IF= 1.731 in 2014)
-  Joontaek Jung, Wonjun Lee, Woojin Kang, Hyeryung Hong, Hi Yuen Song, Inn-yeal Oh, Chul Soon Park, Hongsoo Choi, “A top-crossover-to-bottom addressed segmented annular arrays using piezoelectric micromachined ultrasonic transducers,” Journal of Micromechanics and Microengineering, Vol. 25, 115024 (10pp), Aug. 2015. (IF= 1.731 in 2014)
-  Joontaek Jung, Venkateswarlu Annapureddy, Geon-Tae Hwang, Youngsup Song, Wonjun Lee, Woojin Kang, Jungho Ryu, and Hongsoo Choi, “31-mode piezoelectric micromachined ultrasonic transducer with PZT thick film by granule spraying in vacuum process” Applied Physics Letters, Vol. 110, 212903 (5pp), May 2017. (IF= 3.412 in 2015)
-  Wonjun Lee, Samhwan Kim, Joontaek Jung, Woojin Kang, Eunjung Shin, Cheil Moon, Hongsoo Choi, “Mechanosensitive Channel Stimulation System using Low-Intensity Ultrasound by Piezoelectric Micromachined Ultrasonic Transducer Array,” IEEE International Ultrasonics Symposium (IEEE IUS 2016), France, September 2016.
Patients with sensorineural hearing loss can restore their hearing by using a cochlear implant (CI). However, there is a need to develop next-generation CI to overcome the limitations of the conventional CIs that caused by extracorporeal device. Recently, artificial basilar membranes (ABMs) have been actively studied for the next-generation CI. The ABM is an acoustic transducer that mimic mechanical frequency selectivity of the basilar membrane and acoustic-to-electric energy conversion of hair cells.
Figure 1. Schematic drawings of the conventional cochlear implant (left) and the next-generation cochlear implant
Various types of ABM have been developed in BMR group [1-5]. Beam or cantilever arrays were fabricated for clear frequency selectivity. Also, the piezoelectric or triboelectric effect were used as transduction mechanism for self-powered devices. To demonstrate the potential of ABM as a next generation CI, we measured the electrically-evoked auditory brainstem response from deafened guinea pigs.
Figure 2. Various types of the artificial basilar membrane (ABM). (a) piezoelectric AlN beam array  (b) piezoelectric AlN cantilever array  (c) SU-8 based flexible ABM  (d) triboelectric-based ABM 
Figure 3. Schematics of the experimental set-up to measure auditory brainstem response using the ABM .
-  Sangwon Kim, Won Joon Song, Jongmoon Jang, JeongHun Jang, and Hongsoo Choi, “Mechanical frequency selectivity of an artificial basilar membrane using a beam array with narrow supports,” Journal of Micromechanics and Microengineering, Vol. 23, No. 9, 095018, Sep. 2013. (IF= 1.731 in 2014)
-  Jongmoon Jang, Sangwon Kim, David J. Sly, Stephen J. O’leary, Hongsoo Choi, “MEMS Piezoelectric Artificial Basilar Membrane with Passive Frequency Selectivity for Short Pulse Width Signal Modulation,” Sensors and Actuators A: Physical, Vol. 203, pp. 6–10, Dec. 2013. (IF= 1.903 in 2014)
-  Jongmoon Jang, Jangwoo Lee, Seongyong Woo, David J. Sly, Luke J. Campbell, Jin-Ho Cho, Stephen J. O’Leary, Min-Hyun Park, Sungmin Han, Ji-Wong Choi, Jeong Hun Jang and Hongsoo Choi, “A microelectromechanical system artificial basilar membrane based on a piezoelectric cantilever array and its characterization using an animal model,” Scientific Reports, Vol.5, No. 12447 (13pp), Jul. 2015. (IF= 5.578 in 2014)
-  Jongmoon Jang, Jang Woo Lee, Jeong Hun Jang, and Hongsoo Choi, “A triboelectric-based artificial basilar membrane to mimic cochlear tonotopy,” Advanced Healthcare Materials, Oct. 2016. (Featured as a front cover page, Introduced at TVs and newspapers) (IF= 5.760 in 2014)
-  Jongmoon Jang, Jeong Hun Jang, and Hongsoo Choi, “MEMS flexible artificial basilar membrane fabricated from piezoelectric aluminum nitride on an SU-8 substrate” Journal of Micromechanics and Microengineering, Vol. 27, No. 7, 075006 (10pp), Jun. 2017. (IF= 1.768 in 2015)