Sensors and Actuators A 216 (2014) 257–265 Contents lists available at ScienceDirect Sensors and Actuators A: Physical j ourna l h o mepage: www.elsevier.com/locate/sna In vitro and in vivo characterization of SU-8 flexible neuroprobe: From mechanical properties to electrophysiological recording Shun-Ho Huang a , Shu-Ping Lin b,c , Jia-Jin Jason Chen a,d,e,∗ a Department of Biomedical Engineering, National Cheng Kung University, Tainan, Taiwan b Graduate Institute of Biomedical Engineering, National Chung Hsing University, Taichung, Taiwan c Biomedical Engineering and System Bioinformatics Research Center, Kaohsiung Medical University, Taiwan d Medical Device Innovation Center (MDIC), National Cheng Kung University, Tainan, Taiwan e National Applied Research Laboratories, Taipei 106, Taiwan a r t i c l e i n f o Article history: Received 3 March 2014 Received in revised form 12 May 2014 Accepted 6 June 2014 Available online 14 June 2014 Keywords: Neural implant Flexible neuroprobe SU-8 Biocompatibility Tissue reaction a b s t r a c t Flexible neuroprobe with better tissue compliance exhibits unique mechanical characteristics in main- taining stability of neural implant in vivo. In this study, a flexible neuroprobe using SU-8 was designed and fabricated for in vitro and in vivo electrical sensing to show the improved tissue compatibility com- pared to that of the traditional rigid neuroprobe. The validation of neuroprobe was achieved by in vitro mechanical and cytotoxicity tests as well as in vivo neural recording and immunohistological staining. The fabrication process consisted of the creation of a backbone structure using photolithography, pho- topatterning of evaporated metal, and insulating of the electrode trace. The results of mechanical test of our fabricated SU-8 neuroprobe showed four times of physical stress (18.77 mN) than the insertion force (4.69 mN) to sustain resistance from brain tissue during implantation. The in vitro cytotoxicity assay showed well neuronal survival and proved the sufficient surface biocompatibility of the SU-8 neuroprobe. Further in vivo immunohistological staining showed no obvious glia aggregation around the implanta- tion site indicating suitable biocompatibility compared with that of a rigid neuroprobe. Our in vitro and in vivo studies showed SU-8 neuroprobe possessed enough stress to complete the implantation in brain tissue and remained flexibility to comply micromovement of soft tissue with minor immune responses to achieve in vivo electrophysiological recordings at a signal-noise-ratio of greater than 7. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The tissue compliance of an implanted neuroprobe is a criti- cal issue for the short-, long-term stability and functionality of electrophysiological recording [1,2]. For probe strength or capac- ity of standard silicon fabrication process, implantable neuroprobe, such as the Utah microelectrode array and the Michigan probe, are made of rigid silicon-based materials, and could thus cause severe damage during inserting into the brain tissue [3]. Research has shown that the mechanical mismatch between a rigid silicon probe (whose Young’s modulus is about 170 GPa) and soft brain tis- sue (whose Young’s modulus is about 10 kPa) can induce chronic ∗ Corresponding author at: Department of Biomedical Engineering, National Cheng Kung University, No. 1 University Road, Tainan, Taiwan. Tel.: +886 6 2757575x63423; fax: +886 6 2343270. E-mail address: jasonbiolab@gmail.com (J.-J.J. Chen). inflammation around the implantation site [4,5]. Furthermore, the changes in the mechanical properties of the surrounding envi- ronment cause specific tissue responses [6–8]. A glial sheath proliferated from glial cells such as astrocytes and microglia could encapsulate the probes and eventually isolate the electrodes from surrounding neurons after implantation [9,10]. Polymer materials with two ranges of mechanical properties have been used for fab- ricating flexible neuroprobe which include polydimethylsiloxane (PDMS) [11] with Young’s modulus of 1–10 MPa and polyimide (PI) [12], parylene [13] and SU-8 [38] with the range of 1–10 GPa. These polymer materials have better tissue compliance compared to the rigid material like silicon or glass with Young’s modulus ≥10 GPa [4,5]. In contrast with a silicon-based neuroprobe, a flexible PI probe can minimize the tissue response due to its lower strain force at the probe tip, as observed in finite element modeling [14] and in chronic tissue inflammation assay of animal study [15]. Flexible materials such as PI [16,17], parylene [18], and SU- 8 [19–22] have been adopted for the fabrication of neuroprobe http://dx.doi.org/10.1016/j.sna.2014.06.005 0924-4247/© 2014 Elsevier B.V. All rights reserved.