Table 1.

Various Types of 3D MEAs for in vitro and in vivo applications

Type of 3D MEA		Fabrication Method	Materials	Geometry / Layout / Features	Ref
In vitro	Cardiac	Photolithography Ion beam etching	Multilayered Ti–Pt–Ti–Au–Ti–SiO 2-coated glass substrate	2 μm-wide in diameter of nanoring 10- to 20-nm-thick Au nanoring Multilayered wall of the nanovolcano	(Desbiol les et al., 2019)
		Photolithography	TiN, Au, SU-8	Free standing/ flexible/ mesh 32 electrodes (50 × 50 μm2) Recording/stimulating with same electrodes Folded device	(Feiner et al. 2016)
		3D printing	Carbon black, TPU	Cantilever geometry Stretching a soft strain gauge embedded in the cantilever	(Lind et al., 2017)
	Neural	Photolithography/ Wet / dry etching	Ti / Au / Ti on polyimide	20 nm Ti / 250 nm Au / 20 nm Ti in thickness 50 μm diameter individual electrodes Flexible polyimide probes of length 1100 μm and width 90 μm contain eight 50 μm diameter individual electrodes Probe would bend during actuation	(Soscia et al., 2020)
		Photolithography/ E-beam lithography	Ti / Ni / Ti / Ni	30 / 200 / 50 / 200 nm (adhesion/conduction/diffusionbarrier/silicidation) Nanowire geometries Independent electrical addressability	(Liu et al., 2017)
		Photolithography Wet / dry etching E-beam lithography Electrodeposition	PtB, Ti, Pt,	20 nm Ti, 200 nm Pt PtB deposition on the Pt electrodes 4,096 platinum-black nanowire electrodes array	(Abbott et al., 2020)
In vivo	Heart	Photopatteming	PVA-coated Au gel electrodes on polyrotaxane matrix	12 × 12 transistors Polymer thickness: 1.4 mm 3D wrinkles for strain absorption (> 100 %) Adhesive to biological surfaces	(Lee et al., 2014)
		Micromachining/ Photoetching	Au microneedles Thin PDMS membrane	Barb width: 790 – 1170 mm Shaft width: 230 – 480 mm Height: ~ 1.5 mm Microneedles with 3D arrowhead shapes Stretchability up to ~ 63% strain	(Guvana sen et al., 2017)
		Galvanic-free epitaxial deposition	Ag–Au core–sheath nanowire composite mesh	Nanowire diameter: 140 nm Nanowire thickness: 35 nm Highly stretchable (> 800 % strain) Electrically stable to strains (> 100%)	(Choi et al., 2018)
	Brain	Photolithography Electroplating	Pt-coated Au-TiO2 nanowires	Electrode count: 32 Electrode size: ~ 3000 mm2 Highly soft and stretchable to enable craniotomies smaller than the probe's size	(Tybran dt et al. 2018)
		Vapor deposition Electroplating	Cr/Au electrode; PEDOT:PSS/Pt-black wrinkles	Wrinkle wavelength 11.2 - 35.7 mm Enhanced interfacial surface area Stable ECoG recording and optical stimulation	(Ji et al., 2019)
		Photolithography	SU-8 insulation Pt/Au electrodes	30 mm × 30 mm/ 10 mm × 20 mm Ultraflexibility to conform with brain tissue Stable performances in mice for extended time	(Luan et al., 2017)
		Electron-beam lithography	Au electrodes SU-8 insulation	Au electrode: 100 nm Total thickness: ~ 1 mm Careful handling for stable chronical recording and nondegrading tissue–probe interface	(Wei et al., 2018)
		Photolithography	Pt electrode Au interconnect Top/bottom SU-8 layers	Width: 2 mm Electrode Diameter: 10-20 mm Unit cell: 333 × 125 mm Match the subcellular feature sizes and mechanical properties of neurons	(Yang et al., 2019)