Stretchable microelectrode arrays (stretchable MEAs or sMEAs) (also referred to as stretchable multielectrode arrays) are a specialized type of microelectrode array (MEA) with a key advantage; they can be deformed, stretched, bent, and twisted while maintaining electrical functionality whereas standard MEAs break upon mechanical loading. Flexible MEAs (flexMEA), which are often confounded with stretchable MEAs, lie in between stretchable MEAs and standard MEA in terms of their mechanical properties because they bend and twist to some degree, but not stretch. Just like traditional MEAs, stretchable MEAs consist of a few thousand microelectrodes that allow recording or stimulation of electrical signals from cells (neurons, muscles, etc.), and are used in vivo in a living being or in vitro with cell cultures.
Theory A stretchable conductor typically consists of two components: an elastomeric insulator and an electrical conductor. There are several approaches to producing stretchable and electrical conducting materials that fall into two categories: structural design and material innovation.
Material innovation Electronic Fillers: This is the oldest approach to making an elastomeric material elastically stretchable. In principle, rigid and electrically conductive materials and mixed with an elastomeric polymer before curing to create stretchable composites. If the concentration of the electrically conductive filler is high enough they form a mesh-like percolation network that facilitates the free movement of charge carriers (ions, electrons) through contact junctions. The minimum concentration of the electronic filler material that is required to create conductive pathways for charge carrier transport through the elastomer is called the percolation threshold. The percolation threshold is usually indicated as weight percentage (wt%) or volume percentage (vol%) of the filler material, and ranges from less than 1wt% for high aspect ration carbon nanotubes to over 15wt%. The type of filler materials ranges from metals in powder or nanowire form, carbon as graphite or nanotubes, to electrically conducting polymers. 'Wavy' Nanowires and Nanoribbons: The spontaneous formation of wavy patterns of aligned buckles that is caused by the deposition of a thin gold film on the surface of the elastomer polydimethylsiloxane (PDMS) was first described by the group of George Whitesides at Harvard University in 2000. The gold was deposited on warmed PDMS (100 °C), and, upon cooling and the associated thermal shrinkage of the elastomer, the gold film comes under compressive stress which is relieved by creating buckles. In subsequent years, the group of John Rogers at the University of Urbana Champaign (now at Northwestern University) has developed the technology to bond very thin silicon ribbons to a pre-stretched PDMS membrane. Upon relaxation of the per-stretch, the compressive mechanical stress in the silicon ribbons is relieved by creating wavy buckles in the PDMS. As silicon is a brittle material, the ribbons need to very thin (about 100 nm) to stay intact during buckling. Liquid Metals: A metal or alloy that is liquid at room temperature can be enclosed in PDMS and used as a stretchable conductor. Mercury is the only pure metal that is liquid at room temperature but has limited application due to its neurotoxicity. Cesium melts at 28.5 °C, but reacts violently when exposed to air and is therefore not suitable for this application. Most researchers therefore use an eutectic mixture of Indium and Gallium, so called EGaIn, which has a melting point is 15.7 °C and consists of 75.5% Gallium and 24.5% Indium. A eutectic mixture of Ga (68.5%), In (21.5%) and Sn (10.0%), also known as Galinstan, is another popular choice and has a melting point of 10.5 °C. Microcracked gold thin film: When a thin gold film is deposited on PDMS under certain conditions, the gold film adopts a microcracked morphology which makes the gold stretchable. The maximum strain of the film decreases with the length and increases with the width of the conductor.
Structural design Geometric patterning, fractal patterns: Metal traces are deposited in specific patterns, such as meandering or serpentine shapes, within a stretchable elastomeric substrate to accommodate strain. The resulting structure is akin to a 2-dimensional spring. The University of Ghent and IMEC in Belgium have pioneered the approach to using Meander shaped metallic structures. The group of John Rogers increased the maximum strain in devices created by this approach using fractal-based structures. These fractal patterns are characterized by self-similarity, i.e., a small sections of the structure yields pieces with geometries that resemble the whole structure. These fractal patterns include (i) Koch, Peano, Hilbert lines, (ii) Moore, Vicsek loops, and (iii) Greek crosses. Origami-inspired structures, and kirigami cuts: Intrinsically rigid or inelastic flexible materials can be turned into stretchable materials by applying origami technology and kirigami cuts.
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