|
==='''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<ref>{{cite journal |last1=Kyrylyuk |first1=Andriy V. |last2=van der Schoot |first2=Paul |title=Continuum percolation of carbon nanotubes in polymeric and colloidal media |journal=Proceedings of the National Academy of Sciences |date=17 June 2008 |volume=105 |issue=24 |pages=8221–8226 |doi=10.1073/pnas.0711449105 }}</ref> is called the [[percolation threshold]].<ref>{{cite book |doi=10.1016/B978-1-895198-95-9.50011-X |chapter=Structure and Distribution of Non-Migrating Antistatics |title=Handbook of Antistatics |date=2016 |pages=117–127 |isbn=978-1-895198-95-9 |editor1-first=Jürgen |editor1-last=Pionteck |editor2-first=George |editor2-last=Wypych }}</ref> 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]] [[PDMS]] was first described by the group of George Whitesides at Harvard University in 2000.<ref>{{cite journal |last1=Huck, |first1=Wilhelm T. S., et|last2=Bowden al|first2=Ned |last3=Onck |first3=Patrick |last4=Pardoen |first4=Thomas |last5=Hutchinson |first5=John W. |last6=Whitesides |first6=George M. "|title=Ordering of Spontaneously Formed Buckles on Planar Surfaces." |journal=Langmuir, vol.|date=April 2000 |volume=16, no. |issue=7, 2000,|pages=3497–3501 pp|doi=10.1021/la991302l 3497-3501.}}</ref> 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 100nm) to stay intact during buckling.<ref>{{cite journal |last1=Kim, Dae-Hyeong,|first1=Dae‐Hyeong and|last2=Rogers |first2=John A. Rogers. "|title=Stretchable Electronics: Materials Strategies and Devices." |journal=Advanced Materials, vol.|date=17 December 2008 |volume=20, no. |issue=24, 2008,|pages=4887–4892 pp|doi=10.1002/adma.200801788 4887-4892.}}</ref>
* '''Liquid Metals''': A [[metal]] or [[alloy]] that is liquid at room temperature can be enclosed in [[PDMS]] and used as a stretchable [[conductor]]. [[Mercury (element)|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,<ref>Stretchable{{cite andjournal Foldable|last1=Graudejus Silicon|first1=Oliver Integrated|last2=Görrn Circuits."|first2=Patrick |last3=Wagner |first3=Sigurd |title=Controlling the Morphology of Gold Films on Poly(dimethylsiloxane) |journal=ACS Publications,Applied AmericanMaterials Chemical& Society,Interfaces pubs.acs.org/|date=28 July 2010 |volume=2 |issue=7 |pages=1927–1933 |doi/abs/=10.1021/am1002537. Accessed 10 Nov. 2024.}}</ref>, the gold film adopts a microcracked morphology<ref>{{cite journal |last1=Lacour, |first1=Stéphanie P., et|last2=Chan al.|first2=Donald "|last3=Wagner |first3=Sigurd |last4=Li |first4=Teng |last5=Suo |first5=Zhigang |title=Mechanisms of Reversiblereversible Stretchabilitystretchability of Thinthin Metalmetal Filmsfilms on Elastomericelastomeric Substrates."substrates |journal=Applied Physics Letters, vol.|date=15 88,May no.2006 |volume=88 |issue=20, 2006, p|doi=10. 2041031063/1.2201874 }}</ref> which makes the gold stretchable. The maximum [[Strain (mechanics)|strain]] of the film decreases with the length and increases with the width of the conductor.<ref>{{cite journal |last1=Graudejus |first1=O. |last2=Jia |first2=Zheng |last3=Li |first3=Teng |last4=Wagner |first4=S. |title=Size-dependent rupture strain of elastically stretchable metal conductors |journal=Scripta Materialia |date=June 2012 |volume=66 |issue=11 |pages=919–922 |doi=10.1016/j.scriptamat.2012.02.034 }}</ref>
==='''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 (mechanics)|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.<ref>{{cite journal |last1=Gonzalez, |first1=Mario, et|last2=Vandevelde al|first2=Bart |last3=Christiaens |first3=Wim |last4=Hsu |first4=Yung-Yu |last5=Iker |first5=François |last6=Bossuyt |first6=Frederick |last7=Vanfleteren |first7=Jan |last8=Sluis |first8=Olaf van der |last9=Timmermans |first9=P.H.M. "|title=Design and Implementationimplementation of Flexibleflexible and Stretchablestretchable Systems."systems |journal=Microelectronics Reliability, vol.|date=June 2011 |volume=51, no. |issue=6, 2011,|pages=1069–1076 pp|doi=10. 1069-10761016/j.microrel.2011.03.012 }}</ref>
</ref>
** 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.<ref>{{cite journal |last1=Fan, |first1=Jonathan A., et|last2=Yeo al|first2=Woon-Hong |last3=Su |first3=Yewang |last4=Hattori |first4=Yoshiaki |last5=Lee |first5=Woosik |last6=Jung |first6=Sung-Young |last7=Zhang |first7=Yihui |last8=Liu |first8=Zhuangjian |last9=Cheng |first9=Huanyu |last10=Falgout |first10=Leo |last11=Bajema |first11=Mike |last12=Coleman |first12=Todd |last13=Gregoire |first13=Dan |last14=Larsen |first14=Ryan J. |last15=Huang |first15=Yonggang |last16=Rogers |first16=John A. "|title=Fractal Designdesign Conceptsconcepts for Stretchablestretchable Electronics."electronics |journal=Nature Communications, vol.|date=7 5,February 2014, p.|volume=5 3266|issue=1 |doi=10.1038/ncomms4266 }}</ref>
* '''Origami-inspired structures, and [[kirigami]] cuts''': Intrinsically rigid or inelastic flexible materials can be turned into stretchable materials by applying [[origami]] technology<ref>{{cite journal |last1=Chen |first1=Xingru |last2=Li |first2=Yongkai |last3=Wang |first3=Xiaoyi |last4=Yu |first4=Hongyu |title=Origami Paper-Based Stretchable Humidity Sensor for Textile-Attachable Wearable Electronics |journal=ACS Applied Materials & Interfaces |date=10 August 2022 |volume=14 |issue=31 |pages=36227–36237 |doi=10.1021/acsami.2c08245 }}</ref> and kirigami cuts.<ref>{{cite journal |last1=Xu |first1=Renxiao |last2=Zverev |first2=Anton |last3=Hung |first3=Aaron |last4=Shen |first4=Caiwei |last5=Irie |first5=Lauren |last6=Ding |first6=Geoffrey |last7=Whitmeyer |first7=Michael |last8=Ren |first8=Liangjie |last9=Griffin |first9=Brandon |last10=Melcher |first10=Jack |last11=Zheng |first11=Lily |last12=Zang |first12=Xining |last13=Sanghadasa |first13=Mohan |last14=Lin |first14=Liwei |title=Kirigami-inspired, highly stretchable micro-supercapacitor patches fabricated by laser conversion and cutting |journal=Microsystems & Nanoengineering |date=3 December 2018 |volume=4 |issue=1 |doi=10.1038/s41378-018-0036-z }}</ref>
</ref>
* '''Origami-inspired structures, and [[kirigami]] cuts''': Intrinsically rigid or inelastic flexible materials can be turned into stretchable materials by applying [[origami]] technology<ref>Chen, Xingru, et al. "Origami Paper-Based Stretchable Humidity Sensor for Textile Attachable Wearable Electronics." ACS Applied Materials & Interfaces, vol. 14, no. 32, 2022, pp. 36227-36237.
</ref> and kirigami cuts.<ref>Xu, Renxiao, et al. "Kirigami-inspired, Highly Stretchable Micro-supercapacitor Patches Fabricated by Laser Conversion and Cutting." Microsystems & Nanoengineering, vol. 4, 2018, Article 36. </ref>
==History==
The first time the term stretchable multielectrode array (sMEA) [[File:Manually Stretching Microelectrode Array.jpg|thumb|left|Manually stretching sMEA]] Understanding how cells convert [[stimulus (physiology) | mechanical stimuli]] appeared in the literature was in a conference proceeding in 2002 from the Lawrence Livermore National Laboratory.<ref>Maghribi, M.,{{cite etbook al|doi=10.1109/MMB.2002.1002269 "|chapter=Stretchable Micromicro-Electrodeelectrode Array."array 2002[for retinal prosthesis] |title=2nd Annual International IEEE/-EMBS Special Topic Conference on MicrotechnologyMicrotechnologies in Medicine and Biology,. IEEE,Proceedings (Cat. No.02EX578) |date=2002, ISBN:|last1=Maghribi |first1=M. |last2=Hamilton |first2=J. |last3=Polla |first3=D. |last4=Rose |first4=K. |last5=Wilson |first5=T. |last6=Krulevitch |first6=P. |pages=80–83 |isbn=0-7803-7480-0. }}</ref> This paper described the [[fabrication]] of an sMEA for a retinal [[prosthesis]], but no biological material was used, i.e., functionality to record or stimulate [[neural activity]] was not attempted. The first description of sMEAs being used to record [[neural activity]] in biological samples was in 2006 when the research group of Barclay Morrison at Columbia University and Sigurd Wagner at Princeton University reported recording of spontaneous activity in organotypic [[hippocampal]] tissue slices.<ref>Yu,{{cite Z.,book et al|doi=10.1109/IEMBS.2006.260933 "|chapter=Stretchable Microelectrodemicroelectrode Arrays:arrays Aa Tooltool for Discoveringdiscovering Mechanismsmechanisms of Functionalfunctional Deficitsdeficits Underlyingunderlying Traumatictraumatic Brainbrain Injuryinjury and Interfacinginterfacing Neuronsneurons with Neuroprosthetics."neuroprosthetics Conference Proceedings: Annual|title=2006 International Conference of the IEEE Engineering in Medicine and Biology Society, vol|date=2006 |last1=Yu |first1=Zhe |last2=Tsay |first2=Candice |last3=Lacour |first3=Stephanie P. |last4=Wagner |first4=Sigurd |last5=Morrison |first5=Barclay |volume=Suppl, 2006,|pages=6732–6735 pp.|pmid=17959498 6732|isbn=1-6735.4244-0032-5 }}</ref> Neither the electrodes nor the tissue appears to have been stretched in these experiments. In 2008, a paper from the [[Georgia Institute of Technology]] and [[Emory University]] described the use of sMEAs in stimulating a [[explant]] of a rat [[spinal cord]]. <ref>{{cite journal |last1=Meacham, |first1=Kathleen W., et|last2=Giuly al|first2=Richard J. |last3=Guo |first3=Liang |last4=Hochman |first4=Shawn |last5=DeWeerth |first5=Stephen P. "|title=A Lithographicallylithographically-Patternedpatterned, Elasticelastic Multimulti-Electrodeelectrode Arrayarray for Surfacesurface Stimulationstimulation of the Spinalspinal Cord."cord |journal=Biomedical Microdevices, vol.|date=April 2008 |volume=10, no. |issue=2, 2008,|pages=259–269 pp|doi=10. 2591007/s10544-269.007-9132-9 }}</ref> The sMEA was wrapped around the spinal cord, but not stretched, and the cells were electrically stimulated but not used in recording electrophysiological activity. In 2009, another paper of the Morrison/Wagner groups described for the first time the use of an sMEA with a biological sample being stretched as well as [[electrical stimulation]] and recording of [[electrophysiological]] activity being carried out before and after stretching.<ref>{{cite journal |last1=Yu, |first1=Zhe, et|last2=Graudejus al|first2=Oliver |last3=Tsay |first3=Candice |last4=Lacour |first4=Stéphanie P. "|last5=Wagner |first5=Sigurd |last6=Morrison |first6=Barclay |title=Monitoring Hippocampus Electrical Activity In Vitro on an Elastically Deformable Microelectrode Array." |journal=Journal of Neurotrauma, vol.|date=July 2009 |volume=26, no. |issue=7, 2009,|pages=1135–1145 pp|doi=10.1089/neu.2008.0810 1135-1145.}}</ref> sMEAs for in vitro application are commercially available from BioMedical Sustainable Elastic Electronic Devices.<ref>BMSEED. "Integrated Biomechanics, Imaging, & Electrophysiology." BMSEED, www.bmseed.com. Accessed 10 Nov. 2024.</ref> {{fact}}
In subsequent years, the number of research papers that describes different approaches to fabricating sMEAs and their use for [[in vitro]] and [[in vivo]] research has increased immensely.
* '''Spacing between microelectrodes''': The spacing between microelectrodes (center-to-center) is typically larger than 300μm for sMEAs and 200 μm for glass MEAs. but can be less than 20μm in CMOS MEAs.
The reason for these differences is that sMEAs are fabricated using soft elastomeric materials such as [[PDMS]] as substrate and [[encapsulation]] which have a much higher coefficient of thermal expansion and lower Young’s Modulus than rigid MEAs that are built on glass, plastic or silicon (CMOS) substrates. These properties make it more challenging to align and bond small features. In addition, the maximum [[Strain (mechanics)|strain]] that the electrodes can tolerate decreases for narrower electrodes, which is why the electrodes leads are often wide, thus limiting the number electrodes.<ref>{{cite journal |last1=Graudejus, |first1=O., et|last2=Jia |first2=Zheng |last3=Li al|first3=Teng |last4=Wagner |first4=S. "|title=Size-Dependentdependent Rupturerupture Strainstrain of Elasticallyelastically Stretchablestretchable Metalmetal Conductors."conductors |journal=Scripta Materialia, vol.|date=June 2012 |volume=66, no. |issue=11, 2012,|pages=919–922 pp|doi=10. 919-9221016/j.scriptamat.2012.02.034 }}</ref> sMEAs for in vitro applications are only available commercially from BioMedical Elastic Electronic Devices.<ref>BMSEED. "Integrated Biomechanics, Imaging, & Electrophysiology." BMSEED, www.bmseed.com. Accessed 10 Nov. 2024.</ref>{{fact}}
====Advantages====
====Disadvantage====
The main disadvantage of sMEAs compared to rigid MEAs are related to the different technologies that are used to manufacture these devices. sMEAs have usually up to 60 electrodes with diameters of between 50μm and 100μm where rigid CMOS based MEAs{{fact}} <ref>A Cell-Electronic Interface for Deep Access of Organoids and Tissues." 3Brain, www.3brain.com. Accessed 10 Nov. 2024.</ref>can have thousands of electrodes with diameters of 10μm. This means that sMEAs are not suitable for studying [[sub-cellular]] structures.
===In vivo stretchable MEAs===
Stretchable MEAs have many benefits for [[Implant (medicine)| implantable]] in vivo applications for recording and stimulation of electrophysiological activity from electrogenic biological tissues (most commonly neurons and muscles). Some applications involve only recording of electrophysiological activity, e.g., on the surface of the brain ,<ref>Weltman,{{cite journal |last1=Graudejus |first1=Oliver |last2=Barton |first2=Cody |last3=Ponce Wong |first3=Ruben D |last4=Rowan |first4=Cami C |last5=Oswalt |first5=Denise |last6=Greger |first6=Bradley |title=A., etsoft al.and stretchable bilayer electrode array with independent "Flexiblefunctional MEAlayers for Retinalthe Recordingnext andgeneration Stimulation."of brain machine interfaces |journal=Journal of Neural Engineering, vol.|date=October 2020 |volume=17, no|issue=5 |pages=056023 |doi=10.1088/1741-2552/abb4a5 5,}}</ref> 2020the spinal cord,<ref>{{cite journal |last1=Meacham |first1=Kathleen pW. 056025|last2=Giuly |first2=Richard J. IOP|last3=Guo Science,|first3=Liang iopscience|last4=Hochman |first4=Shawn |last5=DeWeerth |first5=Stephen P.iop.org/article/ |title=A lithographically-patterned, elastic multi-electrode array for surface stimulation of the spinal cord |journal=Biomedical Microdevices |date=April 2008 |volume=10 |issue=2 |pages=259–269 |doi=10.10881007/1741s10544-2552007-9132-9 }}</abb4a5/metaref> some involve only stimulation of electrophysiological activity, and some both.<ref>{{cite Accessedjournal 10|last1=Rowan Nov|first1=Cami C. 2024|last2=Graudejus |first2=Oliver |last3=Otchy |first3=Timothy M. |title=A Microclip Peripheral Nerve Interface (μcPNI) for Bioelectronic Interfacing with Small Nerves |journal=Advanced Science |date=January 2022 |volume=9 |issue=3 |doi=10.1002/advs.202102945 }}</ref>
</ref>, the spinal cord <ref>Meacham, Kathleen W., et al. "A Lithographically-Patterned, Elastic Multi-Electrode Array for Surface Stimulation of the Spinal Cord." Biomedical Microdevices, vol. 10, no. 2, 2008, pp. 259-269. Springer Link, link.springer.com/article/10.1007/s10544-007-9132-9. Accessed 10 Nov. 2024.</ref>, some involve only stimulation of electrophysiological activity, and some both <ref>Rowan, Cami C., et al. "A Microclip Peripheral Nerve Interface (μcPNI) for Bioelectronic Interfacing with Small Nerves." Advanced Science, vol. 8, no. 24, 2021, p. 2102945. Wiley Online Library, onlinelibrary.wiley.com/doi/full/10.1002/advs.202102945. Accessed 10 Nov. 2024.</ref>.
====Advantages====
==Conclusion==
Stretchable microelectrode arrays represent an advancement in biomedical engineering, with potential applications in neural interfaces, cardiac monitoring, in vitro research, and soft robotics. Research and development efforts continue to focus on overcoming existing challenges to fully realize the potential of these devices.
For further reading, visit the comprehensive overview of stretchable microelectrode arrays on the [BMSEED website]( https://www.bmseed.com/stretchable-meas-for-in-vitro-research).
==See Also==
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