Editing Brain–computer interface (section)

==== Technical challenges ====
There exist a number of technical challenges to recording brain activity with invasive BCIs. Advances in [[CMOS]] technology are pushing and enabling integrated, invasive BCI designs with smaller size, lower power requirements, and higher signal acquisition capabilities.<ref>{{Cite journal| vauthors = Zhang M, Tang Z, Liu X, Van der Spiegel J |date= April 2020 |title=Electronic neural interfaces |journal=Nature Electronics |language=en |volume=3 |issue=4 |pages=191–200 |doi=10.1038/s41928-020-0390-3 |s2cid= 216508360 |issn=2520-1131 }}</ref> Invasive BCIs involve electrodes that penetrate brain tissue in an attempt to record [[action potential]] signals (also known as spikes) from individual, or small groups of, neurons near the electrode. The interface between a recording electrode and the electrolytic solution surrounding neurons has been modelled using the [[Hodgkin-Huxley model]].<ref>{{cite journal | vauthors = Hodgkin AL, Huxley AF | title = A quantitative description of membrane current and its application to conduction and excitation in nerve | journal = The Journal of Physiology | volume = 117 | issue = 4 | pages = 500–544 | date = August 1952 | pmid = 12991237 | pmc = 1392413 | doi = 10.1113/jphysiol.1952.sp004764 }}</ref><ref name="Revealing neuronal function through">{{cite journal | vauthors = Obien ME, Deligkaris K, Bullmann T, Bakkum DJ, Frey U | title = Revealing neuronal function through microelectrode array recordings | journal = Frontiers in Neuroscience | volume = 8 | pages = 423 | date = 2015 | pmid = 25610364 | doi = 10.3389/fnins.2014.00423 | pmc = 4285113 | doi-access = free }}</ref>

Electronic limitations to invasive BCIs have been an active area of research in recent decades. While [[Patch clamp|intracellular recordings]] of neurons reveal action potential voltages on the scale of hundreds of millivolts, chronic invasive BCIs rely on recording extracellular voltages which typically are three orders of magnitude smaller, existing at hundreds of microvolts.<ref name=":8">{{Cite journal| vauthors = Harrison RR |date= July 2008 |title=The Design of Integrated Circuits to Observe Brain Activity |journal=Proceedings of the IEEE|volume=96|issue=7|pages=1203–1216|doi=10.1109/JPROC.2008.922581|s2cid= 7020369 |issn=1558-2256}}</ref> Further adding to the challenge of detecting signals on the scale of microvolts is the fact that the electrode-tissue interface has a high [[capacitance]] at small voltages. Due to the nature of these small signals, for BCI systems that incorporate functionality onto an integrated circuit, each electrode requires its own [[amplifier]] and [[Analog-to-digital converter|ADC]], which convert analog extracellular voltages into digital signals.<ref name=":8" /> Because a typical neuron action potential lasts for one millisecond, BCIs measuring spikes must have sampling rates ranging from 300&nbsp;Hz to 5&nbsp;kHz. Yet another concern is that invasive BCIs must be low-power, so as to dissipate less heat to surrounding tissue; at the most basic level more power is traditionally needed to optimize [[signal-to-noise ratio]].<ref name="Revealing neuronal function through"/> Optimal battery design is an active area of research in BCIs.<ref>{{Cite book| vauthors = Haci D, Liu Y, Ghoreishizadeh SS, Constandinou TG |title= 2020 IEEE 11th Latin American Symposium on Circuits & Systems (LASCAS) |chapter= Key Considerations for Power Management in Active Implantable Medical Devices |date= February 2020 |pages=1–4|doi=10.1109/LASCAS45839.2020.9069004|isbn= 978-1-7281-3427-7 |s2cid= 215817530 |chapter-url= https://discovery.ucl.ac.uk/id/eprint/10090175/ }}</ref>[[File:Invasive and partially invasive BCIs.png|thumb|Illustration of invasive and partially invasive BCIs: electrocorticography (ECoG), endovascular, and intracortical microelectrode.|248x248px]]Challenges existing in the area of [[material science]] are central to the design of invasive BCIs. Variations in signal quality over time have been commonly observed with implantable microelectrodes.<ref>{{cite journal | vauthors = Downey JE, Schwed N, Chase SM, Schwartz AB, Collinger JL | title = Intracortical recording stability in human brain-computer interface users | journal = Journal of Neural Engineering | volume = 15 | issue = 4 | pages = 046016 | date = August 2018 | pmid = 29553484 | doi = 10.1088/1741-2552/aab7a0 | bibcode = 2018JNEng..15d6016D | s2cid = 3961913 }}</ref> Optimal material and mechanical characteristics for long term signal stability in invasive BCIs has been an active area of research.<ref>{{cite journal | vauthors = Szostak KM, Grand L, Constandinou TG | title = Neural Interfaces for Intracortical Recording: Requirements, Fabrication Methods, and Characteristics | journal = Frontiers in Neuroscience | volume = 11 | pages = 665 | date = 2017 | pmid = 29270103 | doi = 10.3389/fnins.2017.00665 | pmc = 5725438 | doi-access = free }}</ref> It has been proposed that the formation of [[glial scar]]ring, secondary to damage at the electrode-tissue interface, is likely responsible for electrode failure and reduced recording performance.<ref name=":10" /> Research has suggested that [[blood-brain barrier]] leakage, either at the time of insertion or over time, may be responsible for the inflammatory and glial reaction to chronic microelectrodes implanted in the brain.<ref name=":10">{{cite journal | vauthors = Saxena T, Karumbaiah L, Gaupp EA, Patkar R, Patil K, Betancur M, Stanley GB, Bellamkonda RV | display-authors = 6 | title = The impact of chronic blood-brain barrier breach on intracortical electrode function | journal = Biomaterials | volume = 34 | issue = 20 | pages = 4703–4713 | date = July 2013 | pmid = 23562053 | doi = 10.1016/j.biomaterials.2013.03.007 }}</ref><ref>{{cite journal | vauthors = Nolta NF, Christensen MB, Crane PD, Skousen JL, Tresco PA | title = BBB leakage, astrogliosis, and tissue loss correlate with silicon microelectrode array recording performance | journal = Biomaterials | volume = 53 | pages = 753–762 | date = 2015-06-01 | pmid = 25890770 | doi = 10.1016/j.biomaterials.2015.02.081 }}</ref> As a result, flexible<ref>{{cite journal | vauthors = Robinson JT, Pohlmeyer E, Gather MC, Kemere C, Kitching JE, Malliaras GG, Marblestone A, Shepard KL, Stieglitz T, Xie C | display-authors = 6 | title = Developing Next-generation Brain Sensing Technologies - A Review | journal = IEEE Sensors Journal | volume = 19 | issue = 22 | pages = 10163–10175 | date = November 2019 | pmid = 32116472 | doi = 10.1109/JSEN.2019.2931159 | pmc = 7047830 }}</ref><ref>{{cite journal | vauthors = Luan L, Wei X, Zhao Z, Siegel JJ, Potnis O, Tuppen CA, Lin S, Kazmi S, Fowler RA, Holloway S, Dunn AK, Chitwood RA, Xie C | display-authors = 6 | title = Ultraflexible nanoelectronic probes form reliable, glial scar-free neural integration | journal = Science Advances | volume = 3 | issue = 2 | pages = e1601966 | date = February 2017 | pmid = 28246640 | pmc = 5310823 | doi = 10.1126/sciadv.1601966 | bibcode = 2017SciA....3E1966L }}</ref><ref>{{cite journal | vauthors = Frank JA, Antonini MJ, Anikeeva P | title = Next-generation interfaces for studying neural function | journal = Nature Biotechnology | volume = 37 | issue = 9 | pages = 1013–1023 | date = September 2019 | pmid = 31406326 | pmc = 7243676 | doi = 10.1038/s41587-019-0198-8 }}</ref> and tissue-like designs<ref name=":9">{{cite journal | vauthors = Hong G, Viveros RD, Zwang TJ, Yang X, Lieber CM | title = Tissue-like Neural Probes for Understanding and Modulating the Brain | journal = Biochemistry | volume = 57 | issue = 27 | pages = 3995–4004 | date = July 2018 | pmid = 29529359 | pmc = 6039269 | doi = 10.1021/acs.biochem.8b00122 }}</ref><ref>{{cite journal | vauthors = Viveros RD, Zhou T, Hong G, Fu TM, Lin HG, Lieber CM | title = Advanced One- and Two-Dimensional Mesh Designs for Injectable Electronics | journal = Nano Letters | volume = 19 | issue = 6 | pages = 4180–4187 | date = June 2019 | pmid = 31075202 | pmc = 6565464 | doi = 10.1021/acs.nanolett.9b01727 | bibcode = 2019NanoL..19.4180V }}</ref> have been researched and developed to minimize [[foreign-body reaction]] by means of matching the [[Young's modulus]] of the electrode closer to that of brain tissue.<ref name=":9" />