Bioptential Electrodes：生物電的電極

1、biopotential electrodeselectrode electrolyte interface electrode electrolyte (neutral charge)c+, a- in solutionccca-a-c+c+e-e-current flowc+ : cationa- : anione- : electron fairly common electrode materials: pt, carbon, , au, ag,electrode metal is use in conjunction with salt, e.g. ag-agcl, pt-pt bl

2、ack, or polymer coats (e.g. nafion, to improve selectivity)electrode electrolyte interfacemeaaneccmngeneral ionic equationsa) if electrode has same material as cation, then this material gets oxidized and enters the electrolyte as a cation and electrons remain at the electrode and flow in the extern

3、al circuit.b) if anion can be oxidized at the electrode to form a neutral atom, one or two electrons are given to the electrode.a)b)current flow from electrode to electrolyte : oxidation (loss of e-)current flow from electrolyte to electrode : reduction (gain of e-)the dominating reaction can be inf

4、erred from the following :half cell potentiala characteristic potential difference established by the electrode and its surrounding electrolyte which depends on the metal, concentration of ions in solution and temperature (and some second order factors) . half cell potential cannot be measured witho

5、ut a second electrode.the half cell potential of the standard hydrogen electrode has been arbitrarily set to zero. other half cell potentials are expressed as a potential difference with this electrode.reason for half cell potential : charge separation at interfaceoxidation or reduction reactions at

6、 the electrode-electrolyte interface lead to a double-charge layer, similar to that which exists along electrically active biological cell membranes.measuring half cell potentialnote: electrode material is metal + salt or polymer selective membranesome half cell potentialsstandard hydrogen electrode

7、note: ag-agcl has low junction potential & it is also very stable - hence used in ecg electrodes!polarizationif there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization.overpotentialdifference between observed and zero-curren

8、t half cell potentialsresistancecurrent changes resistance of electrolyte and thus, a voltage drop results.concentrationchanges in distributionof ions at the electrode-electrolyte interfaceactivationthe activation energy barrier depends on the direction of current and determines kineticsacrpvvvvnote

9、: polarization and impedance of the electrode are two of the most important electrode properties to consider.nernst equationbadcaaaanfrteeln0when two aqueous ionic solutions of different concentration are separated by an ion-selective semi-permeable membrane, an electric potential exists across the

10、membrane.for the general oxidation-reduction reactionnedcbathe nernst equation for half cell potential is where e0 : standard half cell potential e : half cell potential a : ionic activity (generally same as concentration) n : number of valence electrons involvednote: interested in ionic activity at

11、 the electrode(but note temp dependencepolarizable and non-polarizable electrodesperfectly polarizable electrodesthese are electrodes in which no actual charge crosses the electrode-electrolyte interface when a current is applied. the current across the interface is a displacement current and the el

12、ectrode behaves like a capacitor. example : ag/agcl electrodeperfectly non-polarizable electrodethese are electrodes where current passes freely across the electrode-electrolyte interface, requiring no energy to make the transition. these electrodes see no overpotentials. example : platinum electrod

13、eexample: ag-agcl is used in recording while pt is use in stimulation use for recordinguse for stimulationag/agcl electrodeeagagagclclagag+cl-cl2relevant ionic equationsgoverning nernst equationclsagaknfrteeln0solubility product of agclfabrication of ag/agcl electrodes1. electrolytic deposition of a

14、gcl2. sintering process forming pellet electrodesequivalent circuitcd : capacitance of electrode-eletrolyte interfacerd : resistance of electrode-eletrolyte interfacers : resistance of electrode lead wireecell : cell potential for electrodefrequency responsecorner frequencyrd+rsrselectrode skin inte

15、rfacesweat glandsand ductselectrodeepidermisdermis andsubcutaneous layerruehersrdcdgelreeseeprpcpcestratum corneumskin impedance for 1cm2 patch:200k 1hz200 1mhzalter skin transport (or deliver drugs) by:pores produced by laser, ultrasound or by iontophoresis100 m100 mnerve endingscapillarymotion art

16、ifactwhywhen the electrode moves with respect to the electrolyte, the distribution of the double layer of charge on polarizable electrode interface changes. this changes the half cell potential temporarily. whatif a pair of electrodes is in an electrolyte and one moves with respect to the other, a p

17、otential difference appears across the electrodes known as the motion artifact. this is a source of noise and interference in biopotential measurementsmotion artifact is minimal for non-polarizable electrodesbody surface recording electrodes1. metal plate electrodes (historic)2. suction electrodes(h

18、istoric interest)3. floating electrodes4. flexible electrodeselectrode metalelectrolytethink of the construction of electrosurgical electrodeand, how does electro-surgery work? commonly used biopotential electrodesmetal plate electrodes large surface: ancient, therefore still used, ecg metal disk wi

19、th stainless steel; platinum or gold coated emg, eeg smaller diameters motion artifacts disposable foam-pad: cheap!(a) metal-plate electrode used for application to limbs. (b) metal-disk electrode applied with surgical tape. (c)disposable foam-pad electrodes, often used with ecgcommonly used biopote

20、ntial electrodessuction electrodes- no straps or adhesives required- precordial (chest) ecg- can only be used for short periods floating electrodes- metal disk is recessed- swimming in the electrolyte gel- not in contact with the skin - reduces motion artifactsuction electrodedouble-sidedadhesive-ta

21、peringinsulatingpackagemetal diskelectrolyte gelin recess(a)(b)(c)snap coated with ag-agclexternal snapplastic cuptackplastic diskfoam padcapillary loopsdead cellular materialgerminating layergel-coated spongecommonly used biopotential electrodesfloating electrodesreusabledisposable(a) carbon-filled

22、 silicone rubber electrode. (b) flexible thin-film neonatal electrode.(c) cross-sectional view of the thin-film electrode in (b). commonly used biopotential electrodesflexible electrodes- body contours are often irregular- regularly shaped rigid electrodes may not always work.- special case : infant

23、s - material : - polymer or nylon with silver - carbon filled silicon rubber (mylar film)internal electrodesneedle and wire electrodes for percutaneous measurement of biopotentials(a) insulated needle electrode. (b) coaxial needle electrode. (c) bipolar coaxial electrode. (d) fine-wire electrode con

24、nected to hypodermic needle, before being inserted. (e) cross-sectional view of skin and muscle, showing coiled fine-wire electrode in place.the latest: bion implanted electrode for muscle recording/stimulationalfred e. mann foundationfetal ecg electrodeselectrodes for detecting fetal electrocardiog

25、ram during labor, by means of intracutaneous needles (a) suction electrode. (b) cross-sectional view of suction electrode in place, showing penetration of probe through epidermis. (c) helical electrode, which is attached to fetal skin by corkscrew type action.electrode arraysexamples of microfabrica

26、ted electrode arrays. (a) one-dimensional plunge electrode array, (b) two-dimensional array, and (c) three-dimensional arraycontactsinsulated leads(b)baseag/agcl electrodesag/agcl electrodesbaseinsulated leads(a)contacts(c)tinesbaseexposed tipmicroelectrodeswhy measure potential difference across ce

27、ll membranerequirements small enough to be placed into cell strong enough to penetrate cell membrane typical tip diameter: 0.05 10 micronstypes solid metal - tungsten microelectrodes supported metal (metal contained within/outside glass needle) glass micropipette - with ag-agcl electrode metalintrac

28、ellularextracellularmetal microelectrodesextracellular recording typically in brain where you are interested in recording the firing of neurons (spikes).use metal electrode+insulation - goes to high impedance amplifiernegative capacitance amplifier!microns!rcmetal supported microelectrodes(a) metal

29、inside glass(b) glass inside metalglass micropipettea glass micropipet electrode filled with an electrolytic solution (a) section of fine-bore glass capillary. (b) capillary narrowed through heating and stretching. (c) final structure of glass-pipet microelectrode. intracellular recording typically

30、for recording from cells, such as cardiac myocyteneed high impedance amplifiernegative capacitance amplifier!heatpullfill with intracellular fluid or 3m kclag-agcl wire+3m kcl has very low junction potential and hence very accurate for dc measurements (e.g. action potential)electrical properties of

31、microelectrodesmetal microelectrode with tip placed within cellequivalent circuitsmetal microelectrodeuse metal electrode+insulation - goes to high impedance amplifiernegative capacitance amplifier!electrical properties of glass intracellular microelectrodesglass micropipette microelectrodestimulati

32、ng electrodes cannot be modeled as a series resistance and capacitance (there is no single useful model) the body/electrode has a highly nonlinear response to stimulation large currents can cause cavitation cell damage heatingtypes of stimulating electrodes1. pacing2. ablation3. defibrillationfeatur

33、esplatinum electrodes:applications: neural stimulationmodern day pt-ir and other exotic metal combinations to reduce polarization, improve conductance and long life/biocompatibilitysteel electrodes for pacemakers and defibrillatorsintraocular stimulation electrodesreference : lutz hesse, thomas scha

34、nze, marcus wilms and marcus eger, “implantation of retina stimulation electrodes and recording of electrical stimulation responses in the visual cortex of the cat”, graefes arch clin exp ophthalmol (2000) 238:840845in vivo neural microsystems (fibe): challengein vivo neural microsystems (fibe): bio

35、compatibility - variantin vivo neural microsystems (fibe): state of the artneural microelectrodesmems - microsystemsinstrumentation for neurophysiologyneural microsystemsintroduction: neural microsystemsexternal electrodessubdural electrodesmicro-electrodesmicrosensorshuman levelanimal leveltissue s

36、lice levelcellular levelintroduction: types of neural microsystems applicationsin vivo applicationsin vitro applicationsmicroelectronic technologyfor microelectrodesbonding padssi substrateexposed tipslead viachannelselectrodesilicon probesilicon chipminiatureinsulatingchambercontactmetal filmholesi

37、o2 insulatedau probessilicon probeexposedelectrodesinsulatedlead vias(b)(d)(a)(c)different types of microelectrodes fabricated using microfabrication/mems technologybeam-lead multiple electrode.multielectrode silicon probemultiple-chamber electrodeperipheral-nerve electrodemichigan probes for neural

38、 recordingsneural recording microelectrodesreference :http:/www.acreo.se/acreo-rd/images/publications/proceedings/abstract-kindlundh.pdfin vivo neural microsystems: 3 examplesuniversity of michigansmart comb-shape microelectrode arrays for brain stimulation and recordinguniversity of illinois at urbana-cham