CA2189866A1 - Novel hydrogel for biosensors and electrodes - Google Patents

Abstract

The invention provide novel organic polymer hydrogels comprising oligo(polyhydroxylalkyl)silylmethacrylate, particularly tetraethy-lene glycol .alpha.-.omega. bis(dimethoxysilylpropylmethacrylate), crosslinkers for use in constructing organic polymers, especially polyacrylates, particularly polyacrylamides. A preferred class of polymers are particularly suited to coating biomedical electrodes and sensors. Generally, these polymers are formed from a solution of 2-50 % (w/v) monomer and 0.1-20 % (w/v) crosslinker and 20-98 % (w/v) an electrically conductive solution of a salt in water or a water-miscible solvent. These usually acrylic polymers generally have bulk resistivity of less than 1,000 Ohms at 1 kHz, tack of between 20 and 150 g, hardness between 100 g and 500 g, 10 to 70 % stress relaxation, thermal stability over a -40 to 70 °C range, structural stability to saturating salt concentrations, and are capable of between 150 and 1,500 % elongation.
Coatings of these polymers are chemically compatible with Al, Sn, Ag and Au conductors in biomedical electrodes and sensors.

Description

2 1 8986~
wo 9~131491 F~lllx,,~

NoYel ~lydrogelfor Biosensors and Elearodes INTRODUCTION
This invention relates to organic polymer ~ ' ' , organic polymers and medical electrodes and sensors; ~ ; c, such polymers.
Medical electrodes and sensors often comprise a conductor coated or 5 covered with a material which interfaces between the conductor and the target site.
Where the target site is human slcin, various electrically conductive hydrogels have proved convenient interfacing materials. U.S. Patent Nos. 4,109,648, 4,515,162, 4,458,696 and 4,777,954 collectively disclose a number of medical electrodes coated with conductive adhesives.
A preferred electrode coating will provide In~.. ,.. ~ .~ I,;.~c.. ,.. ~ lity, thermal stability, excellent electrical properties, durable a~ a, strong ,Ol~ a~ unlimited castability, optimal cc~nfnrn~hility and softness, low ~ rrrtihiljty to drying, and chemical rnmr~tihility with conductor substrates.
ullr~.li 'y, none of the prior art electrodes provide near optimum 15 . l, - ,.. t. . ;~

Wo95/31491 21 89866 ~",, ~ ~
SUMMARY OF THE INVENTION
The invention provides novel organic polymers comprising a crosslinker of tbe general formula:
Il OR3 CH2=C, --(cHR2~l--Si~R4 Rl O
C(o~
CH C'C`o-(a~3)n4--I i--OR6 wherein each of Rl-R9 comprises a one to four earbon alkyl group or hydrogen 15 and eaeh of nl-n4 is an integer from one to five, inclusive. Generally, at least one Rl-R9 alkyl group is substituted with an amine, hydroxyl, or earboxyl group.
Preferred ,,,~,``I, L_ ` are at least 0.1% (w/v) soluble in water. In a partieularly preferred ~ o~ Rl, R3, R4, R6, R7 and R8 are eaeh methyl groups, R2, R5 and R8 are eaeh hydrogens, nl and n4 are eaeh three, n2 is two, and n3 is 20 four.
The Iu~ ktil~ of the invention find use in UUII~LU~ organie polymers, espeeially ~ lyl,..~ " particularly ~Jul~c~ ~. A preferred elass of polymers are particularly suited to eoating biomedieal eleetrodes and sensors.
Generally, these polymers are formed from a solution of 2-50% (wlv) monomer 25 and 0.1-20% (w/v) erosslinker and 20-98% (w/v) an electrieally eonduetive solution of a salt in water and/or a water-miscible solvent.
These usually aerylie polymers and generally have bulk resistivity of less than 1000 Ohms at I kHz, tack of at least 20 g, hardness between 30 g and 500 g,10 to 100% stress relaxation, thermal stability over a -40 to 7ûCC range, structural 30 stability to saturating salt ~nnrf~nh~tirn~ and are eapable of at least 300%
elongation. Coatings of these polymers are chemically compatible with Al, Sn, Agand Au conductors in biomedical electrodes and sensors.

W095131491 2 1 8 9 8 66 P~
The invention also provides methods of using the subject ClUs~lillkCl~, polymcrs and electrodes, e.g. the subject po~ymers are interposed between the skin of a patient and an electrical conductor to improve electrical current transfer.
S BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 Impedance vs. Time Data Two electrodes (hydrogel coated silver-silver chloride ' hydrogel 15.13 % Acrylamide, 1.89 % t~ y~vllv glycol Av~
hic(~ yl~luu~ vt~ y ' ), 7.21% r~ V ' chloride, 37.81%
Glycerol, 0.08% Ammonium persulfate (APS), 0.08% TEMED and 37.81%
Water) were placed face to face and the impedance was measured as a function of time exposed to air. A ,,..li.,r,..l.,.... y sign wave was imposed on the electrodes and the impedance measured with a Spectrum Analyzer (Stanford Research Systems) Fig. 2 Hardness Data Gels were cast in ~ ;.". vials and hardness measured with an Amtek pressure gauge with a 4.6 square cm probe. The probe was pressed to the gel to a0.2" depth for 15 seconds and then released. The force after 15 seconds and the 20 peak force were recorded.
Fig. 3 Stress Relaxation Data Percent stress relaxation was obtained for gels as described in Figure 2 by dividing the difference between the peak and relaxed values by the peak value.
Fig. 4 Tack Data Gels were cast in a cylindrical shape and pressed against an Amtek pressure gauge's 4.6 square cm probe for 10 seconds, and then gel retracted from the probe to obtain the peak tack value.
Fig. 5 Elongation Data Gels were cast in a cylindrical shape and held by t~o retainers and then stretched until breakage; the percent elongation before breakage was measured.

_ _ _ _ _ _ _ , _ . . . ... . . . . . .. . . ..

WOgS/31491 2 1 8 9 8 6 6 F.l/o~
Fig. 6 Impedance vs. Frequency Data As described in Fig. 1 except as a function of frequency.
DETAILED DESCRIPTION OF THE INVENTION
The invention's organic polymer crosslinkers comprise the general formula:
O
Il OR3 CH2=C, (C~R2)nl--I i~R4 C(o~
~ C'C`O'(C~4--1Si~R6 wherein each of Rl-R9 comprises a one to four carbon alkyl group or hydrogen and each of nl-n4 is an integer from one to five, inclusive. R1-R9 alkyl groups rnay be further substituted, e.g. with hydroxyl, halide, carboxyl, arnine, etc.
groups. A wide variety of chemical . ~.~1;1..';.,..~ may be made for Rl-R9, 20 depending upon specifically desired properties of the crosslinker and resultant polymer. Generally, - ~.~1;1. ;.."c made to provide polymers useful as coatings in biomedical electrodes and sensors. Hence, ~ should yield a crosslinker capable of providing a gel with the physical properties (e.g. tack, hardness, bulk resistivity, etc.) described below. Of course, ~..1.~;1,.1;",.~ must 25 also be chemically compatible with the intended free-radical p~ly~ i~liu..
reaction. Hence, except for those of acrylate derivative groups destined for ~lylll~ ,.., the presence of ~ 1;1"1;., containing unsaturated carbons are minimi~ed and preferably avoided.
Generally, the crosslinker should retain substantial solubility in water, 30 usually at least 0.1%, preferably at least 1%, more preferably at least 10% (w/v).
Accordingly, ~n" integer values and "R" groups providing water soluble ~ aalillh_la are preferred and l.~d.u~ ;c moietiea such as I ' ' long chain alkyl groups are avoided. Generally, at least one Rl-R9 alkyl group wo95/31491 2 1 89 8 66 p~"~ ,C
comprises an alkyl, amine, hydroxyl, or carboxyl group. Preferably, a pluraliey of R groups are methyl or ethyl groups. Individually e ' R groups can be variable within a given crosslinker. For example, where n2 is greater than one, R5 can alternate between a hydrogen and hydroxyl group with each repeat (i.e.
5 CHz-CHOH-CH2-CHOH). Generally, nl and n4 are smaller values when R2/R8 are hydrophilic and larger values (e.g. 4 or 5) where R2/R8 is other than hydrogen or l ' ' alkyl. Similarly, smaller n2 and n3 values are associated with R5 with hydrogen or u~ it 1 alkyls.
In a preferred ~...IJ~ ' t, R1 and R9 are limited to hydrogen or methyl 10 groups; R2 and Rg are hydrogen, methyl or substituted methyl groups such as methanol, formate, methyl amine, etc.; R3, R4, R6 and R7 are hydrogen, short alkyl, short substituted alkyl groups or acrylate derivatives such as Ikylsilylacrylate; R5 is hydrogen or hydroxy. In a more preferred Rl and R9 are methyl or hydrogen; R2 and R8 are hydrogen; R3, 15 R4, R6 and R7 are methyl or ethyl or an amine, carboxyl, or alcohol derivative of either; R5 is a hydrogen; nl, n2, n3 and n4 are 2, 3 or 4, and the product of n2and n3 is less than or equal to 12, and preferably less than or equal to 9. In al~alliuukuly preferred; ' ' t, Rl, R3, R4, R6, R7 and R8 are each methyl groups, R2, R5 and R8 are each hydrogens, nl and n4 are each three, n2 is two, 20 and n3 is four. This particularly preferred crosslinker comprises the following chemical for~nula:
s wo 95131491 2 1 8 9 8 6 6 o H2C=C' `o'CH2~CH2~cH~ 1~1 OCH3 O
Cl H2 Cl H2 o l H2 1S R cl H2 HC=c,C~o,C~2~cH ,CH2~si OCH

The / 1l ' ' of the invention find use in uu.~ u~ lg organic polymer gels, especially pùly~ly' , particularly puly~,lyldl--i~ . Generally, these gelsare formed from a solution comprising 2-60% (w/v) and preferably S to 30% (w/v) monomer (e.g. acrylate monomer or derivative thereof), 0.1-20% (wlv) and preferably 0.5-10% (w/v) crosslinker and 10-98% (wlv) and preferably 50-90%
(wlv) an electrically conductive solution of salt in water or a water-miscible solvent (e.g. glycerol).
A preferred class of polymers are IJdULi~.UIdUIy suited to coating biomedical electrodes and sensors. These hydrogels combine high adhesive properties with good cohesive strength and resistance to drying without uulllplulll;a;l.g the electrical properties of the gel. The precise ~ u~ of the gels depends upon the desired properties which will vary widely with the intended application, selected monomer and selected crosslinker. For example, for skin contacting electrodes, preferred properties of the gels include:

WO95131491 2 1 8~86~ T~l/O~ ~.
Bio ~u~ y~ The gel is non-toxic, non-irritating and non ~ i"~
when used in a physiologic electrode or sensor. When placed in direct contact with human skin, the host ~-1. i. ., ~ no or negligible discomfort, pathology orimmune ~ specific to the chemical fi~rr~ finn of the gel. The gel is 5 non-cytotoxic in in Yi~rO cell culture tests and in vivo animal tests.
Th~ r nAI ~-~hili~: The gel is stable and functional in the subject electrodes from 0C to 40C, and preferably from - 40C to 70C without any phase separation. The water in the gel matrix is tightly bound to the polymer chain and essentially no (less than 10%, preferably less than 1 %) free water in the matrix.
Fl~ tri~ AI Proverties: The polymer matrix can support a wide variety of salts and other electrolyte at high 1~ Salt ~ t~ often exceed 1 M and gels with saturated levels of salts are readily made without COII~
tack, adhesion or c,~"r.", ~ y, in contrast to prior art gels which typically 'salt out' under high, of the electrûlytes. The ~ provides a 15 superior gel with low impedance necessary for sensing and ' These polymers generally have bulk resistivity of less than 1000 Ohms, preferably lessthan 250, more preferably less than 50 Ohms at 1 kHz, Al'h ~ ~ The gel is highly tacky to the skin under various conditions.
The sensors and electrodes made with these ~" I.u~ maintain their tack for 20 extended time periods (e.g. at least about 2, preferably at least about 24, more preferably at least about 72 hours) on the patient. The adhesion is mild enough not to cause any local skin trauma when removed. Gels have tack between about 10 and 200 g, preferably between 20 and 150 g, more preferably between 30 and 100 g.
C '~ The gel maintains highly cohesive polymer network. The nature of the crosslinker described in this invention with its dual mode of action allows one to have a very tacky gel that is integral. The gel does not leave anyresidue on the skin when removed. Gels have tensile strength between about 1 and1000 g, preferably between 10 and 100 g, and are capable of elongation between 100 and 3000%, preferably between 150 and 200%, more preferably between 200 and 1000% .
t-hility: The ~ i"., a conductive hydrogel with superior properties that can be cast in to a wide variety of shapes. Many prior art gels used _ _ _ _ .. . . . _ .. .. .... . ... . . ..

wo 9~131491 2 1 8 9 8 6 6 F~~ Cl --in electrodes are pre-cast into sheets by radiation ,UUIy~ iull. This limits theusefulness of these gels to ~ ",~ 1 planer electrodes. The present allows the ' ,, of columnar electrodes with larger heigh~ to area ratios (e.g. HydroDotn', rl~y~ iA, Sunnyvale CA) and S electrodes (TriGelTY, rl.y~;u~ LI;A).
('rm~rrn~hjlity or Softness: C~-"r.~ y can be maintained even with highly cohesive gel. High ~ lr~ l;ly of the gel also lowers the impedance at a skin interface. The gels have hardness between 30 g and 600 g, preferably between 60 and 500 g, more preferably between 120 and 400 g. and stress 10 relaxation between 5 and 80%, preferably between 10 and 70%, more preferably between 20 and 60%.
Tnwer susceptibilitv to dryine: The water in the polymer matrix formed by the ~ l ;. .l. described is tightly bound. Unlike matrices where the water is loosely held in the pockets created by the polymer, the water in the present 15 .~ l;. requires more energy to remove. This translates to a long shelf life in package (e.g. months to years) and a long (at least 4 hrs, preferably at least 24 hrs) "on-the-patient" use without change in the impedance.
rnn~n~tihility Wjth ~l-hCt-~t~-~- The ~ are compatible with a variety of standard metals used in the medical electrodes and sensors like tin, 20 silver, gold, etc., as well as aluminum. With prior art gels many conductor materials, especially aluminum, experience rapid oxidation or other corrosion. The gel does not promote corrosion of the metal and yet helps to maintain a low impedance. Sensors made with the gel c.. ~ ;l;.... provide excellent traces withhigh signal to noise ratio - usually noise is less than 1%, preferably less than25 0.1%, more preferably less than 0.01% of signal peaks with less than 250, preferably less tham 50, more preferably less than 10 IlV peak-to-pea!A noise.
A wide variety of monomers are useful for these preferred hydrogels jncluding water-soluble unsaturated amides and acids and their derivatives like the acrylamide, ~ ~,LII~,lyl~ ;dc~, N l~yluAylll~,dlylc~,lyl~ auly~ulliLIilc.~ acrylic 30 acid, pulr~LII~ , oxides, etc. Preferred monomers include acrylates and derivatives thereof, especially acrylamide, Ill~Lll~l~lyldlllidl:~ N-llydluAyl.._~;lylc~,lyLIlid~. Since gel macro-properties are S;~ll;rl.,~ltly dependent wo 95/31491 2 ~ 8 9 8 5 6 r~l,o~ ~-upon the molar ratio of the monomer to the crosslinker, the percent by weight monomer typically increases with the molecular weight of the crosslinker.
Preferred crosslinkers include ~ i r~
oLt,u(~u~yl.yd.u"y~lkyl)silyll~.~LI,. ~ ly' These may be formed by a limited S L~ of L i~ uliy~lkylsilylacrylate derivatives with pulyllydIw~ydlkanes. Preferred crosslinkers are capable of linking the linear polymer generated by the bulk monomer by two distinct ' the -r '' 1 moieties cu~Julylll~.li~ and covalently link different strands; and the polar silyl groups with its partially negatively charged oxygen in the adjacent 10 chains are anchored by coordinate bonds in the presence of metal ions. These bonds give the hydrogel very high cohesive strength without CUIIIIJ1~ ' ' v its adhesive strength and softness.
The polymer hydrogels can be formed from a precursor monomer and crosslinker in a polar solvent and various added salt by any convenient way, 15 including 1.l,..~.~ h ., ~-1 and/or cataly~l/ initiated pulylll. .i~viull. The solvent is usually water or a water-based solution in which the water is usuallypresent between 20 and 100% by wt. Alcohols, including polyhydric alcohols (e.g.glycerol, t~ . illylu~l~, and propylene glycol, etc.) andlor other co-solvents may be present in the range of 5 to 80% by wt.
Salts which provide for electrical ~ (e.g. chloride, bromide, iodide, acetate, borate, etc. salts of sodium, potassium, ~ etc.) are typically present in 0.2 to 10% by wt. Typically, the electrolyte comprises at least one metal ion that can accept multiple crosslinker ligands. Such ions are usually multivalent cations such as Mg, Ca, Sn, Fe, Mn, etc. The solution may also contain additional salts and/or buffers (e.g. ethylene diamine tetraacetic acid (EDTA), sodium acetate, Tris (hydlu~.ylll_lllyl) l l.. 1 --~ acetate, etc.; 0 to 10%
by wt.) to maintain the pH of gel within selected range, typically a pH betwcen 5.3 and 8.9. Of course, the pH controlling salt can also be the electrolyte that confers electrical ~.u-~du~,Livi~y to the gel.
rhuLu~o~yl~ e c" - ~ ",c comprise a photo-initiator like Irgacure 184, Durocur 2959, etc., usually at less than 1% by wt When IJDIyll.. .i~LiOI~ is initiated by an initiator/catalyst the ~ -,l usually comprise a peroxy wo95131491 2 1 89866 P~ t~ ~
compound such as potassium persulfate as the initiator and amines like t.lld~ 1. ;hyl. ~ as the catalyst, again at less than 1% by wt.
The gel ~ may also contain ul~ ub;dl agents to guard against the growth of bacteria and fungi (e.g. Hexetidine 99 (Angus)). These anti 5 microbial agents are present in less than 1 % by wt.
The invention also provides for using the subject polymers in eleetrodes and sensors, e.g. the subject polymers are interposed between the skin of a patient and an electrieal conductor to improve electrical current transfer. Copending uull~.ullclltly-filed application Serial No. (Attorney Docket A-59028), describes the 10 produetion, design and use of such electrodes. Such eleetrodes and sensors have a wide variety of ~ in medieine, material seiences, ~ vilUllll~ d scienees, ete. For example, biomedical electrodes and sensors are used in monitoring (e.g.EEG, ete.), discharging current (e.g. surgery, ~rfihril-f lrc, etc.), ete.
The following examples are offered by way of illustration and not by way 15 of limitation.
PREFERRED EMBODIMENTS
I. r ~i ~)uly ' with initiatorleatalyst:
In these examples, the pDI~ .i~liull is triggered by the addition of the 20 amine to the 1~ ,". , solution of the . u~ The gelling time is eontrolled by the amount of the catalyst and the initiator.
Example 1.
In this example, the ~ of the gel forming precursor comprises of 5 to 25 25 % ,I._ly' ' 0.2 to 7 % t. .~ . glycol o~-~bis(d;.l.. ILuAy~ilyl~lu~yl~ J~ ), 0.4 to 8 % of magnesium chloride (or 0.2 to 6% sodium chloride) and the solvent consists 0 to 75 % by wt of glycerol and 25 to 100 % by wt of water. The fnrrn~ finn also contained less than 1% each of Hexetidine 99 (bacteriostat and fungistat), ammonium persulfate (APS) (initiator) 30 and TcLldll~ ;lly~..,..li ""; (TEMED) (eatalyst).
Example 2.
The gel ,:~""~ with controlled pH (4 to 9) consists of WO 9S131491 2 1 ~ 9 8 6 6 P~ 5 C' ~;~
Acrylamide 5 to 25 %
h.l~-~.i.yl~l.~ glycol-~-~ bis(d;~ .tl~w~y~;lylL~Iu,u,y' ' yla~u) 0.2 to 7%
Sodium chloride 0.2 to 6 %
Glycerol 20 to 80 %
tetraacetic acid (EDTA) 0 to 2%
Hexetidine 99 < 1 %
Ammonium persulfate (APS) 1%
1%
Example 3.
Anûther gel ~ . with controlled pH (4 to 9) consists of Acrylamide 5 to 25 %
t~ a~ dl y~lu.l~ glycol-15 ~-~a bis(di-.. ~ tl~u~-y~;lyll~uL~L~ ) 0.2 to 7%
chloride 0.2 to 6 %
Glycerol 20 to 80 %

Hexetidine 99 < 1 %
20 Ammonium persulfate (APS) 1%
TEMED 1 %
Tris(l,yd~u~.y,.. ~ l) Acetate 0 to 3%
Example 4.
25 1ll~ ' ' 5 to 25 %
t~ glycol-! bis(d;".. ~hv~;lylulu~'~ ' ) 0.2 to 7%
Sûdium chloride 0.2 to 6 %
Glycerol 20 to 80 %
30 Hexetidine 99 <1 %
Ammonium persulfate (APS) 1%
TEMED 1 %

WO 95/31491 2 1 ~ 9 8 6 6 Example 5.
uAy~ y;~lll;de 5 to 25 %
glycol C~-~.J bis(methoxy, (2-ethoxy- 2' ~ u~y~ yl) ~ Uj~Y~;IYI~UIU~JYIIII~ IYI~ ) 0.2 to 7%
Sodium chloride 0.2 to 6 %
Glycerol 20 to 80 %
Hexetidine 99 < 1 %
Ammonium persulfate (APS) 1%
TEMED 1 %
Example 6.
Y 5 to 25 %
triethylene glycol cY-~,) bis(~i"-~,.l,uAy,;lyll,.u~Jyllll~ ly~ ` 0.2 to 7%
Sodium chloride 0.2 to 6 %
Glycerol 20 to 80 %
Hexetidine 99 < 1 %
Ammonium persulfate (APS) 1%
TEMED 1 %
20 II. (',~ ul~ ,.;~l with I ' The gels in these examples are formed by use of high intensity tungsten lamp with 3000~W/cm2. Typically the gels are formed in less than 30 sec of exposure Example 7 Acrylamide 5 to 25 %
.. l,yh,ll~ glycol ~-~ bis(u;.,Ll,u,.y~;lylutl.y' ' y' ) 0.2 to 7%
Sodium chloride 0.2 to 6 %
Glycerol 20 to 80 %
Etllyl .1;~ .. tetraacetic acid(EDTA) 0 to 2%
Hexetidine 99 <1 %
Irgacure 184 <1%

~ WO95131491 21 89866 ~I/U~ ~ ~
Exdmple 8 Acrylamide 5 to 25 %
t~,il~.~il,jl.. ~ glycol ~-~ bis(~ luAy~;lyl,u.u~yl,l,.,ll~.. ,l~ ) 0.2 to 7%
r' _ Chloride 0.4 to 8%
Glycerol 20 to 80 %
tetrdacetic acidi'EDTA) 0 to 2%
Hexetidine 99 < 1 %
Irgacure 194 < 1%
10 Example 9.
The gel c~ ,r~ with controlled pH (4 to 9) consists ûf Acrylamide 5 to 25 %
t~,Ll~LI.~ glycol ~-~ bis(~ ,illw~;lyl,ulu~ .,-y' ) 0.2 to 7%
Sodium chloride 0.2 to 6 %
Glycerol 20 to 80 %
~LII~ tetrdacetic acid(EDTA) 0 to 2%
Hexetidine 99 < l %
Irgacure 184 < 1%
20 Example 10.
Another gel ~u~ ;l with controlled pH (4 to 9) consists of Acrylamide 5 to Z5 %
t~,.ldC:~Il.yl~,.l~; glycol ~-~ bis(~lilll~,~llu~ ;lylulu,u~llll.,LII~ ) 0.2 to 7%
' _ chloride 0.2 to 6 %
Glycerol 20 to 80 %
Hexetidine 99 < 1 %
Irgacure 184 < 1%
Tris~ll.rdlu~ ,Lllyl)~ " .. ",, Acetate 0 to 3%
III. r~uLi~,uldlly Preferred Gel t~u~ (rdnge followed by l,~l,l.il.~,.i~l example) Exdmple 11.
Acrylamide 4 to 20 (15.56) %

wo 95131491 2 ~ 8 9 8 6 6 r~ m 1 --ht..~ glycol-~-~ bis(Ji~ u~y~;lyl~u~yl~ ) 0.4 to 8 (1.95)%
Sodium chloride 0.2 to 5 (4.51)%
Glycerol 0 to 80 (38.91)%
Ammonium persulfate (APS) 0.1 to 0.05 (0.08)%
TEMED 0.1 to 0.05 (0.08)%
Water 20 to 75 (38.91)%
Example 12.
10 Acryl~nide 4 to 20 (15.13) %
h~ ,tl.~l~,..., glycol-~-~ bis(~ llu7~yaily~ uL~ l-y ` 0.4 to 8 (1.89)%
r ~ _ chloride 0.2 eo 8 (7.21) %
Glycerol 0 to 80 (37.81)%
Ammonium persulfate (APS) 0.1 to 0.05 (0.08)%
TEMED 0.1 to 0.05 (0.08)%
Water 20 to 75 (37.81)%
All I ' ' and patent A~ cited in this ~1~- ; r; A~ are herein 20 i..cu.~ ' by reference as if each individual publication or patent application were specifically and individually indicated to be , ~ by reference.
Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of -~ , it will be readily apparent to those of ordinary skill in the art in light of the teachings of this25 invention that certain changes and rnr--lifin~irmc may be made thereto without departing from the spirit or scope of the appended claims.

Claims (9)

WHAT IS CLAIMED IS:

1. A organic polymer crosslinker comprising the general formula:

wherein each of R1-R9 comprises a one to four carbon alkyl group or hydrogen and each of n1-n4 is an integer from one to five, inclusive.

2. A crosslinker according to claim 1, wherein R1, R3, R4, R6, R7 and R8 are each methyl groups, R2, R5 and R8 are each hydrogens, n1 and n4 are each three, n2 is two, and n3 is four.

3. A crosslinker according to claim 1, wherein said crosslinker is at least 0.1% (w/v) soluble in water.

4. An organic polymer comprising a crosslinker according to claim 1.

5. An organic polymer according to claim 4, comprising 10-30% acrylamide (wtv), 0.5-5% (w/v) crosslinker and 65-90% (w/v) an electrically conductive solution of a salt in water and/or a water-miscible solvent.

6. An organic polymer according to claim 4, wherein said polymer is chemically compatible with conductors of Al, Sn, Ag and Au.

7. A substantially homogeneous electrically conductive hydrogel formed by thermal or photo induced free radical polymerization of precursor comprising a water soluble unsaturated substituted amide or acid monomer, multifunctional crosslinker according to claim 1, polymerization initiator and electrically conducting solution comprising 10 to 95% by weight water miscible polar solvent and 1 to 10% by weight salt.

8. A method of improving electrical current transfer between the skin of a patient and an electrical conductor said method comprising interposing between said skin and said conductor an organic polymer according to claim 4.

9. A crosslinked organic polymer gel formed from a mixture comprising ethylenic monomers, oligo(polyhydroxyalkyl)silylacrylate crosslinkers and divalent cations, wherein said gel comprises polymers of said monomers covalently crosslinked by said crosslinkers through carbon-carbon bonds, and polymers of said monomers noncovalently crosslinked by said crosslinkers through ionic bonds.