DTIC ADA579943: Dissolvable Films of Silk Fibroin for Ultrathin Conformal Bio-integrated Electronics

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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Dissolvable films of silk fibroin for ultrathin conformal W911NF-07-1-0618
bio-integrated electronics
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
7620AK
6. AUTHORS 5d. PROJECT NUMBER
Dae-Hyeong Kim, Jonathan Viventi, Jason J. Amsden, Jianliang Xiao,
Leif Vigeland, Yun-Soung Kim, Justin A. Blanco, Bruce Panilaitis, Eric 5e. TASK NUMBER
S. Frechette, Diego Contreras, David L. Kaplan, Fiorenzo G. Omenetto,
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Tufts University
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14. ABSTRACT
offer important opportunities for diagnosing and treating disease and for improving brain/machine interfaces. This
article describes a material strategy for a type of bio-interfaced system that relies on ultrathin electronics supported
by bioresorbable substrates of silk fibroin. Mounting such devices on tissue and then allowing the silk to dissolve
and resorb initiates a spontaneous, conformal wrapping process driven by capillary forces at the biotic/abiotic
interface. Specialized mesh designs and ultrathin forms for the electronics ensure minimal stresses on the tissue and
15. SUBJECT TERMS
silk, electronics, devices, biotic/abiotic
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Report Title
Dissolvable films of silk fibroin for ultrathin conformal bio-integrated electronics
ABSTRACT
offer important opportunities for diagnosing and treating disease and for improving brain/machine interfaces. This
article describes a material strategy for a type of bio-interfaced system that relies on ultrathin electronics supported
by bioresorbable substrates of silk fibroin. Mounting such devices on tissue and then allowing the silk to dissolve and
resorb initiates a spontaneous, conformal wrapping process driven by capillary forces at the biotic/abiotic interface.
Specialized mesh designs and ultrathin forms for the electronics ensure minimal stresses on the tissue and highly
conformal coverage, even for complex curvilinear surfaces, as confirmed by experimental and theoretical studies. In
vivo, neural mapping experiments on feline animal models illustrate one mode of use for this class of technology.
These concepts provide new capabilities for implantable and
surgical devices.
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ARO Report Number 53392.31-MS-DRP
Dissolvable films of silk fibroin for ultrathin confo ...
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(DODGARS §32.36). The views, opinions and/or findings contained in this report are those of the author(s) and should not be
construed as an official Department of the Army position, policy or decision, unless so designated by other documentation.
Approved for public release; distribution is unlimited.
ARTICLES
PUBLISHEDONLINE:18APRIL2010|DOI:10.1038/NMAT2745
Dissolvable films of silk fibroin for ultrathin
conformal bio-integrated electronics
Dae-HyeongKimandJonathanViventietal.*
Electronics that are capable of intimate, non-invasive integration with the soft, curvilinear surfaces of biological tissues
offer important opportunities for diagnosing and treating disease and for improving brain/machine interfaces. This article
describesamaterialstrategyforatypeofbio-interfacedsystemthatreliesonultrathinelectronicssupportedbybioresorbable
substrates of silk fibroin. Mounting such devices on tissue and then allowing the silk to dissolve and resorb initiates a
spontaneous,conformalwrappingprocessdrivenbycapillaryforcesatthebiotic/abioticinterface.Specializedmeshdesigns
andultrathinformsfortheelectronicsensureminimalstressesonthetissueandhighlyconformalcoverage,evenforcomplex
curvilinearsurfaces,asconfirmedbyexperimentalandtheoreticalstudies.Invivo,neuralmappingexperimentsonfelineanimal
modelsillustrateonemodeofuseforthisclassoftechnology.Theseconceptsprovidenewcapabilitiesforimplantableand
surgicaldevices.
S
trategies for bio-integrated electronics must overcome Silk is an appealing biopolymer as a temporary, soluble
challenges associated with the mismatch between the hard, supporting substrate for this application because it is optically
planarsurfacesofsemiconductorwafersandthesoft,curvi- transparent13,14, mechanically robust and flexible in thin-film
lineartissuesofbiologicalsystems.Thesedifferencesinmechanics form15–17,compatiblewithaqueousprocessing18,19andamenableto
and form lead, almost invariably, to low-fidelity coupling at the chemicalandbiologicalfunctionalization13,20.Thesilk,inboththe
biotic/abiotic interface and limited long-term tissue health. The non-treated and methanol-treated formats, is biocompatible21,22,
difficulties are most pronounced, and the solutions are perhaps bioresorbable23 and water soluble with programmable rates of
mostimportant,insystemsdesignedforbrain/computerinterfaces dissolution15,16. Moreover, recent work demonstrates the ability
(BCIs). State-of-the-art penetrating microelectrode arrays consist of silk films to serve as a platform for transistors23 and various
ofsharpshanks,typically10×10arraysofpinswithbasewidths classes of photonic devices24,25. The process of preparing silk
∼80µm, lengths ∼1.5mm and pitch ∼400µm (ref. 1). These substrates for the purposes reported here began with material
arrays are rigid and inflexible because of their construction derived from Bombyx mori cocoons, and followed published
from blocks of silicon, which also supports their conventional procedures18,19. Briefly, boiling the cocoons in a 0.02M aqueous
wafer-based electronics. They are valuable for research in BCIs, solution of sodium carbonate for 60min removed sericin, a
but they damage tissue and do not offer long-term electrical water-soluble glycoprotein that binds fibroin filaments in the
interface stability2 because of unwanted biological responses to cocoon butcaninduce undesirableimmunological responses21,26.
the electrodes. Comparable BCI performance can be achieved An aqueous solution of lithium bromide at 60◦C solubilized
withnon-penetrating,surfaceelectrodesystemsthatareminimally the fibres and subsequent dialysis removed the lithium bromide.
invasiveandprovidegreatlyimprovedstability3–5 withminimized Centrifugationfollowedbymicrofiltrationeliminatedparticulates
inflammation.Standardclinicalsubduralelectrodearraysareuseful toyieldsolutionsof8–10%silkfibroinwithminimalcontaminants.
for BCIs (ref. 6) but their widely spaced (∼1cm), large contact Casting a small amount of the solution on a flat piece of
electrodes(∼0.35cmdiameter)spatiallyundersampletheelectrical poly(dimethylsiloxane) (PDMS) followed by crystallization in air
signalspresentonthesurfaceofthebrain7.Decreasingthespacing (∼12h) yielded uniform films (thickness of 20–50µm) (Fig.1a)
andsizeofthemeasurementpointscanimproveBCIperformance that were subsequently removed from the PDMS for integration
byprovidingaccesstohightemporalandspatialfrequencysignals8. withseparatelyfabricatedelectronics.
Suchdesigns,however,demandexcellentconformalcoverageover For the systems described in the following, ultrathin, spin-
the highly convoluted brain surface, to ensure direct coupling cast films of polyimide (PI) served as a support for arrays of
betweenbraintissueandtheelectrodes. electrodes designed for passive neural recording. Control devices
Reducing the thickness of the substrate decreases the bending consistedofotherwisesimilarlayouts,butformedusingstandard
rigidity, thereby improving conformal contact. Unfortunately, photolithographic procedures applied directly on commercial PI
clinicalarraysandeventhethinnestdevicesdesignedforresearch sheetswiththicknessesof25and75µm(SupplementaryFig.S1).
havethicknesses(700µmand>10µm(refs9,10),respectively)that Anisotropic conductive film (ACF) bonded to electrode pads at
are larger than desired to ensure conformal contact. Analogous one end of the arrays provided electrical connection to external
systems based on stretchable substrates have also been explored data acquisition systems (Supplementary Fig. S2). Ultrathin PI
in other neural interfaces11,12 but typically with similar or films, with or without mesh layouts, cannot be manipulated
larger thicknesses. In conventional designs, ultrathin geometries effectively for processing, interconnecting or implanting onto
(that is, <10µm) are impractical, because the films are not the brain because of their extreme flexibility and mechanical
sufficiently self-supporting to be manipulated effectively during fragility. For these cases, the fabrication process exploited layers
fabricationorimplantation. of PI spin-cast onto silicon wafers coated with sacrificial films of
*Afulllistofauthorsandtheiraffiliationsappearsattheendofthepaper.
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ARTICLES NATUREMATERIALS
DOI:10.1038/NMAT2745
a
Silk
Silk film
Cast and dry silk
on PDMS substrate
b
Fabricated device on Dissolve PMMA; Connect ACF;
carrier wafer transfer to silk contact to brain;
(PI/PMMA/Si) substrate using silk solution dissolve silk substrate
ACF 2 mm
pad Silk ACF
c
ACF
Silk
Brain
Open skull, Contact, pour Dissolve silk,
prepare electrode saline solution conformal contact
Figure1|Schematicillustrationandimagescorrespondingtostepsforfabricatingconformalsilk-supportedPIelectrodearrays.a,Castinganddrying
ofsilkfibroinsolutiononatemporarysubstrateofPDMS;5–15-µm-thicksilkfilmafterdryingfor12hatroomtemperature.b,Stepsforfabricatingthe
electrodearrays,transferprintingthemontosilkandconnectingtoACFcable.c,Schematicillustrationofclinicaluseofarepresentativedeviceinan
ultrathinmeshgeometrywithadissolvablesilksupport.
poly(methylmethacrylate) (PMMA) (left frame of Fig.1b). After section. Electrode arrays were implanted by placing them on
electrodefabrication,themeshstructuredevicesunderwentfurther the exposed brain (after craniotomy) and then flushing with
etching to remove unwanted parts of the PI. The processing was saline to dissolve the silk. This procedure induced spontaneous,
completed by dissolving the PMMA layer with acetone, transfer conformalwrappingofthedevice,asillustratedschematicallyfor
printing the entire assembly to a film of silk and connecting the themeshdesigninFig.1c.Afterthemeasurements,theelectrode
ACF, yielding easily manipulated bioresorbable neural recording array can be easily removed, because of the attachment of the
systems. See schematic illustrations and images in Fig.1b. In electrodestotheACF.
all cases, the arrays consisted of 30 measurement electrodes ThesequenceofimagesinFig.2ashowsthedissolutionprocess
(Au, 150nm) in a 6×5 configuration, each with dimensions for a representative case (7-µm-thick PI film, connected to ACF
of 500µm×500µm and spaced by 2mm. Interconnection wires on a silk substrate with a thickness of ∼25µm) inserted into
were protected by a thin (∼1.2µm) overcoat of PI to prevent warm water (∼35◦C). As the silk substrate disappears, the total
contact with the tissue or surrounding fluids. Choosing the bending stiffness, EI, diminishes markedly because of its cubic
thickness of the PI passivation layer to match that of the PI dependenceonthickness.ComputedresultsappearinFig.2band
substratelocatestheinterconnectsattheneutralmechanicalplane, Supplementary Fig. S3c for PI thicknesses of 2.5 and 7µm. To
therebyminimizingthepotentialforbending-inducedmechanical highlight the benefits of reduced thickness, the inset shows the
fracture. Details of the fabrication steps appear in the Methods ratio of EI for these two cases. Through programmed control
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a
1 cm Water No silk
Silk film
Dissolve
b 2.5 108
25
o 20
2.0 108 ness rati 1105 27.5 µ µ m m s tsitfiffnfneessss Silk
4m) Stiff 5
.µPa 1.5 108 00 5 10 15 20 25 30
s (G Silk thickness ( µ m) 7 µ m
s
e
n
g stiff 1.0 108
n
di
en 2.5 µ m
B
5.0 107
0
0 5 10 15 20 25 30
Silk thickness ( µ m)
c 100 2.5 108
7 µ m
ning silk (%) 486000 .µ4ss (GPa m) 211...005 111000888 2.5 µ m
Remai 20 Stiffne 5.0 107
0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30
Time (min) Time (min)
Figure2|Time-dependentchangesasthesilksubstratedissolves.a,Dissolutionofthesilkthroughsubmersioninwarmwater.b,Totalbendingstiffness
of7µmand2.5µmelectrodearraysonsupportingsilkfilmsasafunctionofthethicknessofthesupportingsilkfilm.Inset:Theratioofbendingstiffness
between7µmand2.5µm.c,Time-dependentchangeinthevolumeofasilkfilmduringdissolution(leftframe)andbendingstiffnesscalculatedforsilk
treatedin70%ethanolfor5sfortwodifferentarraythicknesses(rightframe).The5sethanoltreatmentincreasesthedissolutiontimefromminutesto
about1h.
of the dissolution rate by modifications of the silk protein SupplementaryFigsS5andS6.Torevealtheunderlyingmechanics,
secondary structure15,16, these changes in EI can be designed wecarriedoutsystematicandquantitativestudiesonwell-defined
to occur over periods of time ranging from seconds to years, surfacesthatcapturecertainbasicfeaturesofthecurvatureofthe
depending on requirements. Figure2c shows, as an example, the brain.Thefirstsetofexperimentsexploredwrappingthedevices
dissolution rate of silk film slightly treated with ethanol (left onisolatedandoverlappedcylindricalsurfaces.Figure4ashowsthe
frame) and the computed time dependence of EI in devices that simplestcaseofadevicewithbendingstiffnessEI,thicknessh,width
employmorethoroughethanoltreatment(rightframe).Theerror b and length 2L, wrapped on a cylinder with radius R. Analytical
rangeforsilkthicknessmeasurementis±7%.SeeSupplementary expressions for EI can be written for the multilayer structures of
Informationfordetailedconditions.Thisdissolutiontimecanbe Fig.1intermsofmaterialspropertiesandgeometries,asdescribed
lengthened even more by extending the treatment time to days in the Supplementary Information. For the wrapped state to be
or weeks15; the corresponding time dependence of EI appears in energeticallyfavourable,
SupplementaryFig.S4.
Toexaminetheabilityofthesesystemstoconformtorelevant γ ≥γ = EI (1)
surfaces,wecarriedoutexperimentsusingahumanbrainmodel, c 2R2b
followingthebasicstepsshowninFig.1c.Figure3providesimages
forvariouscasesafterwashingwithsaline,includingrelativelythick where γ is the adhesion energy per unit area. The bottom frame
control devices that do not incorporate silk. Clearly, the extent ofFig.4acomparestheaboverelationwithaseriesofexperiments
of conformal coverage increases with decreasing thickness; the (Supplementary Fig. S7). The data, the error range of which is
meshdesignprovidesfurtherimprovements,asshowninFig.3d, ±5%,areconsistentwithγ ∼10mJm−2,whichiscomparableto
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a radiusr ,attheangularpositionθ =sin−1[d/(R+r )].Thecontact
0 0 0
angleofathinfilmwithonecylinder,θ,canbeshowntobe
Rsinθ dRθcosθ (cid:18)γ (cid:19)(cid:18) d dθcosθ(cid:19)
+ − −1 1− + =0
d−Rsinθ (d−Rsinθ)2 γ Rsinθ Rsin2θ
c (2)
Dfroemcr e7a6s e µ tm hi ctok n2e.5ss µ m Cfrohman gshee settr utcot mureesh whereγcisgiveninequation(1).Thesolutionofequation(2)takes
b 1 cm 76 µ m 26 µ m tahteθf=orm0,θan=dθth(de/fRilm,γ/dγoce)s.nFoortγwr<apγca,rothuenedntehrgeycyhlainsdaemrsi.nPimarutimal
wrappingoccurstoacontactangleofθ (thatis,contactforangles
between0andθ<θ )forγ ≤γ <γ(cid:48),whereγ(cid:48) isobtainedfrom
0 c c c
equation(2)withθ=θ asgivenintheSupplementaryInformation.
0
For γ ≥γ(cid:48), wrapping is complete (that is, conformal contact for
c
angles between 0 and θ ). By comparing equation(2) with the
0
Poor contact Poor contact experimentinSupplementaryFig.S8,theextractedadhesionenergy
2.5 µ m 7.0 µ m per unit area is γ =10mJm−2. Results appear in the bottom
frame of Fig.4b, where the parameters correspond roughly to
features on the brain model: R=6.14mm, d =5.93mm and
r =1.72mm.Theerrorrangeofthedatais±5%.(Experimental
0
images appear in Supplementary Fig. S8.) By substituting θ with
θ in equation(2), the critical thickness for conformal contact is
0
obtained as h =4.9µm for the present system; that is, devices
Good contact Good contact thinner than ∼04.9µm achieve conformal contact on this surface.
c 0.3 cm 76 µ m 26 µ m Theexperimentalresultsareconsistentwiththiscalculation.
Cylindrical surfaces such as those of Fig.4a,b are developable;
thebrainisnot.Asamodelnon-developablesurface,weexamined
the case of a hemispherical substrate. Figure4c shows results
for electrode arrays with sheet designs at thicknesses of 7 and
2.5µmandwithanopenmeshlayout28 at2.5µm,eachonaglass
hemispherewitharadiusofcurvatureof6.3mm.Withonlywater
2.5 µ m 7.0 µ m
capillarityastheadhesionforce,themeshelectrodearrayachieves
excellentconformalcontact.Thesheetsshowcomparativelypoor
contact, with large wrinkles, even for the thinnest case (that
is, 2.5µm). Mechanical analysis of a simple model reveals the
underlyingphysics.TheleftframeofFig.4dshowsthemechanics
model for the sheet design, which consists of a circular film with
d 0.2 cm radius r+w wrapped onto a sphere with radius R. The central
greenpartdenotesaPIplateofradiusr,tensionstiffness(Eh) and
PI
Mesh type, 2.5 µ m equi-biaxialbendingstiffness(EI) .Theyellowringcorresponds
PI
toamultilayerstructureofPIandAu,ofwidthw,tensionstiffness
(Eh) andequi-biaxialbendingstiffness(EI) .Forthe
composite composite
filmtowraparoundthesphere,therequiredminimumadhesion
energyperunitareaisobtainedanalyticallyas
Silk (EI) (Eh) (cid:90) r(cid:18) R x(cid:19)2
γsheet = PI+ PI 1− sin xdx
Contact c R2 r2 0 x R
electrode PI mesh 2w(EI) w(Eh) (cid:18) R r(cid:19)2
+ composite+ composite 1− sin (3)
Figure3|Neuralelectrodearraysofvaryingthicknessonsimulated rR2 r r R
brainmodelstoillustrateflexibility.a,Schematicillustrationoftrendsin
thicknessandstructurethatimproveconformalcontact.b,Seriesof Amechanicsmodelforthemeshdesignappearsintherightframe
picturesillustratinghowthethicknessoftheelectrodearraycontributesto ofFig.4d,whichconsistsofonlyacircularstripofacorresponding
conformalcontactonabrainmodel.c,Magnifiedviewofthepicturesinb. multilayer of PI and Au. In this case, the minimum adhesion
d,Imageofanelectrodearraywithameshdesignonadissolvablesilk energyperunitareais
substrate.Thearrowsindicatestrutsinthemeshthathelptostabilizethe (cid:32) (cid:114) (cid:33)2
Auinterconnectsafterdissolutionofthesilk.Theinsetillustratesthehigh γmesh=(EI)composite+w2(Eh)composite 1− 1−r2 (4)
degreeofconformalcontactthatcanbeachievedonthebrainmodelonce c R2 24r2 R2
thesilksubstratehasbeendissolved.
For the case that w (cid:28)r, γsheet in equation(3) is always larger
c
reportedvaluesforwetinterfaces27.Reducingthethicknessprovides thanγmesh inequation(4);thatis,γsheet>γmesh.Theinferenceis
c c c
clearbenefits,forexample,wrappingcylindersusingonlycapillary thattheopenmeshdesignrequiresamuchloweradhesionenergy
adhesionforcesispossibleforR∼1cmwhenh<∼15µm. thanthecorrespondingsheet,therebyleadingtogreatlyimproved
A pair of overlapped cylinders represents a simple model for ability for conformal coverage, as shown in Supplementary Fig.
a gyrus of the brain. Figure4b shows cylinders with radii R, a S9a. Supplementary Fig. S9c shows critical adhesion energies for
centre-to-centreseparationof2dandconnectedbyasmootharcof filmswiththicknessesupto80µm.Forathicknessof2.5µmand
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w/r =4, γcsheet=29.1mJm−2 for the sheet, which is more than a 2L b 2L
12timeslargerthanthemeshγcmesh=2.4mJm−2.Inaddition,the h h
ma efashctdoersiogfniwn/vro,lvceosmmpeamrebdranweitshtrasihnesetthsatwairthesmsimalillearr,btyhricokungehslsy. R r0 θ0
For the experimental mesh systems, this ratio is of the order R 2d
of 1/4. As a result, for a representative critical wrinkling strain
of 0.1%, nearly two thirds of the sheet will wrinkle. Under the
sameconditions,theentiremeshgivesperfect,conformalcontact.
θ
Finally,thenormal(peeling)interfacialstressforthemeshisonly
1/4ofthatforthesheet(SupplementaryFigsS9bandS9d),leading
toimprovedadhesionandreducedforcesappliedtothesubstrate.
SeeSupplementaryInformationfordetails.
120 60
In vivo neural monitoring experiments on a feline animal Modelling Modelling
mmftdihuxoeaerctdodhetsaoilunnmdbieactymess.netoxedTnprehesodeotsrteat2adetx8esai◦tdcs2×tai×hpn2epv32opa◦crlrmvaaoetcufdtriescsagapanilnoaicdnameno.iptafselAcisecnotyahretteiiseontxifnizo.tseiTcadoulhfsceectaedrhtaleenowscneitiortfthoaaodvmmioetsuoyanrrharaieatbnaoyldders μThickness ( m) 9630000 UnwrappEexdWpeγrraimp=pe 1en0dt mJ m¬2 θ(degree)3410055 Conform. wrapPartial wrap NEox wperaripment
covered much of the visual cortex as shown in the left frames 0 1 2 3 4 0 20 40 60 80
ofFig.5a–c.Visualstimuliconsistedoffull-fielddriftinggratings Diameter (cm) Thickness ( μ m)
presented for 1s at 2Hz with a spatial frequency of 0.5cycles
c
per degree. Gratings were presented at two different directions 0.3 cm
overeightdifferentorientations(16uniquestimuli).However,the
responses obtained from all 16 unique stimuli were averaged to
obtainthelargestpossiblesignal-to-noiseratio.
Three kinds of electrode array were used for comparison:
76-µm- and 2.5-µm-thick sheets and a 2.5µm-thick mesh. The 76 μ m 2.5 μ m
second two included dissolvable silk supports. The left images of
Fig.5a–c illustrate the progressively improved conformal contact 0.2 cm
withreducedthickness(thatis,76µmto2.5µm,inFig.5aandb,
respectively) and with introduction of the mesh (that is, Fig.5c).
The right frames of Fig.5a–c demonstrate the effectiveness of
decreasing the electrode thickness and the mesh structure on
physiologicalmeasurementsofbrainactivity.
In particular, these frames show the average evoked response
measuredateachelectrode,eachplottedinaspatialarrangement
that corresponds to the images in the left frames. Prominent
visuallyevokedpotentialsareobserved,particularlyastrongP100
response. The P100 response is a ‘positive’ evoked response
Mesh
typicallyoccurringat100msafterthestimulationonset29.TheP100
d
responsesshowninFig.5areplottedpositivedown,byconvention.
The background colour of each plot illustrates a quantitative r r
measure of the evoked response signal quality. This measure of
R w w
signalqualitywascalculatedbydividingtherootmeansquare(rms) Side Top Side Top
amplitudeofeachaverageelectroderesponseinthe200mswindow
immediately after the presentation of the visual stimulus by the
rmsamplitudeoftheaverage1.5swindowimmediatelypreceding Figure4|Mechanicalmodelling,theoreticalpredictionsandmeasured
thestimuluspresentation.ThecolourbaratthebottomofFig.5c properties.a,AthinfilmwrappedaroundacylinderofradiusR.The
providesthenumericalscaleforallofthecoloursusedinFig.5a–c. unwrappedandwrappedstatesappearinthetopandcentreframes,
Thismeasurementservesasaquantitativemetricoftheelectrode respectively.Thebottomframecomparesthemechanicsmodeland
performance, because the uniform nature of the stimulation is experiments.b,Athinfilmwrappedaroundtwooverlappedcylinders.The
expectedtoevokesimilarresponsesacrosstheentirevisualcortex. topandcentreframesshowtheunwrappedandwrappedstates,
Ineachcase,28ofthe30electrodechannelswererecordedand respectively.Thebottomframeshowsacomparisonbetweenthe
evaluatedforevokedpotentialresponse,ascolouredingreentored. mechanicsmodelandexperiments.c,Imagesofelectrodearrays(76µm
Twochannels,indicatedingrey,werereservedaslocalreferences, sheetinlefttop,2.5µmsheetinrighttopand2.5µmmeshinbottom
asrequiredbytherecordingapparatus,andwerenotevaluated.The panel)wrappedontoaglasshemisphere.d,Mechanicsmodelsforsheet
channelswithhighandlowrmsamplituderatiosarecolouredgreen (leftframe)andmesh(rightframe)designs.
andred,respectively.The76µm(Fig.5a)electrodearrayexhibited
thelowestperformancewithameanrmsamplituderatioofall28 with the brain and recorded weak responses. The 2.5µm mesh
channels of 3.6±1.8. This was due to poor contact at many of electrode (Fig.5c) showed the best performance, with nearly all
theelectrodes.The2.5µmarray(Fig.5b)showedbetterconformal channels in good contact and a still higher mean rms amplitude
contact and correspondingly a higher mean rms amplitude ratioof5.7±3.0.Thelowerstandarddeviationofthe2.5µmarray
ratio of 5.2±3.9. However, the higher standard deviation and illustratesthatmostoftheelectrodesrecordedgoodresponses.
correspondinglywidespectrumoftheredandgreenchannelson Figure5dshowsrepresentativesingle-channeldatafromoneof
thearrayindicatethatalthoughmanyelectrodesrecordedexcellent the2.5µmmeshelectrodes.Asleepspindleisobservedwithgood
signals,approximatelyhalfoftheelectrodesstillhadpoorcontact signal amplitude and signal-to-noise ratio. This collective set of
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ARTICLES NATUREMATERIALS
DOI:10.1038/NMAT2745
a observations is consistent with the systematic mechanics studies
76 µ m describedpreviously.Wedidnotobserveanyevidenceofimmune
response. Histology data from related types of device implanted
undertheskinexhibitednoinflammationafter4weeks,asshown
inSupplementaryFig.S10.
Althoughpurelypassiveelectrodesystemsservetodemonstrate
the advantages and underlying aspects of these systems, the
same approaches are compatible with fully active electronics and
optoelectronics. This technology allows intimate integration of
finelyspacedelectrodesystemswithlivingtissue,enablingthekind
ofreliablebiotic/abioticinterfacewithmoving,biologicalstructures
that will be required for chronically implanted, high-resolution
medical devices. This improved electrode/tissue interface has the
potential for a positive impact on human health in many modes
b
2.5 µ m ofuse.
Methods
Thickelectrodearray(>25µM)fabrication. CommercialPIfilms(Kapton,
Dupont)withthicknessesof25and75µmwereattachedtoatemporarycarrier
substrateconsistingofaglassslidecoatedwithPDMS.Aftercleaningthesurfaces
withacetone,isopropylalcoholanddeionizedwater,electronbeamevaporation
formeduniformcoatingsofmetal(Cr/Au,50/1450A).Photolithographyand
patternedetchingyieldedarraysofinterconnectlines.ThinlayersofPI(thickness
∼1.2µm)spin-castandpatternedbyreactiveionetchingleftonlytheendsofthe
linesexposed.Furtherdepositionandpatterningdefinedsquaremetalelectrode
padsattheselocations.PeelingtheseawayfromthePDMS-coatedglassslideand
bondingthemtoanACFcable,usingproceduresdescribedinaseparatesection,
completedthefabrication.SupplementaryFig.S1providesaschematicdiagram
andimagesoftheprocess.
c
Mesh
Thinelectrodearray(<10µM)fabrication. Thefabricationinthiscaseused
acarriersiliconwafercoatedwithathin(∼1.2µm)spin-castlayerofPMMA
(A2,MicroChem).ThedevicesubstrateconsistedofafilmofPI(SigmaAldrich)
spin-castontothePMMA.Proceduressimilartothosedescribedforthickdevices
formedthemetalelectrodesandPIovercoat.Afterfabrication,theultrathindevices
werereleasedbydissolvingthesacrificialPMMAlayer.Transferprintingwitha
PDMSstampdeliveredthedevicestodrysilkfilmsubstrates,coatedwith∼9%silk
solutionasanadhesive.ThefinalstepinvolvedbondingofanACFcable.
Meshelectrodearray(<10µM)fabrication. Thefirstandlastpartsofthe
fabricationsequencewereidenticaltothestepsoutlinedintheprevioussection.
Theonlydifferencewastheadditionofasteptoremovecertainregionsof
thepolymerlayers(thatis,PIandunderlyingPMMA)byoxygenreactive
V ionetchingthroughamask(designinSupplementaryFig.S3)todefinethe
0.2 cm 2 m meshstructure.Detaileddimensionsareasfollows:thickness∼2.5µm,contact
0. 200 ms electrodesize500µm×500µm,meshwidth∼250µm.(Seemoredetailsin
SupplementaryFig.S5.)Thisetchingimmediatelyfollowedtheformation
oftheelectrodepads.
0 2 4 6 8
d ACFconnection. Thecontactpadsontheelectrodearraywerefirstalignedwith
theACFcable.Metalclipswereusedtoapplypressure,spreadevenlyoverthe
mV contactpadareausingapieceofPDMSinsertedbetweentheACFandtheclips.
1 Next,theclampedsampleandACFwereplacedinanovenpreheatedto∼150◦C
200 ms for∼15min.Thisprocessformedastrongmechanicalbondbetweentheelectrode
arrayandtheACFwithlowelectricalresistance.
Figure5|Photographsanddatafromanimalvalidationexperiments. Dataacquisitionandprocessing. Theelectrodearrayswereconnectedtoa
a–c,Imageofanelectrodearrayonafelinebrain(left)andtheaverage NeuralynxDigitalLynxdataacquisitionsystemthroughtheACFandacustom
evokedresponsefromeachelectrode(right)withthecolourshowingthe electrodeinterfaceboard.TheboardappearsinSupplementaryFig.S11.
SupplementaryFig.S2showstheconnectedelectrodearray,ACFribbonand
ratioofthermsamplitudeofeachaverageelectroderesponseinthe
circuitboard.ThesignalsweresampledatthestandardDigitalLynxsampling
200mswindow(plotted)immediatelyafterthepresentationofthevisual
rateof32,556Hzperchannel.Thehigh-passfilterwassetat1Hzandlow-pass
stimulustothermsamplitudeoftheaverage1.5swindow(notshown) filterat9kHz.CustomMATLABsoftware(TheMathWorks)wasusedforoffline
immediatelyprecedingthestimuluspresentationfora76µm(a),2.5µm processing.Signalsweredown-sampledto2,713Hzandfurtherlow-passfilteredat
(b)and2.5µmmesh(c)electrodearray.Thestimuluspresentationoccurs 50Hz.Responseswereaveragedforeachstimulusandplottedperelectrode.
attheleftedgeoftheplottedwindow.Inallthreeimages,theoccipitalpole
Animalexperiments. Experimentswereconductedinaccordancewiththeethical
isatthebottomoftheframeandthemedialisattheright.Thescalebars guidelinesoftheNationalInstitutesofHealthandwiththeapprovalofthe
atthebottomofcindicatethespatialscalefortheleftframesandthe InstitutionalAnimalCareandUseCommitteeoftheUniversityofPennsylvania.
voltageandtimescalesfortherightframesofa–c.Thecolourbaratthe Surgicalandstimulationmethodswereasdescribedindetailpreviously29–31.Briefly,
bottomofcprovidesthescaleusedintherightframesofa–cto adultcats(2.5–3.5kg)wereanaesthetizedwithintravenousthiopentalwitha
continuousinfusion(3–10mgkg−1h−1)andparalysedwithgallaminetriethiodide
indicatethermsamplituderatios.d,Representativevoltagedatafroma
(Flaxedil).Heartrate,bloodpressure,end-tidalCO andelectroencephalograph
singleelectrodeina2.5µmmeshelectrodearrayshowinga weremonitoredthroughouttheexperimenttoassu2redepthandstabilityof
sleepspindle. anaesthesiaandrectaltemperaturewaskeptat37–38◦Cwithaheatingpad.The
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NATUREMATERIALS ARTICLES
DOI:10.1038/NMAT2745
surfaceofthevisualcortexwasexposedwithacraniotomycentredatHorsley 19. Perry,H.,Gopinath,A.,Kaplan,D.L.,Negro,L.D.&Omenetto,F.G.
Clarkeposterior4.0,lateral2.0. Nano-andmicropatterningofopticallytransparent,mechanicallyrobust,
Forvisualstimulation,thecorneaswereprotectedwithcontactlensesafter biocompatiblesilkfibroinfilms.Adv.Mater.20,3070–3072(2008).
dilatingthepupilswith1%ophthalmicatropineandretractingthenictitating 20. Murphy,A.R.,John,P.S.&Kaplan,D.L.Modificationofsilkfibroinusing
membraneswithphenylephrine(Neosynephrine).Spectaclelenseswerechosen diazoniumcouplingchemistryandtheeffectsonhMSCproliferationand
bythetapetalreflectiontechniquetooptimizethefocusofstimuliontheretina. differentiation.Biomaterials29,2829–2838(2008).
Thepositionofthemonitorwasadjustedwithanx–ystagesothatthearea 21. Altman,G.H.etal.Silk-basedbiomaterials.Biomaterials24,401–416(2003).
centralaewerecentredonthescreen.StimuliwerepresentedonanImageSystems 22. Santin,M.,Motta,A.,Freddi,G.&Cannas,M.Invitroevaluationof
(Minnetonka)modelM09LVmonochromemonitoroperatingat125framess−1at theinflammatorypotentialofthesilkfibroin.J.Biomed.Mater.Res.46,
aspatialresolutionof1,024×786pixelsandameanluminanceof47cdm−2. 382–389(1999).
23. Kim,D-H.etal.Siliconelectronicsonsilkasapathtobioresorbable,
Received 26 November 2009; accepted 10March2010; implantabledevices.Appl.Phys.Lett.95,133701–133703(2009).
24. Amsden,J.J.etal.Spectralanalysisofinducedcolourchangeonperiodically
publishedonline18April2010;correctedonline 23April2010
nanopatternedsilkfilms.Opt.Express17,21271–21279(2009).
25. Parker,S.T.etal.Biocompatiblesilkprintedopticalwaveguides.Adv.Mater.
References
21,2411–2415(2009).
1. Kim,S.etal.IntegratedwirelessneuralinterfacebasedontheUtahelectrode 26. Soong,H.K.&Kenyon,K.R.Adversereactionstovirginsilksuturesincataract
array.Biomed.Microdevices11,453–466(2009). surgery.Ophthalmology91,479–483(1984).
2. Ryu,S.I.&Shenoy,K.V.Humancorticalprostheses:Lostintranslation? 27. Chaudhury,M.K.&Whitesides,G.M.Directmeasurementof
Neurosurg.Focus27,E5(2009). interfacialinteractionsbetweensemisphericallensesandflatsheetsof
3. Andersen,R.A.,Musallam,S.&Pesaran,B.Selectingthesignalsfora poly(dimethylsiloxane)andtheirchemicalderivatives.Langmuir 7,
brain–machineinterface.Curr.Opin.Neurobiol.14,720–726(2004). 1013–1025(1991).
4. Mehring,C.etal.Inferenceofhandmovementsfromlocalfieldpotentialsin 28. Someya,T.etal.Conformable,flexible,large-areanetworksofpressureand
monkeymotorcortex.NatureNeurosci.6,1253–1254(2003). thermalsensorswithorganictransistoractivematrixes.Proc.NatlAcad.
5. Ball,T.etal.Towardsanimplantablebrain–machineinterfacebasedon Sci.USA102,12321–12325(2005).
epicorticalfieldpotentials.Biomed.Tech.49,756–759(2004). 29. Padnick,L.B.&Linsenmeier,R.A.Propertiesoftheflashvisualevoked
6. Wilson,J.A.,Felton,E.A.,Garell,P.C.,Schalk,G.&Williams,J.C. potentialrecordedinthecatprimaryvisualcortex.VisionRes.39,
ECoGfactorsunderlyingmultimodalcontrolofabrain–computerinterface. 2833–2840(1999).
IEEETrans.NeuralSyst.Rehabil.Eng.14,246–250(2006). 30. Cardin,J.A.,Palmer,L.A.&Contreras,D.Stimulusfeatureselectivityin
7. Freeman,W.J.,Rogers,L.J.,Holmes,M.D.&Silbergeld,D.L.Spatialspectral excitatoryandinhibitoryneuronsinprimaryvisualcortex.J.Neurosci.27,
analysisofhumanelectrocorticogramsincludingthealphaandgammabands. 10333–10344(2007).
J.Neurosci.Methods95,111–121(2000). 31. Cardin,J.A.,Palmer,L.A.&Contreras,D.Cellularmechanismsunderlying
8. Kellis,S.S.,House,P.A.,Thomson,K.E.,Brown,R.&Greger,B.Human stimulus-dependentgainmodulationinprimaryvisualcortexneuronsinvivo.
neocorticalelectricalactivityrecordedonnonpenetratingmicrowirearrays: Neuron59,150–160(2008).
Applicabilityforneuroprostheses.Neurosurg.Focus27,E9(2009).
9. Rubehn,B.,Bosman,C.,Oostenveld,R.,Fries,P.&Stieglitz,T.A Acknowledgements
MEMS-basedflexiblemultichannelECoG-electrodearray.J.NeuralEng. WethankT.BanksandJ.A.N.T.SoaresforhelpusingfacilitiesattheFrederick
6,036003(2009). SeitzMaterialsResearchLaboratory.Thismaterialisbasedonworksupportedbya
10. Hollenberg,B.A.,Richards,C.D.,Richards,R.,Bahr,D.F.&Rector,D.M. NationalSecurityScienceandEngineeringFacultyFellowshipandtheUSDepartment
AMEMSfabricatedflexibleelectrodearrayforrecordingsurfacefield ofEnergy,DivisionofMaterialsSciencesunderAwardNo.DEFG02-91ER45439,
potentials.J.Neurosci.Methods153,147–153(2006). throughtheFrederickSeitzMRLandCenterforMicroanalysisofMaterialsatthe
11. Yu,Z.etal.Monitoringhippocampuselectricalactivityinvitroonanelastically UniversityofIllinoisatUrbana-Champaign.Theaspectsoftheworkrelatingtosilkare
deformablemicroelectrodearray.J.Neurotrauma26,1135–1145(2009). supportedbytheUSArmyResearchLaboratoryandtheUSArmyResearchOfficeunder
12. Meacham,K.W.,Giuly,R.J.,Guo,L.,Hochman,S.&DeWeerth,S.P. contractnumberW911NF-07-1-0618andbytheDARPA-DSOandtheNIHP41Tissue
Alithographically-patterned,elasticmulti-electrodearrayforsurface EngineeringResourceCenter(P41EB002520).WorkattheUniversityofPennsylvania
stimulationofthespinalcord.Biomed.Microdev.10,259–269(2008). issupportedbytheNationalInstitutesofHealthGrants(NINDSRO1-NS041811-04,
13. Lawrence,B.D.,Cronin-Golomb,M.,Georgakoudi,I.,Kaplan,D.L.& R01NS48598-04),andtheKlingensteinFoundation.J.A.R.issupportedbyaNational
Omenetto,F.G.Bioactivesilkproteinbiomaterialsystemsforopticaldevices. ScienceSecurityandEngineeringFacultyFellowship.
Biomacromolecules9,1214–1220(2008).
Authorcontributions
14. Omenetto,F.G.&Kaplan,D.L.Anewrouteforsilk.NaturePhoton.2,
641–643(2008). D-H.K.,J.V.,J.J.A.,J.X.,L.V.,Y-S.K.,D.C.,D.L.K.,F.G.O.,Y.H,K-C.H.,M.R.Z.,B.L.and
15. Jin,H-J.etal.Water-stablesilkfilmswithreducedβ-sheetcontent.Adv.Funct. J.A.R.designedtheexperiments.D.H.K.,E.S.F.,J.V.,J.J.A.,J.X.,L.V.,Y-S.K.,B.P.and
Mater.15,1241–1247(2005). J.A.B.carriedoutexperimentsandanalysis.D-H.K.,J.V.,J.J.A.,J.X.,L.V.,J.A.B.,D.C.,
16. Lu,Q.etal.Water-insolublesilkfilmswithsilkIstructure.ActaBiomater.6, D.L.K.,F.G.O.,Y.H.,B.L.andJ.A.R.wrotethepaper.
1380–1387(2009).
Additionalinformation
17. Jiang,C.etal.Mechanicalpropertiesofrobustultrathinsilkfibroinfilms.
Adv.Funct.Mater.17,2229–2237(2007). Theauthorsdeclarenocompetingfinancialinterests.Supplementaryinformation
18. Sofia,S.,McCarthy,M.B.,Gronowicz,G.&Kaplan,D.L.Functionalized accompaniesthispaperonwww.nature.com/naturematerials.Reprintsandpermissions
silk-basedbiomaterialsforboneformation.J.Biomed.Mater.Res.54, informationisavailableonlineathttp://npg.nature.com/reprintsandpermissions.
139–148(2001). CorrespondenceandrequestsformaterialsshouldbeaddressedtoB.L.orJ.A.R.
Dae-HyeongKim1†,JonathanViventi2†,JasonJ.Amsden3,JianliangXiao4,LeifVigeland5,Yun-SoungKim1,JustinA.Blanco2,
BrucePanilaitis3,EricS.Frechette6,DiegoContreras5,DavidL.Kaplan3,FiorenzoG.Omenetto3,YonggangHuang4,
Keh-ChihHwang7,MitchellR.Zakin8,BrianLitt2,6*andJohnA.Rogers1*
1DepartmentofMaterialsScienceandEngineering,BeckmanInstituteforAdvancedScienceandTechnologyandFrederickSeitzMaterialsResearch
Laboratory,UniversityofIllinoisatUrbana-Champaign,Urbana,Illinois61801,USA,2DepartmentofBioengineering,UniversityofPennsylvania,
Philadelphia,Pennsylvania19104,USA,3DepartmentofBiomedicalEngineering,TuftsUniversity,Medford,Massachusetts02155,USA,4Departmentof
MechanicalEngineeringandDepartmentofCivilandEnvironmentalEngineering,NorthwesternUniversity,Evanston,Illinois60208,USA,5Department
ofNeuroscience,UniversityofPennsylvaniaSchoolofMedicine,215StemmlerHall,Philadelphia,Pennsylvania19104,USA,6DepartmentofNeurology,
HospitaloftheUniversityofPennsylvania,3WestGates,3400SpruceStreet,Philadelphia,Pennsylvania19104,USA,7AML,DepartmentofEngineering
Mechanics,TsinghuaUniversity,Beijing100084,China,8DefenseAdvancedResearchProjectsAgency,Arlington,Virginia22203,USA.†Theseauthors
contributedequallytothiswork.*e-mail:[email protected];[email protected].
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