fabrication of the passive electrode arrays began with solid state phosphorus doping ( ph-1000n source , saint gobain , usa , 1000 c for 10 minutes ) of p - type device si on a si on insulator ( soi , top si ~300 nm , soitec , france ) wafer . removing the buried oxide layer of the soi by wet etching with concentrated hf released the device si as a si nm , retrieved with a slab of the elastomer poly(dimethylsiloxane ) ( pdms ) and transfer - printed to a spin - cast bilayer of poly(methylmethacrylate ) ( pmma , ~800 nm thick ) and polyimide ( pi , ~300 nm thick ) on a si wafer . photolithography and reactive ion etching ( rie ) defined a pattern of electrodes and interconnects in the si nms . plasma enhanced chemical vapor deposition ( pecvd ) formed a layer of sio2 ( thickness 100 nm ) as encapsulation . patterned etching with buffered oxide etchant removed the sio2 from the electrode regions . spin casting and patterning a top coating of pi ( ~300 nm thick ) placed the si nm electrodes and interconnects near the neutral mechanical plane . patterning a mesh structure across the multilayer ( i.e. pi , sio2 , pi and pmma ) by rie followed by immersion in buffered oxide etchant exposed the base layer of pmma to allow its dissolution in acetone . retrieval onto a slab of pdms enabled removal of the bottom exposed layer of pi by rie . transfer onto a film of plga ( ~30 m thick ) , facilitated by heating to temperatures close to the glass transition of the plga ( 55~60 c , lactide / glycolide ratio of 75:25 composition ) , followed by elimination of the top layer of pi by rie completed the fabrication . bonding an acf cable to the terminal regions of the si nm interconnects yielded connection points for interfaces to external data acquisition ( daq ) systems . the fabrication began with growth of 200 nm of thermal oxide on a p - type soi wafer ( top si ~320 nm , soitec ) , photolithography and immersion in buffered oxide etchant to create a mask for solid state phosphorus diffusion ( 1000 c for 6 minutes ) to define the source and drain contacts . releasing , retrieving and transferring the doped si nms onto a temporary substrate , consisting of si wafer with a bilayer coating of pi / pmma , followed procedures similar to those described for passive electrode arrays . photolithography and rie etching patterned the si nms into geometries for an 88 array of unit cells , each consisting of two transistors connected in series for purpose of actively multiplexed readout . a thin layer of sio2 ( pecvd at 220 c , thickness ~100 nm ) served as the gate dielectric . buffered oxide etching through a photolithographically patterned mask formed openings through the sio2 to expose the source and drain contact regions . photolithography and lift off in acetone defined a patterned layer of mo ( sputter deposited , thickness ~300 nm ) for the gate electrodes and metal interconnects . deposition of a trilayer of sio2 ( ~300 nm)/si3n4 ( ~400 nm)/sio2 ( ~300 nm ) by pecvd formed the interlayer dielectric . photolithography and buffered oxide etching created vertical interconnect access ( via ) holes for electrical connections between layers . an additional layer of mo ( thickness ~300 nm ) patterned by photolithography and liftoff defined column select lines . another trilayer of sio2 ( ~300 nm)/si3n4 ( ~400 nm)/sio2 ( ~300 nm ) served as encapsulation , with openings at the locations of the sensing electrodes and peripheral contact pads for interfacing to an external daq system . selective rie and buffered oxide etching through these multilayer stacks ( diluted pi / trilayers of inorganic materials / trilayers of inorganic materials / diluted pi ) formed mesh structures that enabled release of active layers from the temporary substrate by dissolving the pmma layer in acetone . transfer printing steps followed , according to procedures similar to those for the passive electrode fabrication . . 2 is representative of four different acute experiments , each of which lasted 5 - 6 hours . the procedures , which were approved by the institutional care and use committee of the university of pennsylvania , involved an anaesthetized rat with its head fixed in a stereotaxic apparatus . the animal was anesthetized with initially ketamine / xylazine and then isoflurane throughout the craniotomy and neural recordings . a craniotomy exposed a 48 mm region of cortex in either left or tight or in both hemispheres . all recordings were taken in reference to a distant stainless steel bone screw inserted through the skull during the surgery . a commercial stainless steel microwire electrode ( ~100 um stainless steel wire from california fine wire ) placed at 0.5 mm depth from the cortical surface in close proximity to the bioresorbable electrodes served as a control during acute recordings . neural data was acquired by a fhc multi - channel neural amplifier ( fhc inc , bowdoin , me , usa ) and an acquisition system ( 16 bit axon instruments digidata 1322a , axon instruments , foster city , ca ) . neural recording data were analyzed offline using clampfit software ( axon instruments ) and custom matlab software for neural signal analysis . the following procedures were approved by the institutional care and use committee of the university of pennsylvania . one 150-g , sprague - dawley rat was anesthetized with a ketamine ( 60 mg / kg ) , dexdomitor ( 0.25 mg / kg ) solution and placed in a stereotaxic frame . a skull screw was placed in the left frontal bone to serve as the reference electrode for the recordings . a pair of needle stimulating electrode were inserted into the left mystacial pad at various locations . brief electrical currents ( ~250 - 600 a , 1 ms / phase , biphasic pulse ) were passed between the electrodes to activate the intrinsic muscles of the vibrissae , causing a visible protraction of the whiskers . current amplitude and electrode spacing was adjusted for focal activation , usually 1 - 4 whiskers . an adult long evans rat was anesthetized with isoflurane and placed in a stereotactic frame ( david kopf instruments tujunga , ca ) . body temperature was maintained with a heating blanket and the eyes were covered with ointment to prevent drying . the skull was exposed and a large craniotomy ( 4  8 mm ) was made between bregma and lambda and laterally to the midline . the electrode was placed on the exposed dura and a slurry of gel foam and saline was layered on top of the electrode . a screw electrode was placed contralaterally to the experimental array , with another such electrode placed posterior to lambda as a ground and reference . the skull and electrodes were then covered with dental cement and the connecting plug was secured on top . the rat was given meloxicam for postoperative pain and allowed to recover on a heating pad . after 1 week the animal was placed in a cage for video / eeg recording . eeg signals were collected continuously from 3 channels on the array and from the screw . the signals were amplified and low pass filtered at 600 hz ( multichannel systems , reutlingen , germany ) and sampled at 2000 hz with a 16 bit digitizer ( national instruments , austin , tx ) . rats ( n=14 ) were anesthetized and transcardiac perfusion was performed using phosphate buffered saline ( pbs 10x , cat . # bm-220 , boston bioproducts , ashland , ma ) , followed by 4% paraformaldehyde ( pfa , cat . whole brains were then removed and post - fixed overnight at 4c in the same 4% pfa solution . # 57 - 50 - 1 , sigma - aldrich , saint louis , mo ) at 4c and coronal sections were cut at 20 m using a leica cm3050 s cryostat ( leica biosystems inc . ) . serial sections , spanning the entire craniotomy site , were mounted on charged slides and stored at 20c until use . for immunostaining , slides were first immersed in an antigen retrieval solution ( 0.1 m citrate buffer , ph 6.0 , cat # ab64214 , abcam , cambridge , ma ) and placed in a water bath at 95c for 10 minute . after cooling , sections were rinsed in distilled water , incubated in a blocking solution containing 0.1% triton x-100 ( cat . # 9002 - 93 - 1 , sigma - aldrich ) and 5% normal goat serum ( cat . kerrville , tx ) for one hour at room temperature ( rt ) and then incubated overnight at 4 c with the following primary antibodies : anti- glial fibrillary acidic protein ( gfap , 1:1000 , cat . # smi-22r , covance , princeton , nj ) , and anti - ionized calcium binding adapter molecule 1 ( iba-1 , 1:1000 , cat . after 320-minute washes in pbs , sections were incubated with the corresponding fluorescent secondary antibodies ( alexa fluor 488 goat anti - mouse igg2b , 1:1000 , cat . # a-21141 , and alexa fluor 568 goat anti - rabbit igg , 1:1000 , cat . after the final washes ( 3  20 min in pbs ) , the slides were cover - slipped with an anti - fade medium containing the nuclear stain dapi ( fluoromount - g+dapi , cat . control sections were incubated with omission of one or both primary antibodies , adding only the secondary antibodies to exclude false - positive labeling . slides were examined on an epifluorescence microscope ( zeiss axioscope , germany ) and images were acquired with a 20 objective and a spot rt3 digital camera , using the spot software 5.1 ( diagnostic instruments , sterling heights , mi ) . digital images were processed using adobe photoshop 12.0 ( adobe systems , san jose , ca ) . fabrication of the passive electrode arrays began with solid state phosphorus doping ( ph-1000n source , saint gobain , usa , 1000 c for 10 minutes ) of p - type device si on a si on insulator ( soi , top si ~300 nm , soitec , france ) wafer . removing the buried oxide layer of the soi by wet etching with concentrated hf released the device si as a si nm , retrieved with a slab of the elastomer poly(dimethylsiloxane ) ( pdms ) and transfer - printed to a spin - cast bilayer of poly(methylmethacrylate ) ( pmma , ~800 nm thick ) and polyimide ( pi , ~300 nm thick ) on a si wafer . photolithography and reactive ion etching ( rie ) defined a pattern of electrodes and interconnects in the si nms . plasma enhanced chemical vapor deposition ( pecvd ) formed a layer of sio2 ( thickness 100 nm ) as encapsulation . patterned etching with buffered oxide etchant removed the sio2 from the electrode regions . spin casting and patterning a top coating of pi ( ~300 nm thick ) placed the si nm electrodes and interconnects near the neutral mechanical plane . patterning a mesh structure across the multilayer ( i.e. pi , sio2 , pi and pmma ) by rie followed by immersion in buffered oxide etchant exposed the base layer of pmma to allow its dissolution in acetone . retrieval onto a slab of pdms enabled removal of the bottom exposed layer of pi by rie . transfer onto a film of plga ( ~30 m thick ) , facilitated by heating to temperatures close to the glass transition of the plga ( 55~60 c , lactide / glycolide ratio of 75:25 composition ) , followed by elimination of the top layer of pi by rie completed the fabrication . bonding an acf cable to the terminal regions of the si nm interconnects yielded connection points for interfaces to external data acquisition ( daq ) systems . the fabrication began with growth of 200 nm of thermal oxide on a p - type soi wafer ( top si ~320 nm , soitec ) , photolithography and immersion in buffered oxide etchant to create a mask for solid state phosphorus diffusion ( 1000 c for 6 minutes ) to define the source and drain contacts . releasing , retrieving and transferring the doped si nms onto a temporary substrate , consisting of si wafer with a bilayer coating of pi / pmma , followed procedures similar to those described for passive electrode arrays . photolithography and rie etching patterned the si nms into geometries for an 88 array of unit cells , each consisting of two transistors connected in series for purpose of actively multiplexed readout . a thin layer of sio2 ( pecvd at 220 c , thickness ~100 nm ) served as the gate dielectric . buffered oxide etching through a photolithographically patterned mask formed openings through the sio2 to expose the source and drain contact regions . photolithography and lift off in acetone defined a patterned layer of mo ( sputter deposited , thickness ~300 nm ) for the gate electrodes and metal interconnects . deposition of a trilayer of sio2 ( ~300 nm)/si3n4 ( ~400 nm)/sio2 ( ~300 nm ) by pecvd formed the interlayer dielectric . photolithography and buffered oxide etching created vertical interconnect access ( via ) holes for electrical connections between layers . an additional layer of mo ( thickness ~300 nm ) patterned by photolithography and liftoff defined column select lines . another trilayer of sio2 ( ~300 nm)/si3n4 ( ~400 nm)/sio2 ( ~300 nm ) served as encapsulation , with openings at the locations of the sensing electrodes and peripheral contact pads for interfacing to an external daq system . selective rie and buffered oxide etching through these multilayer stacks ( diluted pi / trilayers of inorganic materials / trilayers of inorganic materials / diluted pi ) formed mesh structures that enabled release of active layers from the temporary substrate by dissolving the pmma layer in acetone . transfer printing steps followed , according to procedures similar to those for the passive electrode fabrication . 2 is representative of four different acute experiments , each of which lasted 5 - 6 hours . the procedures , which were approved by the institutional care and use committee of the university of pennsylvania , involved an anaesthetized rat with its head fixed in a stereotaxic apparatus . the animal was anesthetized with initially ketamine / xylazine and then isoflurane throughout the craniotomy and neural recordings . a craniotomy exposed a 48 mm region of cortex in either left or tight or in both hemispheres . all recordings were taken in reference to a distant stainless steel bone screw inserted through the skull during the surgery . a commercial stainless steel microwire electrode ( ~100 um stainless steel wire from california fine wire ) placed at 0.5 mm depth from the cortical surface in close proximity to the bioresorbable electrodes served as a control during acute recordings . neural data was acquired by a fhc multi - channel neural amplifier ( fhc inc , bowdoin , me , usa ) and an acquisition system ( 16 bit axon instruments digidata 1322a , axon instruments , foster city , ca ) . neural recording data were analyzed offline using clampfit software ( axon instruments ) and custom matlab software for neural signal analysis . the following procedures were approved by the institutional care and use committee of the university of pennsylvania . one 150-g , sprague - dawley rat was anesthetized with a ketamine ( 60 mg / kg ) , dexdomitor ( 0.25 mg / kg ) solution and placed in a stereotaxic frame . a skull screw was placed in the left frontal bone to serve as the reference electrode for the recordings . a pair of needle stimulating electrode were inserted into the left mystacial pad at various locations . brief electrical currents ( ~250 - 600 a , 1 ms / phase , biphasic pulse ) were passed between the electrodes to activate the intrinsic muscles of the vibrissae , causing a visible protraction of the whiskers . current amplitude and electrode spacing was adjusted for focal activation , usually 1 - 4 whiskers . an adult long evans rat was anesthetized with isoflurane and placed in a stereotactic frame ( david kopf instruments tujunga , ca ) . body temperature was maintained with a heating blanket and the eyes were covered with ointment to prevent drying . the skull was exposed and a large craniotomy ( 4  8 mm ) was made between bregma and lambda and laterally to the midline . the electrode was placed on the exposed dura and a slurry of gel foam and saline was layered on top of the electrode . a screw electrode was placed contralaterally to the experimental array , with another such electrode placed posterior to lambda as a ground and reference . the skull and electrodes were then covered with dental cement and the connecting plug was secured on top . the rat was given meloxicam for postoperative pain and allowed to recover on a heating pad . after 1 week the animal was placed in a cage for video / eeg recording . eeg signals were collected continuously from 3 channels on the array and from the screw . the signals were amplified and low pass filtered at 600 hz ( multichannel systems , reutlingen , germany ) and sampled at 2000 hz with a 16 bit digitizer ( national instruments , austin , tx ) . rats ( n=14 ) were anesthetized and transcardiac perfusion was performed using phosphate buffered saline ( pbs 10x , cat . # bm-220 , boston bioproducts , ashland , ma ) , followed by 4% paraformaldehyde ( pfa , cat . whole brains were then removed and post - fixed overnight at 4c in the same 4% pfa solution . # 57 - 50 - 1 , sigma - aldrich , saint louis , mo ) at 4c and coronal sections were cut at 20 m using a leica cm3050 s cryostat ( leica biosystems inc . ) . serial sections , spanning the entire craniotomy site , were mounted on charged slides and stored at 20c until use . for immunostaining , slides were first immersed in an antigen retrieval solution ( 0.1 m citrate buffer , ph 6.0 , cat # ab64214 , abcam , cambridge , ma ) and placed in a water bath at 95c for 10 minute . after cooling , sections were rinsed in distilled water , incubated in a blocking solution containing 0.1% triton x-100 ( cat . # 9002 - 93 - 1 , sigma - aldrich ) and 5% normal goat serum ( cat . # gs-0500 , equitech - bio inc . , kerrville , tx ) for one hour at room temperature ( rt ) and then incubated overnight at 4 c with the following primary antibodies : anti- glial fibrillary acidic protein ( gfap , 1:1000 , cat . # smi-22r , covance , princeton , nj ) , and anti - ionized calcium binding adapter molecule 1 ( iba-1 , 1:1000 , cat . after 320-minute washes in pbs , sections were incubated with the corresponding fluorescent secondary antibodies ( alexa fluor 488 goat anti - mouse igg2b , 1:1000 , cat . # a-21141 , and alexa fluor 568 goat anti - rabbit igg , 1:1000 , cat . # a-11011 , invitrogen by life technologies , grand island , ny ) . after the final washes ( 3  20 min in pbs ) , the slides were cover - slipped with an anti - fade medium containing the nuclear stain dapi ( fluoromount - g+dapi , cat . control sections were incubated with omission of one or both primary antibodies , adding only the secondary antibodies to exclude false - positive labeling . slides were examined on an epifluorescence microscope ( zeiss axioscope , germany ) and images were acquired with a 20 objective and a spot rt3 digital camera , using the spot software 5.1 ( diagnostic instruments , sterling heights , mi ) . digital images were processed using adobe photoshop 12.0 ( adobe systems , san jose , ca ) .
bioresorbable silicon electronics technology offers unprecedented opportunities to deploy advanced implantable monitoring systems that eliminate risks , cost and discomfort associated with surgical extraction . 
 applications include post - operative monitoring and transient physiologic recording after percutaneous or minimally invasive placement of vascular , cardiac , orthopedic , neural or other devices . 
 we present an embodiment of these materials in both passive and actively addressed arrays of bioresorbable silicon electrodes with multiplexing capabilities , that record in vivo electrophysiological signals from the cortical surface and the subgaleal space . 
 the devices detect normal physiologic and epileptiform activity , both in acute and chronic recordings . 
 comparative studies show sensor performance comparable to standard clinical systems and reduced tissue reactivity relative to conventional clinical electrocorticography ( ecog ) electrodes . 
 this technology offers general applicability in neural interfaces , with additional potential utility in treatment of disorders where transient monitoring and modulation of physiologic function , implant integrity and tissue recovery or regeneration are required .