Acta Biomaterialia 10 (2014) 960–967

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Acta Biomaterialia

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Lifetime assessment of atomic-layer-deposited Al2O3–Parylene C bilayercoating for neural interfaces using accelerated age testing andelectrochemical characterization

1742-7061/$ - see front matter � 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.actbio.2013.10.031

⇑ Corresponding authors. Tel.: +1 240 5154419 (S. Minnikanti). Address: Electri-cal and Computer Engineering Department, George Mason University, 4400University Dr. MSN 1G5, Fairfax, VA 22030, USA. Tel.: +1 703 7283083 (N. Peixoto).

E-mail addresses: sminnika@masonlive.gmu.edu (S. Minnikanti), npeixoto@gmu.edu (N. Peixoto).

Saugandhika Minnikanti a,⇑, Guoqing Diao b, Joseph J. Pancrazio a,c, Xianzong Xie d, Loren Rieth d,Florian Solzbacher d, Nathalia Peixoto a,c,⇑a Electrical and Computer Engineering Department, George Mason University, 4400 University Dr. MSN 1G5, Fairfax, VA 22030, USAb Department of Statistics, George Mason University, 4400 University Dr., Fairfax, VA 22030, USAc Bioengineering Department, George Mason University, 4400 University Dr. MSN 1G5, Fairfax, VA 22030, USAd Electrical and Computer Engineering, University of Utah, 50 S. Central Campus Dr., Salt Lake City, UT 84112, USA

a r t i c l e i n f o

Article history:Received 13 June 2013Received in revised form 18 October 2013Accepted 24 October 2013Available online 1 November 2013

Keywords:Parylene CAl2O3

Electrochemical impedance spectroscopyAccelerated lifetime testingInterdigitated electrode arrays

a b s t r a c t

The lifetime and stability of insulation are critical features for the reliable operation of an implantableneural interface device. A critical factor for an implanted insulation’s performance is its barrier propertiesthat limit access of biological fluids to the underlying device or metal electrode. Parylene C is a materialthat has been used in FDA-approved implantable devices. Considered a biocompatible polymer with bar-rier properties, it has been used as a substrate, insulation or an encapsulation for neural implant technol-ogy. Recently, it has been suggested that a bilayer coating of Parylene C on top of atomic-layer-depositedAl2O3 would provide enhanced barrier properties. Here we report a comprehensive study to examine themean time to failure of Parylene C and Al2O3–Parylene C coated devices using accelerated lifetime testing.Samples were tested at 60 �C for up to 3 months while performing electrochemical measurements tocharacterize the integrity of the insulation. The mean time to failure for Al2O3–Parylene C was 4.6 timeslonger than Parylene C coated samples. In addition, based on modeling of the data using electrical circuitequivalents, we show here that there are two main modes of failure. Our results suggest that failure of theinsulating layer is due to pore formation or blistering as well as thinning of the coating over time. Theenhanced barrier properties of the bilayer Al2O3–Parylene C over Parylene C makes it a promising candi-date as an encapsulating neural interface.

� 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Implantable neuroprosthetic devices offer the promise ofrestoring neurological function to disabled individuals [1–3]. Priorwork has demonstrated that arrays of microelectrodes implantedin the motor cortex yield single unit and local field potentialrecordings that encode information on intended movement [4].By capturing electrical signals corresponding to volitional move-ment, these devices have clinical implications as communicationdevices for locked-in patients [5], as well as control signal sourcesfor paralyzed muscles on prosthetic limbs.

Currently implantable electrode arrays under investigation canbe categorized into three basic structures [6]: arrays built of micro-

wires [7], silicon-based micromachined [8] and flexible polymericarrays [9]. Substrates for microwire-based arrays are usually tung-sten [10], stainless steel [11,12], platinum [13], platinum–iridium[14] or gold [15]. The insulation materials used for microwiresare Teflon [16], polyimide [10] or Parylene C [17]. The two mostpopular silicon-based structures are the Utah electrode array(UEA) [18] and Michigan electrode arrays [19]. UEAs comprise abed of electrically isolated conductive silicon needles. The tips ofthe silicon needles are coated with either platinum or iridiumoxide. Michigan arrays are planar structures with electrode sites(Ir/Pt/Au) on a silicon-based shank. Commonly used insulationfor silicon-based arrays are polyimide [20] or Parylene C [21]. Flex-ible multielectrode arrays are non-silicon-based structures wherethe metallic substrate is sandwiched between layers of a polymersuch as polyimide [9,22] or Parylene [23].

A major barrier for clinical translation is the lack of device reli-ability. Microwire arrays typically fail 18 months after implantation[7]. Nevertheless, implanted microwires have been shown to mea-sure neuronal activity successfully for a period of 7 years. However,

S. Minnikanti et al. / Acta Biomaterialia 10 (2014) 960–967 961

over time the measurable single unit activity reduced, with moreelectrodes detecting signals from multiple neurons [24]. Theauthors mentioned the possibility of an increase in electrode sur-face area [24], probably caused by changes in the insulation coating.While device performance depends on the tissue response [25],there is a growing recognition that device materials may not with-stand exposure to the harsh ionic environment in vivo [26].

A widely used encapsulation layer for implantable devices isParylene C (poly(dichloro-p-xylylene)) [27-30], a semicrystallinepolymer that belongs to the family of thermoplasts known as poly-paraxylylene (PPX). Parylene C has several advantageous proper-ties including a low dielectric constant and non-cytotoxicity [31],and can be deposited as a conformal, pinhole-free film to accom-modate extreme contours including sharp edges and crevices[32]. Nevertheless, failure of Parylene C as an encapsulation andinsulator has been reported [33]. In fact, long term in vivo mea-surements with Parylene C coated microelectrode arrays revealeddecreased impedance and concurrent bioelectrical signal loss, sug-gesting degradation of the insulating layer [5].

Since Parylene C is a common choice as a chronic implantablepolymer [34], age accelerated lifetime testing (ALT) of Parylene Con different substrates and structures has been investigated byvarious groups, with reported lifetimes from 6 months [35] to1 year [21]. Parylene C coating, although an excellent insulator,has poor adhesion properties towards inorganic and metallic sub-strates [34,36]. Various techniques such as adhesion promoterprimers [33], chemical modification of the polymer (reactive Paryl-ene C) [37] and thermal [38] and plasma treatments [36] are usedto improve its adhesion to the underlying substrate. The conceptbehind treatments based on primer (Silane), plasma or chemicalmodification is to introduce or increase radical sites available forcovalent bonding [39] between Parylene C and substrate. Heattreatments on the other hand anneal Parylene C to increase theefficiency of the physical interlock with the substrate [40,41]. Inaddition, Parylene C coatings are permeable to water diffusion ata rate of 15 lm min�1 [42]. In an effort to improve encapsulationperformance, Xie et al. recently reported a bi-layer encapsulationscheme which combines atomic-layer-deposited (ALD) Al2O3 fol-lowed by Parylene C [43]. Thin Al2O3 coatings are reported as giv-ing pinhole-free [43] and conformal coverage, even on roughsurfaces [44]. This combination of Al2O3 and Parylene C createsdual moisture barriers and may enhance device lifetime, as sug-gested by preliminary accelerated aging experiments [43].

In this paper, we present a comprehensive comparison of Paryl-ene C and ALD-Al2O3–Parylene C encapsulation lifetimes underaccelerated aging conditions. We use electrochemical methodsincluding electrochemical impedance spectroscopy (EIS), leakagecurrent analysis and cyclic voltammetry on coated interdigitatedelectrodes (IDEs) to monitor electrochemical correlates of insulat-ing layer integrity. Our results demonstrate a statistically signifi-cant improvement in encapsulation layer integrity with ALD-Al2O3–Parylene C over Parylene C alone. In addition electricalequivalent models of the electrode insulation electrolyte interfaceoffer insights into two distinct mechanisms of device failure overtime. Lastly, our work establishes a statistical framework for com-paring insulating layer lifetimes in future studies.

2. Material and methods

2.1. Materials

IDE test structures were fabricated by our group at the Univer-sity of Utah and the fabrication process has been described else-where [43]. The IDE structure consists of 500 lm thick fusedsilica substrates (2.7 � 0.6 cm2) with electrode width and pitch of

130 lm (Fig. 1). Owing to its higher resistivity compared with sil-icon substrate, fused silica with a relative thin oxide layer is recom-mended in order to isolate the performance of Parylene C [45]. Theelectrodes are layered of Ti (100 nm)/Pt(150 nm)/Au(150 nm).Electrical access to the IDEs was provided by soldering wires tothe contact pads. A thin layer of Al2O3 (52 nm) covering the entiresurface of the IDE was deposited using plasma-assisted atomiclayer deposition. The Al2O3 layer was then silanized with gas-phaseadhesion promoter Silane A-174, followed by a 6 lm thick Paryl-ene C layer deposited using the standard Gorham process [46].The control IDE test structures were coated only with adhesionpromoter Silane A-174 and 6 lm Parylene C. A 6 ml glass vialserved as the sample holder where the IDEs were sealed to the vialcap.

2.2. Accelerated lifetime testing (ALT)

The process for ALT is presented in Fig. 2. Samples from eachcoating, Parylene C and Al2O3 Parylene C, were initially inspectedin air and in phosphate buffered saline (PBS (1�), pH 7.4, 2.7 mMKCl and 137 mM NaCl) at room temperature to rule out false pos-itives due to fabrication, handling or transportation damage. Thistest was followed by samples being age-accelerated in vials con-taining PBS. Vials were sealed and kept at 60 �C in a thermal bath.A USB temperature sensor (Go!Temp, from Vernier, Beaverton, OR)was enclosed in a separate PBS-filled 6 ml vial and also placed inthe thermal bath. The temperature was monitored via Logger Litesoftware (Vernier, Beaverton, OR) and variations were below ± 2 �Cthroughout the experiment (for 3 months). Silicone oil was used inthe bath to avoid evaporation. The samples were completely sub-merged in PBS throughout the experiment. PBS (pH 7.4) is thephysiological media for biological investigations and is used tradi-tionally for impedance measurements of neural electrodes [47].The glass transition temperature (Tg) for Parylene C as reportedin the literature varies between 55 and 95 �C [48]. ASTM F1980(American Society for Testing and Materials Standard guide foraccelerated aging of sterile medical device packages) recommendsthat aging temperature do not exceed 60 �C to avoid non-linearvariations in the rate of reaction [49], therefore we maintained aconstant 60 �C during our experiments. As high impedance mea-surements are prone to noise, all measurements were conductedwithin a Faraday cage. Electrochemical testing was only inter-rupted for �30 min every 2 weeks to replace PBS in the vials.

2.3. Electrochemical characterization

Electrochemical characterization of IDE structures consisted ofmeasuring EIS, DC leakage current and cyclic voltammetry (CV)across the fingers of the IDE structure. A 16 channel CHI660D(CH instruments Inc., Austin, TX) potentiostat was used for all elec-trochemical testing at room temperature and at 60 �C. A two-elec-trode setup was employed where counter and reference terminalsof the potentiostat were shorted and connected to one of the IDEfingers (Fig. 1). The working and working sense were connectedto the second IDE finger. PBS (pH 7.4) was used to fill the 6 mlIDE sample holders. The frequency range chosen for EIS was from0.01 Hz to 10 kHz with a 50 mVrms AC sinusoidal waveform. Themeasured frequencies were 12 points per decade on a logarithmicscale, and each impedance reported as the average over three cy-cles of the AC sinusoid. Chronoamperometry was used for measur-ing leakage currents. The technique involves applying a step pulse,in this case 5 V for 150 s, and the measured response was consid-ered as DC leakage current. The DC current was recorded with asample interval of 15 s. In the case of CV, the excitation voltagewas ramped from �0.6 to 0.6 V at a rate of 50 mV s–1.

Fig. 1. The interdigitated electrode structure consists of Ti/Pt/Au microelectrodes coated with either Al2O3–Parylene C (52 nm/6 lm) or Parylene C (6 lm). The width andpitch of the interdigitated electrodes are both 130 lm. Wires connected to the solder bond pads provide electrical connections to the IDE structure. Electrochemicalcharacterization is performed using a two-electrode setup where the working electrode/working sense (WE/WS) and the reference electrode/counter electrode (RE/CE)terminals of the potentiostat are connected to the two fingers of the IDE.

Fig. 2. Process for accelerated lifetime testing of Al2O3–Parylene C (52 nm/6 lm) and Parylene C samples (6 lm). The samples are initially inspected via electrochemicalcharacterization (EIS and DC leakage currents) in air and immersed in PBS. All samples that pass the room temperature inspection are placed in the thermal bath for ageacceleration. Electrochemical characterization (EIS, DC leakage currents and CVs) are measured every 6 h throughout the course of the experiment. This is followed by MTTFestimation as well as equivalent circuit modeling of the measured EIS spectra.

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2.4. Equivalent circuit modeling

As the insulation coating fails the impedance decreases, and dy-namic transitions in phase are observable in the measured EISspectra [50]. Changes in electrical equivalent circuits based onthe EIS spectra provide insight into the possible mechanism of fail-ure [51]. In these experiments, the IDEs were connected such thatthe IDE fingers served as the working and reference electrode. Withthis setup, the lateral impedance between the coated IDE fingerswas measured. An intact insulation can be modeled as a constantphase element (QL) in parallel with a layer resistance (RL) [51].Thus, the parallel combination of QL and RL was used to modelthe intact coating covering the IDE fingers, as shown by the insetof Fig. 3a. The impedance of the CPE is mathematically expressedas ZCPE = Qo

�1(jx)�a, where Qo is a constant, j =p�1, x = 2pf and

a is a constant between 0 and 1. When a = 1, Qo acts as an idealcapacitor [52]. The CPE accounts for the frequency-dependentcapacitance seen in the experimental data, which is attributed tosurface inhomogeneity and to the non-ideal dielectric characteris-tics of the insulation [53]. RL accounts for any conductive pathwaysthrough the coating. As the CPE constant accounts for the layercapacitance, henceforth it will be referred to as QL.

We used ZSimpWin (EChem Software, Ann Arbor, MI) to fit theEIS data to equivalent circuit models. ZSimpWin employs thedown-hill simplex algorithm to optimize the fits by evaluatingthe impedance function until a local minimum is reached. Thissoftware also provides good initial estimates, which are highlyessential in regression-based fits. All fits used in this study havechi-squared values lower than 0.001, and error in parameter esti-mates lower than 20%.

2.5. Mean time to failure (MTTF)

Three metrics were used here to define insulation failure: (a)leakage current above 1 nA, (b) impedance modulus below0.1 GX for any frequency greater than 1 Hz and (c) impedancephase larger than �80� for any frequency greater than 1 Hz. Thesecriteria were derived from our preliminary experiments as well asfrom the literature [43,54]. In our experiments, we noticed that the1 pA leakage current was typical of insulation that was not failing.Many devices withstood leakage currents of 1 pA for over4 months. Once failure was noticed through the impedance spec-troscopy, the leakage current concomitantly increased, suggestingthe underlying metal was accessible to the electrolyte, establishinga low resistance path between the IDE fingers. For the purposes ofthis paper, a ‘‘failed’’ device or insulation samples complied withthe three metric thresholds. The time to failure for the insulatinglayers was estimated from the time stamp on the initial EIS filespresenting failed spectra. An exponential distribution was assumedfor the failure times for Parylene C and Al2O3–Parylene C IDE sam-ple groups at 60 �C. The MTTF for each sample was expressed asmean ± standard error of the mean (SEM).

This was followed by a hypothesis testing to compare the MTTFbetween the two IDE groups. Specifically, we test the null hypoth-esis being equal MTTF between Parylene C (l1) and Al2O3–ParyleneC (l2) against the one-sided alternative of l2 > l1. All samples thatsurvived the 3 month accelerated aging were included as right cen-sored data. This accounts for the times of failure of the survivingIDEs as being ‘‘right’’ or greater than the 3 month ALT period. Next,the information from this study was used to determine samplesizes for future studies. Specifically, we determined sample sizes

Fig. 3. Complex impedance (Nyquist plots) plots for (a) an intact and (b) a failedcoating with respective equivalent circuit models in inset. The solid line indicatesthe calculated data from the model as proposed in this paper, while the symbol (⁄)represents the calculated impedance with frequency as a parameter, varying from10 mHz to 10 kHz. The intact coating equivalent circuit (a, inset) is composed of QL

(capacitance of the insulation) in parallel with RL (resistance of the insulation). Asthe insulation degrades, the electrolyte penetrates and the underlying metal isexposed, giving rise to the second semicircle in the Nyquist plot (b). The failedequivalent circuit model (b, inset) includes RP (polarization resistance), Cdl, (doublelayer capacitance at the exposed metal interface) and W (Warburg impedance,accounting for diffusion of ions across the interface).

Fig. 4. DC leakage current plotted against time for a stable Al2O3 –Parylene C(52 nm/6 lm) coated sample. Each data point is an average of 180 s of leakagecurrent while 5 VDC is applied across the two electrodes of the same IDE. The samplepresents low DC leakage currents with occasional increase, well below thethreshold of 1000 pA. The sample is kept at 60 �C throughout the experiment.

S. Minnikanti et al. / Acta Biomaterialia 10 (2014) 960–967 963

necessary to achieve a power of 80% at the significance level of 0.05to detect an effect size by using a log-rank test. The effect size usedin the hypothesis testing and sample calculation is the ratio of (l1/l2).

The following formula and assumptions were used to extrapo-late the MTTF to body temperature (37 �C): Q10 = 2 (a 10 �C in-crease in temperature doubles rate of the chemical reaction),TAA = 60 �C (accelerated aging temperature) and TRS = 37 �C (rec-ommended shelf temperature–body temperature). The simulatedage at 37 �C is calculated using the equation below [55]:

Age37 �C ¼ ðAge60 �CÞ � Q ½TAA�TRT �=1010 ð1Þ

Fig. 5. DC leakage current plotted against age-accelerated (60 �C) time of a failingAl2O3–Parylene C (52 nm/6 lm) coated sample. Each data point is an average of150 s of leakage current while 5 VDC is applied across the IDE sample. At 700 h(�29 days) the DC leakage currents exceeds 1000 pA, indicating failure.

3. Results and discussions

3.1. DC leakage current

The DC leakage currents measured in air for Parylene C andAl2O3–Parylene C IDEs were low, with an average of 1 pA(n = 14). An increase in the average DC leakage current to78 ± 37 pA (mean ± SEM, n = 8) for Al2O3–Parylene C and to357 ± 41 pA (mean ± SEM, n = 6) for Parylene C was seen afterimmersion in PBS at room temperature. This is expected, as the ini-tial electrolyte ingress into polymeric coating creates conductivepathways that are absent in air [49]. All coatings on IDEs maintain-ing barrier properties presented with leakage currents lower than

800 pA (Fig. 4) throughout the study period, while leakage currentsexceeded 1 nA for insulation (n = 8) designated as failed for thepurposes of this paper (Fig. 5).

3.2. Equivalent circuit modeling

EIS was measured for all samples in air before immersion inPBS. The estimated average QL for all the IDEs was 62 ± 1 pF(mean ± SEM, n = 14). A bending of the impedance curve in the Ny-quist plots was observed when the electrodes were immersed inPBS at room temperature, suggesting electrolyte penetrationthrough the insulation. A fraction of the Al2O3–Parylene C (2 of10) and Parylene C (4 out 10) samples failed during room temper-ature inspection. These samples were not included in the ALT testsor in the results.

The changes in parameter values as well as the manifestation ofdouble layer capacitance (Cdl), polarization resistance (RP) andWarburg diffusion impedance (W) help us in understanding thebehavior of intact coating as it fails after a period of time. Thus,modeling results for both kinds of samples at day 1 (intact) andat failed time points are summarized in a single table for eachmaterial.

The intact insulation model (Fig. 3a, inset) was used to charac-terize the impedance data for all surviving samples. The average QL

in PBS compared to air increased by ten-fold for Parylene C (Ta-ble 1) and Al2O3–Parylene C IDEs (Table 2). This is due to the pene-trating electrolyte having a higher dielectric constant (er = 80) [56]than the Parylene C (er = 3.15) [43]. As expected, the average RL ofAl2O3–Parylene C (Table 2) was five times greater than that of Par-ylene C (Table 1). After room temperature inspection, the sampleswere kept at 60 �C in PBS. Representative plots from two IDEs

Table 1EIS equivalent circuit model parameters for Parylene C coated IDE samples.

Fit parameters PBS room temp survivingn = 6 (mean ± SEM)

PBS 60 �C @ day failn = 4 (mean ± SEM)

QL (pF) 151 ± 13 312 ± 85a 0.96 ± 0.004 0.95 ± 0.01RL (GX) 5.94 ± 2.4 2.40 ± 0.52Cdl (pF)* – 249 ± 52RP (GX)* – 0.142 ± 0.031W (GX S1/2)* – 0.580 ± .0035

* Applicable to only n = 3 failed electrodes.

Table 2EIS equivalent circuit model Al2O3–Parylene C coated IDE samples.

Fit parameters PBS-room temp survivingn = 8 (mean ± SEM)

PBS 60 �C @ day failn = 4 (mean ± SEM)

QL (pF) 156 ± 18 4090 ± 1920a 0.97 ± 0.006 0.92 ± 0.03RL (GX) 25.9 ± 3.2 3.95 ± 0.43Cdl (nF) – 1.32 ± 1.03Rp (GX) – 0.30 ± 0.22W (GX S1/2)* – 0.38 ± 0.06

* Applicable to only n = 3 failed electrodes.

Fig. 6. Overlay of impedance (a) modulus and (b) phase of a stable Al2O3–ParyleneC (52 nm/6 lm) coated sample from day 1 (solid red line) to over 1700 h (�70 days)(black d) at 60 �C in PBS. The stable impedance spectrum and the phase alwaysaround �80� (capacitive characteristics) indicate that the coating maintained itsbarrier properties throughout the experiment.

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illustrate the stability of an intact coating (Fig. 6) and the transitionin impedance and phase exhibited by a failed coating (Fig. 7). Spe-cifically, the EIS spectra of the failing insulation show a decrease inthe impedance modulus (Fig. 7a) and phase transitioning from�80� (capacitive) towards 0� (resistive) (Fig. 7b), which is consis-tent with the emergence of a conductive pathway through theinsulation. We expect that as the insulating coating degrades, thelateral impedance between the IDE fingers would decrease as soonas underlying metal (Au) on either one of the IDE finger is exposed.Since the reference electrode is a coated IDE finger with highimpedance, it needs to be accounted for in the equivalent circuitof a failed coating. To do so, we developed various models incorpo-rating different failure scenarios. The equivalent circuit model fit-ting the failed insulation accounts for pore formation, allowingelectrolyte to access the underlying metal (Fig. 3b, inset). Cdl ac-counts for capacitance arising due to topographical, structuralproperties of the underlying metal [57], as well as the chargeadsorption and separation occurring at the metal–electrolyte inter-face [51]; the corrosion rate of the exposed metal is described by aresistor (RP) [33]. The accessible metal is thus modeled as a doublelayer capacitance (Cdl) in parallel with polarization resistance (RP)and Warburg diffusion impedance (W) of the metal. Warburgimpedance is included in the model if the degradation process isdiffusion-controlled [51]. It is associated to the mass transport ofelectroactive species such as oxygen and ions of electrolyte.

Of the 14 total IDEs exposed to accelerated aging, four IDEs fromeach group failed within the 3 month study period. The failed insu-lation model (Fig. 3b, inset) fit 75% of both Parylene C and Al2O3–Parylene C (Fig. 3b), suggesting a similar failure mechanism. How-ever, 75% of Parylene C samples failed within the first 24 h of ageacceleration at 60 �C, while Al2O3–Parylene C failed after longerimmersion time (longer than 100 h). The increase in QL for ParyleneC and low RL estimates below 3 GX (Table 1) would suggest in-creased electrolyte penetration. The manifestation of RP indicatedaccess to underlying metal [51]. The estimated a value of failedParylene C (0.96) was nearly identical to the room temperatureestimates (0.95), suggesting the similar surface morphology. TheEIS spectra of the Parylene C IDE that failed after the longestimmersion time (>1500 h) fit the intact insulation model with anincrease in QL and decrease in RL. The increase in QL and decrease

in RL suggest either blistering [58,59] thinning [60] or delamination[51].

Failed Al2O3–Parylene C samples also exhibited lower RL, �GX(Table 2). The Cdl and RP manifestation is clearly distinguishablein 50% of these samples, with appearance of a second semicirclein a low frequency spectrum of the Nyquist plots (Fig. 3b) [51].The high frequency semicircle (Fig. 3b) represents the coating char-acteristics (QL and RL) [51]. Both of these samples failed after 650 hof age acceleration. The large increase in QL for failed Al2O3–Paryl-ene C samples suggests blistering [32], microscopic delamination[25] or thinning [34] of Parylene C coating, while the presence ofCdl and RP would suggest that the Al2O3 layer was exposed andits dissolution [61] led to access of the underlying Au electrodes.The decrease in a for all failed Al2O3–Parylene C samples indicateschanges in surface morphology of the coating [60] (Table 2). Con-sistently, all failed IDE samples presented an increase in leakagecurrent (lA), a decrease in impedance modulus for low frequenciesand an increase in the area under the cyclic voltammetry curve,suggesting exposure of the underlying metal.

3.3. Mean time to failure

An exponential distribution for failure times (Fig. 8) was as-sumed to estimate the MTTF in each group. The 95% confidenceinterval for Parylene C was estimated at (465, 2683) (h) and(1288, 20, 592) (h) for Al2O3–Parylene C. MTTF of Parylene C andAl2O3–Parylene C are listed in Table 3. The p-value for testing thenull hypothesis of equal MTTF between groups (l1 = l2) againstthe one-sided alternative of l2 > l1 is 0.034, with effect size of4.6 (l2/l1), thus demonstrating statistical significance. The obser-vations suggest that the MTTF of Al2O3–Parylene C is �4.6 times

Fig. 7. Overlay of impedance (a) modulus and (b) phase of Al2O3–Parylene C(52 nm/6 lm) sample under test at 60 �C. At day 1 (solid red line) the samplepresents high impedance and phase around –80� (capacitive characteristics) forfrequencies greater than 1 Hz. This characteristics is maintained (blue +) formeasured time points until day 29 (black d); a decrease in impedance modulus isaccompanied by shift in phase towards �30� (conductive characteristics) in themid-frequency range (1–100 Hz), indicating access of electrolyte to the underlyingmetal due to insulation failure.

Fig. 8. The time to failure of IDE samples used to calculate statistics reported for thelifetime comparison between Al2O3–Parylene C (solid line) and Parylene C (dashedline) coated samples.

Table 3Mean time to failure of Parylene C and Al2O3–Parylene C IDE samples.

Sample MTTF (hours) @60 �Cmean ± SEM

MTTF(days) @60 �Cmean ± SEM

MTTF@37 �C(Q10 = 2)

Parylene C (n = 6) 1117 ± 499 49.1 ± 20.7 �8 monthsAl2O3–Parylene C (n = 8) 5150 ± 2683* 214.6 ± 111.8 �36 months

* The lifetime of Al2O3–Parylene C coating > Parylene C coating at 60 �C (p = 0.03).

Fig. 9. Impedance modulus of a failing Al2O3 (52 nm)–Parylene C (6 lm), for threefrequencies (0.01 (blue h), 10 (green e) and 1000 (red ⁄) Hz). The drop inimpedance modulus at 0.01 Hz and at 10 Hz at 700 h indicates failure of insulation.However, the modulus of the impedance at 1 kHz is insensitive to the failure andmaintains its magnitude for 1700 h.

S. Minnikanti et al. / Acta Biomaterialia 10 (2014) 960–967 965

greater than Parylene C. In addition, for the same number of failureevents to occur in both groups, assuming an exponential distribu-tion of lifetimes, the required sample size is eight Parylene C and

18 Al2O3–Parylene C IDEs. The MTTF at 37 �C for both sampleswas extrapolated assuming a Q10 = 2, which allows for a conserva-tive approach for estimating an aging factor for polymers used asbarrier systems of medical devices according to ASTM-F1980. Thus,the range of 74.1–222.4 days at 60 �C is equivalent to 1–3 years at37 �C.

4. Conclusions

Our data strongly suggest that the MTTF of Al2O3–Parylene Ccoating is significantly greater than Parylene C alone. The MTTF37 �C

for Parylene C (8 months) reported here is comparable to values re-ported from other studies using similar coating thickness. Haraet al. reported MTTF37 �C as 6 months for Parylene C sheath micro-electrode probes [35], while Li et al. reported 2 days as MTTF90 �C

for Parylene C [66], showing that Parylene C coated IDEs main-tained barrier properties for more than 1 year at 37 �C. We haveshown that the MTTF37 �C of Al2O3–Parylene C (�36 months) is animprovement over Parylene C (�8 months). Additionally, it is com-parable to liquid crystal polymers (LCPs) tested as encapsulation(�2 years) [62]. LCPs are known for their lower water intake com-pared to Parylene C [64,34].

The EIS spectra and equivalent circuit models suggest that thereare two modes of failure in the tested IDEs The same circuit(Fig. 3a, inset) fits intact IDEs as well as those that fail after longestimmersion time (longer than 1000 h). These failures could be dueto blistering of the Parylene C coating, allowing a large volume ofelectrolyte penetration. The second mode of failure is consistentwith pore formation where the electrolyte penetrates the insula-tion coating enough to expose the underlying metal. These twofailure modes are common for polymeric coatings known forallowing water and salt permeation [65]. The commonly reportedmodes of failure are delamination, blistering and formation of mi-cro-pores [15,17,27,37]. Li et al. reported that Parylene C coatingsfailed due to the magnification of electrical stresses at imperfectionsites causing micro-cracks followed by delamination [66]. Otherpolymeric coatings show similar modes of failure. Parylene N coat-ings failure during soak tests was related to micro-crack formationinstead of delamination. Polyimide sandwiched layers failed due todissolution, delamination and blistering [63], possibly due to ‘‘out-gassing of trapped moisture’’ [67].

Here, intact coatings maintained high impedance over the3 month study period while the failed coatings show a decreasein impedance and phase angle transitioning from �80� (capacitive)towards 0� (resistive), indicative of electrolyte access to the under-lying metal. A common metric to evaluate the ability of a recording

966 S. Minnikanti et al. / Acta Biomaterialia 10 (2014) 960–967

electrode [47], as well as the performance of encapsulating insula-tion for neural interfaces [63], is the impedance modulus at 1 kHz.It is important to recognize that six out of eight failing IDE samplesin this study showed no decrease in the impedance modulus at1 kHz (Fig. 9). The insulation coating has to fail drastically suchthat its value is lower than the dominating capacitive impedanceto reflect changes at higher frequencies [68]. A reduction in the1 kHz impedance of failed IDE was observed when the 0.1 Hzimpedance decreased below 10 MX. Thus, changes in 1 kHzimpedance would reflect failure at much later time points and can-not be the only performance metric used as an indication of mate-rial stability for implantable electrodes.

A decrease in the impedance of implanted neural electrodes dueto insulation damage has been reported in the literature,[17,18,69]. Though tremendously valuable, the in vivo validationis beyond the scope of the present study. The significance of IDEcoating failures in our testing paradigm on the overall functionalityof implanted microelectrode arrays is difficult to extrapolate. In-deed, EIS is very sensitive to minute changes in film integrity. How-ever, the ability of an implanted microelectrode with pinholes ordelamination to continue to record neuronal biopotentials will de-pend on volume conductor conditions. Nevertheless, it is apparentthat the insulating character of Al2O3–Parylene C is significantlyimproved over Parylene C alone, suggesting that this is a promisingmaterial modification for increasing neural interface reliability.

5. Disclosures

Florian Solzbacher has commercial interest in Blackrock micro-systems, which manufactures and sells neural interfaces.

Acknowledgements

This work was sponsored by the Defense Advanced ResearchProjects Agency (DARPA) MTO under the auspices of Dr Jack Judythrough the Space and Naval Warfare Systems Center, PacificGrant/Contract No. N66001-12-1-4026 – Biocompatibility of Ad-vanced Materials for Brain Machine Interfaces.

Appendix A. Figures with essential colour discrimination

Certain figures in this article, particularly Figs. 3 to 7, 9 are dif-ficult to interpret in black and white. The full colour images can befound in the on-line version, at doi:http://dx.doi.org/10.1016/j.actbio.2013.10.031).

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