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Abstract
Parylene-C has been used extensively in neural interface devices as a conformal, biocompatible coating; it has also recently become an integrated part of our parylene-cabled silicon probes. However, it is found over the years that its adhesion capability to silicon may be compromised after thermal treatments, cleaning, handling, bench testing and implantations. This paper explores extensively new techniques including annealing, melting, anchoring, and XeF<sub>2</sub> silicon surface roughening to enhance this adhesion. We report the first quantitative experimental data on the effectiveness of each through tearing, soaking, etching and ASTM peeling tests. The results show various improved adhesion means over the standard A-174 adhesion promotion, which can greatly benefit the use of parylene.
... However, poor adhesion and low thermal stability have restricted its application [4,5]. As most microfabrication techniques employ Si wafers, the poor adhesion between Parylene C and Si not only affects the process reliability and long-term stability of the prepared device [6,7], but it also necessitates the careful use of chemicals, such as HF and BHF [8]. In addition, Si devices with a Parylene C coating cannot withstand the standard steam sterilization process [9,10]. ...
... The current strategies for overcoming the adhesion issues involve both physical and chemical approaches. The physical methods, which aim to increase the contact surface area between Parylene C and Si, include Si surface roughening via reactive ion etching [6] and anchoring Parylene layers on the Si substrate [8]. These procedures unavoidably subject the Si substrate to further processes. ...
... These procedures unavoidably subject the Si substrate to further processes. The chemical approach consists of adding an adhesion layer, such as the A174 promoter [3], hexamethyldisilazane [11], or molten Parylene C [6,12], or increasing the surface energy by attaching functional groups to the Si substrate [7]. These methods introduce more molecules to the Si substrate and additional complexity to the microfabrication process, possibly affecting device performance. ...
Parylene C has been widely used in the fields of microelectromechanical systems (MEMS) and electronic device encapsulation because of its unique properties, such as biocompatibility and conformal coverage. However, its poor adhesion and low thermal stability limit its use in a wider range of applications. This study proposes a novel method for improving the thermal stability and enhancing the adhesion between Parylene and Si by copolymerizing Parylene C with Parylene F. The successful preparation of Parylene copolymer films containing different ratios of Parylene C and Parylene F was confirmed using Fourier-transform infrared spectroscopy and surface energy calculations. The proposed method resulted in the copolymer film having an adhesion 10.4 times stronger than that of the Parylene C homopolymer film. Furthermore, the friction coefficients, cell culture capability, and water and oxygen barrier performances of the Parylene copolymer films were tested. The results indicated no degradation compared with the Parylene C homopolymer film. This copolymerization method significantly expands the applications of Parylene materials.
... With the introduction of multielectrodes, investigations began on the effectiveness of multipolar stimulation from one channel [22]. Both bipolar and tripolar configurations have been shown to reduce current spreading, with tripolar achieving the highest degree of selectivity [23][24][25] at the cost of higher power consumption. ...
... Additionally, with bipolar and tripolar configurations, reducing the spacing between adjacent electrodes can further reduce the spread of current [23,25] ...
... It is likely that the heat caused bonding between the parylene and the silicon in all contact areas. Later studies from the same group further investigated this concept and other adhesion enhancement techniques [23]. To promote adhesion, the released parylene arrays are heated in a nitrogen-purged vacuum oven for 12 hrs. ...
Cochlear implants for the deaf are the most successful neural prostheses;
however, pitch perception remains relatively poor. The size of the scala tympani into
which the electrode array is inserted limits the number of electrodes to about twenty-four
using conventional wire-bundle arrays. Thin-film arrays can offer significant advantages
by increasing the number of sites, reducing damage to residual hearing with smaller and
more flexible arrays, and allowing deeper insertion (greater pitch range).
This thesis creates an active cochlear array that builds on the success of a
previous-generation Michigan array by increasing the reliability, usability and functionality.
A robust and flexible 32-site prototype cochlear electrode array for a 128-site
human array has been developed. Molded and thin-film backings for this array have been
created for positioning the array inside the cochlea and close to the modiolar wall. The
array has been integrated into the commercial molding process of Cochlear Ltd., as well
as into a custom molding process created as part of this thesis. Batch-fabricated backings
that include flexible parylene rings and self-curling parylene layers have also been
created. These backings can achieve a minimum radius of curvature less than 0.5 mm.
Array stiffness can be graded from 0.2 k??N/m2 to 1.4 k??N/m2 with parylene rings to
increase the rigidity seven-fold over that of a flat parylene array.
These parylene arrays have achieved full insertion into cat and guinea pig
cochleae during in-vivo implants. The arrays have achieved the deepest insertions to date
(more than 8mm in some cases). Flexible guinea pig arrays achieved atraumatic
implantations with no visible damage to the scala media.
A 32-site, 4-channel Application Specific Integrated Circuit (ASIC) was realized
in 0.5??m technology to support a wide range of multisite multipolar stimulus
configurations using a distributed Digital to Analog Converter (DAC) architecture. It fits
within the space of the otic bulla having a size of 2.2 mm by 2.5 mm, and operates from a
??2.5 V supply at clock speeds up to 500 kHz. The maximum power consumption of the
ASIC is 2.5 mW when outputting 500 ??A and it is compatible with back voltages
exceeding ??2V.
... Due to the low glass transition and melting temperature of PxC, heating will lead to an intermingling of the polymer chains and therefore increase the adhesion. [92,156]. However, the high temperatures necessary for annealing are unfeasible for some application with temperature sensitive materials. ...
... The silane A-174 is commonly applied as adhesion promoter for PxC [99,156]. It was applied by chemical vapor deposition (CVD) for 3 min in the vacuum chamber of the PxC deposition. ...
The technological implementation of bioelectrical medicine requires small, flexible implants that allow contact between the nerve tissue with high local resolution and exhibit a small mechanical impact on the body. One of the most promising technologies in this field is chip-in-foil systems, as they combine the advantages of flexible foil implants with the computing power of modern electronics. As an application, the use for the therapy of type 2 diabetes mellitus is proposed, in which a closed-loop system uses the activity of the β-cells to determine the blood sugar level and, as a result, achieves insulin release by electrostimulation of the pancreas.
In this thesis, a chip-in-foil system is presented in which a flip-chip-process with silicone as filling material was used in order to maintain a completely leveled surface while embedding various dies. In order to achieve a fully flexible implant, the dies were thinned to a thickness of less than 30 µm and polymers were used as embedding material. For the sensing and stimulation of pancreatic activity, special application-specific circuits were developed which were integrated into a functional demonstrator. In addition to 200 µm thick spacer dies, the corresponding functional dies were glued face-down to the top of a polyimide-coated glass substrate. After these substrates have been covered with uncured silicone, a second glass substrate with a sacrificial layer was applied and pressed onto the spacer dies. After the silicone had cured, the lower substrate could be removed. At this point the dies were embedded with the surface pointing upwards under a polyimide layer. Since the silicone was completely covered by this layer, the classical microsystem technologies could be used for the further process steps. After the surface has been patterned, the edge of the components was cut out by laser and the sacrificial layer was electrochemically dissolved, causing the foil implant to detach from the substrate.
Since the embedding of dies without further encapsulation can lead to previously unknown failure mechanisms, test structures have been developed both on special chips and within the foil substrate in order to investigate the lifetime of the devices. One of the most critical points was the adhesion of the different polymers to each other. In order to test and improve this, the effect of several adhesion promoters on interface adhesion was tested by means of peel-testing in saline solution. Material combinations classified as stable were further investigated in a specially developed setup, in which a repetitive mechanical load was combined with a simulated body environment in order to obtain as realistic an impression as possible over the life of the implants. The automated measurement allowed the installation of 15 samples with freely selectable measurement settings, whereby electrochemical impedance spectroscopy and four-point measurement have been used as the measurement method. Measurements over several weeks at 37 °C have shown that while the highest stability could be achieved with two layers of polyimide, the combination of titanium oxide and the adhesion promoter A-174 also significantly improved the adhesion of Parylene-C to polyimide.
... 21,22 Furthermore, even though Parylene-C is an excellent moisture barrier, in a permanently wet environment water will eventually diffuse through the polymer layer and start to degrade the interface. 11,23,24 This most critical issue for long-term implant applications cannot be solved by the standard adhesion promoter (silane A-174) used for Parylene-C. Huang et al. 24 proposed an alternative adhesion promotion route on silicon substrates via macroscale anchoring. ...
... 11,23,24 This most critical issue for long-term implant applications cannot be solved by the standard adhesion promoter (silane A-174) used for Parylene-C. Huang et al. 24 proposed an alternative adhesion promotion route on silicon substrates via macroscale anchoring. Deep trenches are etched using deep reactive ion etching (DRIE) allowing the Parylene to physically interlock with the substrate. ...
While Parylene-C is known for its excellent mechanical properties and biocompatibility, its applicability for long-term implants is limited by poor adhesion to many commonly used implant alloys. While the commonly adapted route for adhesion promotion offers significant improvement for dry adhesion, it does not ensure long-term stability in permanent contact with fluids. In this work we investigate different synthesis routes for titanium oxide nanostructures on Ti6Al4 V which serve to physically anchor Parylene to the metal surface. The obtained titanium oxide nanostructures have a range of differing morphologies and dimensions and achieve approximately a 550 larger specific surface than an ideally flat surface. The influence of different nanostructures’ characteristics on the wet-adhesion of Parylene-C was investigated by accelerated aging of Parylene at 80 °C for 72 h in Hank’s balanced salt solution (HBSS) and subsequent blisterlike adhesion testing. The aging procedure caused reference samples with polished surfaces and silane A-174 adhesion promoter to experience an 88% reduction in failure load at which the Parylene film delaminates compared to the unaged samples. Nanostructured samples performed equally well before and after aging, corresponding to an 840% failure load increase compared to the reference surfaces. These findings may allow new applications of Parylene-C for long-term applications in liquid environments, which were previously prohibited because of poor adhesion.
... The samples were taken from various locations on the wafer. However, samples that were annealed above 200°C demonstrated strong adhesion forces to the silicon wafer due to recrystallization of Parylene above the T g , which agrees with previously reported results [20,21]. The films annealed above 150°C could not be peeled off, so they could not be tested using a standard tensile test machine and had to be tested using a nano-indenter. ...
... It is important to note that the thermoforming process should be carried out in a vacuum environment to inhibit thermal oxidation of Parylene C at temperatures >125°C [10]. The PSE was thermoformed at 200°C for 48 hours to not only achieve its 3D shape, but also to enhance the adhesion between the Parylene C layers [11]. Here we present the evaluation of the thermoforming process on the chemical and mechanical properties of Parylene C as well as the electrochemical performance of the PSE, to capture any material changes and their possible impacts on the final thermoformed device, and on the feasibility of the PSE as an implantable intracortical neural probe. ...
The chemical, mechanical, and electrochemical attributes of the Parylene sheath electrode (PSE) were evaluated following a post-fabrication thermoforming process to determine its impact on both the polymer and thin film platinum materials. The three-dimensional conical shape of the PSE was formed via thermal molding of a surface micromachined Parylene C microchannel using a custom shape-forming microwire having the desired taper at 200°C for 48 hours under vacuum. Contact angle and Fourier transform infrared spectroscopy measurements indicated that the thermoforming process resulted in no significant changes to the surface and bulk chemistry of Parylene. The thermoformed Parylene samples possessed greater Young's modulus, but retained their flexibility. Electrochemical characterization of electrodes before and after thermoforming revealed a decreased storage charge capacity and increased electrode impedance, however, recording functionality was not lost as resolvable neuronal unit activity was successfully obtained post-implantation.
... However Parylene C and all Parylenes in fact, adheres poorly to surfaces and this has limited its use in harsh environments, particularly in liquids [9], [20]–[24]. Many attempts have been made to improve Parylene's adhesion properties [11], [20]–[23], [25], [26]; however, none of them seem efficient enough to prevent the delamination of the Parylene layers on substrates exposed to a harsh chemical environment for more than a few hours. One of the most common wet etching techniques for the fabrication of silicon-based MEMS devices is the anisotropic potassium hydroxide (KOH) etching. ...
... SEM micrographs of the sheath structures taken a week after the thermoforming procedure demonstrated that structures maintained their shape and did not sag or otherwise revert to the original flat microchannel shape (Fig. 8). Thermoforming at 200 uC also served to anneal the multiple Parylene layers leading to improved Parylene/ Parylene adhesion [31][32][33] and improved sheath probe mechanical robustness. Improved mechanical robustness of the sheath probe was confirmed using a nanoindenter (Berkovich tip, MTS Nano Indenter XP) to determine stiffness of the sheath structure. ...
A Parylene C neural probe with a three dimensional sheath structure was designed, fabricated, and characterized. Multiple platinum (Pt) electrodes for recording neural signals were fabricated on both inner and outer surfaces of the sheath structure. Thermoforming of Parylene was used to create the three dimensional sheath structures from flat surface micromachined microchannels using solid microwires as molds. Benchtop electrochemical characterization was performed on the thin film Pt electrodes using cyclic voltammetry and electrochemical impedance spectroscopy and showed that electrodes possessed low impedances suitable for neuronal recordings. A procedure for implantation of the neural probe was developed and successfully demonstrated in vitro into an agarose brain tissue model. The electrode-lined sheath will be decorated with eluting neurotrophic factors to promote in vivo neural tissue ingrowth post-implantation. These features will enhance tissue integration and improve recording quality towards realizing reliable chronic neural interfaces.
... Pressure generation of several hundred psi is possible by electrolysis but unlikely to be sustained within the transducer structure due to the known weak interfacial adhesion of the Parylene–Parylene interface. Strategies such as annealing, anchoring, or molten Parylene adhesion [40] may be employed to improve adhesion performance; however, a systematic comparison of the improved interfacial adhesion afforded by these techniques is still required. VIII. ...
We report the design, fabrication, and characterization of electrochemical microelectromechanical systems (EC-MEMS) devices featuring encapsulated fluid as the basis for transduction. Parylene microstructures, including discrete chambers (square or circular geometry), are utilized as physical transducers for electrochemically mediated liquid impedance transduction of physical phenomenon such as contact and force. Parylene-based EC-MEMS technologies uniquely leverage advantages in size (<; 500 μm diameter), packaging (no hermetic packaging necessary), power (nanowatts to microwatts), and flexibility to address the physical sensing requirements of in vivo applications. Robust EC impedance (EI) sensor responses (up to 20% from base-line) and discrimination of 200-nm chamber deflections were possible using the EI transduction technique. Additional transducer configurations enabling electrolysis-based out-of-plane actuation and biomimetic mechanotransduction in microfluidic channels are also presented.
Parylene C is a commonly used polymer in the micro-electromechanical systems (MEMS) field because of its excellent barrier property and process compatibility with other microfabrications. Whereas, the poor adhesion of other materials to Parylene C is the urgent challenge that restricts its real applications. This work proposed a strategy to enhance the adhesion between Parylene C or metals and the Parylene C substrate. A short-time oxygen plasma reaction ion etching process with ambient titanium in the etching chamber is introduced between the first layer of Parylene C film deposition (the substrate) and the second Parylene C or metal coatings. Parylene C nanostructures (nanograss) are generated on the substrate because of the oxygen plasma bombarding with sputtered titanium nanoparticles as nanomasks. Different feature sizes of nanograss were successfully obtained by tuning the RF power, oxygen flow rate and etching times. Scanning electron microscopy images showed that both the nanograss density and height (0.61 ± 0.02 μm - 0.76 ± 0.03 μm) were positively proportional to the etching time with low RF power (150 W) and oxygen flowrates (60 sscm). Scratch tests are conducted after the second layer of Parylene C or metal coatings to quantitively analyze the adhesion enhancement. The results indicated that the adhesion of both Parylene C and metal on the Parylene C substrate with nanograss structures were enhanced up to around 8 and 10 times, respectively, compared to those on untreated substrates. This nanograss technique based adhesion enhancement approach is easy-to-realize, robust, chemical-free, precisely controllable, thereby holds promising potentials in various Parylene MEMS applications. Keywords: Nanograss, Parylene C, adhesion enhancement
This paper introduces a batch fabrication method to manufacture Micro-Electro-Mechanical System (MEMS) platforms for 3D integration of sensors spatially-distributed on a 2D plane. In the heart of the concept is a foldable MEMS structure with polymer hinges which is used to support and guide the assembly of the discrete planar sensors by means of folding them into a 3D shape, like origami, providing controlled distribution of sensors in space. Flexible hinges carrying the electrical interconnects are a critical structural element of the platform and material selection study for those is the main focus of this paper. We analyzed different materials for flexible hinges fabrication, including photo-definable polyimide and parylene-C. Three approaches for sensor integration are presented: 1) co-fabrication; 2) sensor drop-in; and 3) transfer bonding. The prototypes of structures with different flexible hinge materials were fabricated and evaluated based on their mechanical flexibility, chemical compatibility, and material outgassing. Parylene-C exhibited similar or better performances compared to polyimide, demonstrating in each of the experiments a higher degree of thermal flexibility up to 350 °C, a superior chemical resistance against hydrofluoric acid, and 2.9 times lower outgassing. [2020-0301]
Parylene-C is a frequently used polymeric thinfilm coating in medical applications and is known for its excellent biocompatibility and flexible deposition process. However, its use in long-term implants is limited due to its poor adhesion to metals in liquid environments. In this work we present a strategy to anchor Parylene-C to medical grade titanium (Ti) by means of nanostructuring the Ti substrates surface prior to Parylene coating. We observe that, after aging in physiological salt solution for 3 days, Parylene coating lose their adhesion to bare titanium surfaces. However, the Parylene films deposited on nanostructured Ti surfaces retain full adhesion, even after aging them in the same solution for 10 days. Additionally, we demonstrate that combining nanostructured surfaces with very thin Parylene coatings provides the additional benefit of accelerating cell proliferation. Nanostructured surfaces showed cell proliferation without the typically required oxygen plasma treatment. Combining plasma treatment and nanostructuring further improved proliferation performance over smooth Parylene surfaces.
Parylene C is a thin-film polymer used as a biocompatible barrier layer in biomedical implants and implantable MEMS; free-film Parylene C may serve as both the substrate and insulation in polymer-based microdevices, a growing branch of biomedical technology. The adhesion of vapor deposited Parylene C, particularly when exposed to wet,
in vivo
environments, is a critical determinant of device lifetime for such polymer-based implants. This paper explores several novel strategies for improving the adhesion of multi-layer Parylene structures, including thermal annealing and the use of several chemical interposer layers. Interfacial adhesion of Parylene-Parylene and Parylene-platinum-Parylene films was examined using a standard T-peel test to quantify adhesion and measure film integrity under chronic exposure to saline up to two years. Improved adhesion and barrier properties in Parylene-Parylene films resulted from the inclusion of diamond-like carbon and ethylene glycol diacrylate layers. Thermal annealing improved Parylene film integrity in wet environments but was insufficient for improving the integrity of Parylene-platinum interfaces. A 100-fold increase in adhesive strength at such interfaces was achieved using a commercially available adhesion promoter, and the corresponding improvements in resistance to moisture driven delamination were observed. X-ray diffraction and X-ray photoelectron spectroscopy results are provided to highlight the role of film morphology and surface composition in adhesion integrity. [2018-0076]
A fabrication approach for ultra-miniature ultra-compliant neural probes with parylene-C insulation that are embedded in biodissolvable insertion needles was previously established by the authors. However, that approach required application of a peeling process to release the probe-needle assembly from its handle wafer. The use of thermal annealing in vacuum to improve encapsulation properties of parylene-C results in increased adhesion to the substrate that undermines the peeling process. In this paper, we introduce a transfer process step that eliminates the peeling process and allows the potential use of a wide range of sacrificial release materials. The transfer step increases the versatility of the overall fabrication approach since it allows the integration of insertion needle and sacrificial release materials that otherwise would not have been compatible with the high-temperature annealing. Several sacrificial release materials, including photoresist, polydimethylsiloxane, mounting adhesive, and liquid wax, are investigated and characterized for suitability in the transfer process. Considering compatibility with the biodissolvable needle attachment, a liquid wax is identified to be an effective material because of its strong adhesion to relevant surfaces, its ability to be spin coated, and its dissolvability in isopropyl alcohol.
The effect of hexamethyldisilazane (HMDS) on adhesion between Parylene-C (Px) and silicon is investigated through the use of peel tests and contact angle tests. These tests allow measurement of the adhesive forces and investigation of the possible mechanisms for variations in adhesion that can be exploited in processes that require devices to be peeled from their substrate to release them after they are fabricated. An analytical framework is used to quantify the upper limit of adhesion on the basis of the geometric and material properties of a device. It was found that the modulation of adhesion between Px and Si by treatment with HMDS is largely due to the increase in dispersive surface energy of treated Si over the range 5.3–18.1 mJ/m2 based on measured contact angles of 46.3°–74.9° with DI water and 36.2°–55.7° with ethylene glycol. Varying the duration of HMDS vapor prime from 0 min to 2 min modulates the average adhesion between Px and Si from 5 mN/cm to 16 mN/cm.
The thermoplastic nature of Parylene C is leveraged to enable the formation of three-dimensional structures using a thermal forming (thermoforming) technique. Thermoforming involves the heating of Parylene films above its glass transition temperature while they are physically confined in the final desired conformation. Micro and macro scale three-dimensional structures composed of Parylene thin films were developed using the thermoforming process, and the resulting chemical and mechanical changes to the films were characterized. No large changes to the surface and bulk chemistries of the polymer were observed following the thermoforming process conducted in vacuum. Heat treated structures exhibited increased stiffness by a maximum of 37% depending on the treatment temperature, due to an increase in crystallinity of the Parylene polymer. This study revealed important property changes resulting from the process, namely (1) the development of high strains in thermoformed areas of small radii of curvature (30–90 µm) and (2) ~1.5% bulk material shrinkage in thermoformed multilayered Parylene–Parylene and Parylene–metal–Parylene films. Thermoforming is a simple process whereby three-dimensional structures can be achieved from Parylene C-based thin film structures with tunable mechanical properties as a function of treatment temperature.
Non-planar, three dimensional structures, not possible with conventional microfabrication processes, were achieved using post-fabrication thermal annealing of thin film Parylene-C structures facilitated by a mold (“thermoforming”). We demonstrate thermoforming of Parylene-Parylene and Parylene-metal-Parylene (PMP) structures for increased structural and mechanical functionality such as strain relief, formation of open-lumen sheath structures, and conformation-matching of curved surfaces that broaden applications for Parylene MEMS. Characterization of the material and mechanical properties as a function of thermoforming temperature is also presented. Thermoformed Parylene consistently retained bulk/surface chemical material properties following the treatment regardless of temperature, and thermoforming at higher temperatures increased structural stiffness, which is attributed to increased crystallinity of the polymer. By varying the thermoforming process parameters, the final shaped structure can be mechanically and structurally tuned for broad range of applications, most notably, structured implantable neural interfaces with integrated channels for tissue ingrowth and improved integration.
Parylene-C has been extensively used in biomedical devices as a conformal and biocompatible coating. It is also a good material for implantation. Unfortunately, serious delamination between parylene-C and other materials is often found even during standard MEMS processes such as lift-off and sacrificial photoresist releasing. Therefore, surface treatments before parylene deposition that can enhance the interface adhesion between the deposited parylene and the coated surface are highly desirable. Interestingly, the chemical structure of Parylene-C does suggest that parylene-C deposition on a clean hydrophobic surface favors a good interface adhesion. This work is then to study various surface treatments, especially on silicon and another parylene-C surfaces, by hexane, toluene, propylene carbonate (P.C.), and tetrafluoromethane (CF4) plasma. Our hypothesis is then good surface cleaning can lead to interface adhesion. We report the results here using parameters such as peeling force, soaking undercut rate, and vertical attack bubble density (VABD) to quantify the effectiveness of these adhesion treatments.
Parylene-C has become a more and more popular material for BioMEMS implant applications due to its good biomedical properties [1, 2]. It was also used as an intermediate layer for silicon wafer bonding [3, 4]. However, the bonding between parylene-C and silicon is still problematic. In this paper, a low-temperature bonding between parylene-C and silicon using photo-patternable adhesives is presented. This method can not only determine the bonding pads but also reduce the residual stress in the packaging. Its application on high-density multi-channel chip integration is also demonstrated. Two commercially available photo-patternable materials, i.e., SU-8 and AZ-4620, with stable characteristics are chosen to demonstrate this method. The processing conditions are optimized in terms of bonding temperature, pressure, time, and surface treatment. The peeling force is measured by ASTM peeling tests under various bonding conditions. The results show that the epoxy-based SU-8 is better than AZ-4620 as an adhesive material with a peeling force up to 8.4 N/cm2. This low-temperature bonding technique allows selectively local area bonding. Besides, bonding without applying a high electric field is especially suitable for the integration with microelectronics in MEMS packaging.
Objective:
Reliable chronic recordings from implanted neural probes remain a significant challenge; current silicon-based and microwire technologies experience a wide range of biotic and abiotic failure modes contributing to loss of signal quality.
Approach:
A multi-prong alternative strategy with potential to overcome these hurdles is introduced that combines a novel three dimensional (3D), polymer-based probe structure with coatings. Specifically, the Parylene C sheath-based neural probe is coated with neurotrophic and anti-inflammatory factors loaded onto a Matrigel carrier to encourage the ingrowth of neuronal processes for improved recording quality, reduce the immune response, and promote improved probe integration into brain tissue for reliable, long-term implementation compared to its rigid counterparts.
Main results:
The 3D sheath structure of the probe was formed by thermal molding of a surface micromachined Parylene C microchannel, with electrode sites lining the interior and exterior regions of the lumen. Electrochemical characterization of the probes via cyclic voltammetry and electrochemical impedance spectroscopy was performed and indicated suitable electrode properties for neural recordings (1 kHz electrical impedance of ∼200 kΩ in vitro). A novel introducer tool for the insertion of the compliant polymer probe into neural tissue was developed and validated both in vitro using agarose gel and in vivo in the rat cerebral cortex. In vivo electrical functionality of the Parylene C-based 3D probes and their suitability for recording the neuronal activity over a 28-day period was demonstrated by maintaining the 1 kHz electrical impedance within a functional range (<400 kΩ) and achieving a reasonably high signal-to-noise ratio for detection of resolvable multi-unit neuronal activity on most recording sites in the probe. Immunohistochemical analysis of the implant site indicated strong correlations between the quality of recorded activity and the neuronal/astrocytic density around the probe.
Significance:
The provided electrophysiological and immunohistochemical data provide strong support to the viability of the developed probe technology. Furthermore, the obtained data provide insights into further optimization of the probe design, including tip geometry, use of neurotrophic and anti-inflammatory drugs in the Matrigel coating, and placement of the recording sites.
Neural interfaces, devices that interact with nervous system, have been developed to help patients with neural disorders to restore lost neural function. The neural interface device requires a conformal and biocompatible encapsulation layer to protect the device during chronic implantation, and to electrically isolate individual electrodes. Parylene-C thin films deposited by a chemical vapour deposition system were studied as an encapsulation layer for neural interface devices. Leakage current tests were used to investigate the encapsulation performance of Parylene-C films, and the results showed hermetic protection as well as long-term (>100 days) stability of the films. The adhesion between Parylene-C and the silicon substrate after several thermal treatments was studied by ASTM tape adhesion tests. Results from these tests suggested that thermal stress may degrade the adhesion force. Parylene samples were subjected to accelerated lifetime testing (85 % relative humidity (RH) and 8C) for 20 days, and the film did not show appearance changes as observed by optical microscopy. However, X-ray diffractograms show that the film crystallinity increased during this test.
Parylene has been used as encapsulation material since several decades, e.g. for wires, needles and pacemaker coatings. Within this paper, its use as insulation and pro-tection layer for flexible biomedical microimplants is de-scribed. Investigations on the deposition, structuring with reactive ion etching and the cytotoxic behavior of the etched layers led to promising results. Parylene seems to be an appropriate material to protect hybrid assemblies of microimplants from water and ions.
This paper describes a new technique for strongly anchoring parylene (poly-para-xylylene) layers on a silicon substrate. Parylene has gained interest for MEMS applications due to its excellent properties. More specifically, because of its flexibility (Young's modulus of 4 GPa), its chemical barrier properties, its conformal deposition and its biocompatibility, parylene is of great interest for microfluidics and BioMEMS. One of the issues with parylene processing is adhesion and delamination problems, occurring during fabrication or during device operation. Here, we report a new technique for anchoring parylene films on silicon using DRIE-etched trenches and anchors. We demonstrate a new way to completely protect the adhesion of parylene even when exposed to aggressive chemicals.
This paper presents the first use of recrystallized
parylene as masking material for silicon chemical etch.
Recrystallized parylene was obtained by melting parylene C at
350°C for 2 hours. The masking ability of recrystallized parylene
was tested in HNA (hydrofluoric acid, nitric acid and acetic acid)
solution of various ratios, KOH (potassium hydroxide) solution
and TMAH (tetramethylammonium hydroxide) at different
temperatures and concentrations. It is found that interface
between parylene and the substrate can be attacked, which
results in undercuts. Otherwise, recrystallized parylene exhibited
good adhesion to silicon, complete protection of unexposed silicon
and silicon etching rates comparable to literature data.
Neural interface devices have been developed for neural science and neuroprosthetics applications to record and stimulate neural signals. Chemical-vapor-deposited Parylene-C films were studied as an encapsulation material for such an implantable device. The surface morphology of an implant affects its biocompatibility; thus, the Parylene-C surface morphology was investigated as a function of the precursor sublimation rate by atomic force microscopy. We found that high precursor sublimation rates resulted in slightly higher root-mean-square surface roughnesses (from 5.78 to 9.53 nm for deposition rates from 0.015 to 0.08 g/min). The crystallinity affects the physical properties of semicrystalline polymers, and various heat treatments were found to modify the crystallinity of Parylene-C films, as assessed by X-ray diffraction (XRD). The XRD peak at 2θ = ∼14.5° increased in intensity and decreased in full width at half maximum with increasing annealing temperature, indicating an increase in film crystallinity. Poor adhesion would compromise the protection offered by Parylene-C coatings. The adhesion between Parylene-C and silicon, amorphous silicon carbide, and boron silicate glass substrates were evaluated using the standard tape adhesion test from the American Society for Testing and Materials (ASTM) in an attempt to minimize the occurrence of delamination failures. The tape adhesion tests indicated strong adhesion for all the as-deposited Parylene films with the application of an adhesion promoter (Silquest A-174® silane). However, annealing the deposited films at temperatures from 85 to 150°C in air for 20 min reduced film adhesion, and also the adhesion testing procedure used significantly affects the results obtained. Supporting evidence suggested that the thermal stress generated in the films weakened the adhesive force. We concluded that the Parylene-C film properties (surface morphology, crystallinity, and adhesion) changed during deposition and thermal annealing, suggesting that the Parylene-C film properties can be tailored and that, with care, failure due to film delamination can be avoided.
In recent years, there has been great interest in using parylene C as a structural and packaging material to protect MEMS devices and electronics for biomedical applications. However, parylene packaging characteristics have not been fully investigated and understood. In this paper, accelerated-lifetime soak tests were done to study parylene packaging characteristics, utilizing parylene packaged metal thin film resistors. Three sets of samples fabricated with different conditions were tested in hot saline both passively and actively, and their failure mechanisms were determined using optical and electron-beam metrologies. The mean time to failure at body temperature was extrapolated using the Arrhenius model, and the preliminary results showed that parylene packaging can remain intact at 37 degC for over 60 years, which is very promising for biomedical implantations. This experimental data also demonstrated that parylene packaging performance can be improved by increasing the thickness of parylene, or by high temperature annealing.
The dehydration and the dehydroxylation of the surface of amorphous silica have been investigated by using the mass spec- trometric thermal analysis method with the employment of tempera- ture-programmed desorption.
In this paper, we present a novel packaging technique that utilizes a simple, flexible parylene (chip) pocket on silicon substrate with metal pads. This pocket can house an IC chip or a discrete component inside and provide electrical connections to it. On the other hand, recent achievement in silicon probes implantation in the parietal cortex enables technological advances in neural prosthesis research. However, most of these technologies suffer from high signal-to-noise ratio and expensive integration scheme with IC chips or lack thereof. As a demonstration, this work uses this technique to produce an 8-shank silicon probe array integrated with a fully functional 16-channel amplifier CMOS chip.
A retinal prosthesis was permanently implanted in the eye of a completely blind test subject. This report details the results from the first 10 weeks of testing with the implant subject. The implanted device included an extraocular case to hold electronics, an intraocular electrode array (platinum disks, 4 x 4 arrangement) designed to interface with the retina, and a cable to connect the electronics case to the electrode array. The subject was able to see perceptions of light (spots) on all 16 electrodes of the array. In addition, the subject was able to use a camera to detect the presence or absence of ambient light, to detect motion, and to recognize simple shapes.
Recent advances in the field of neural prosthetics have demonstrated the thought control of a computer cursor. This capability relies primarily on electrode array surgically implanted into the brain as an acquisition source of neural activity. Various technologies have been developed for signal extraction; however most suffer from either fragile electrode shanks and bulky cables or inefficient use of surgical site areas. Here we present a design and initial testing results from high electrode density, silicon based arrays system with an integrated parylene cable. The greatly reduced flexible rigidity of the parylene cable is believed to relief possible mechanical damages due to relative motion between a brain and its skull.