Accelerated hermeticity testing of biocompatible moisture barriers used for the encapsulation of implantable medical devices

Barrier layers for the long-term encapsulation of implantable medical devices play a crucial role in the devices’ performance and reliability. Typically, to understand the stability and predict the lifetime of barriers (therefore, the implantable devices), the device is subjected to accelerated testing at higher temperatures compared to its service parameters. Nevertheless, at high temperatures, reaction and degradation mechanisms might be different, resulting in false accelerated test results. In this study, the maximum valid temperatures for the accelerated testing of two barrier layers were investigated: atomic layer deposited (ALD) Al2O3 and stacked ALD HfO2/Al2O3/HfO2, hereinafter referred to as ALD-3. The in-house developed standard barrier performance test is based on continuous electrical resistance monitoring and microscopic inspection of Cu patterns covered with the barrier and immersed in phosphate buffered saline (PBS) at temperatures up to 95 °C. The results demonstrate the valid temperature window to perform temperature acceleration tests. In addition, the optimized ALD layer in combination with polyimide (polyimide/ALD-3/polyimide) works as effective barrier at 60 °C for 1215 days, suggesting the potential applicability to the encapsulation of long-term implants

Ultra-long-term reliable encapsulation using an atomic layer deposited Hfo2/Al2o3/Hfo2 triple-interlayer for biomedical implants

Long-term packaging of miniaturized, flexible implantable medical devices is essential for the next generation of medical devices. Polymer materials that are biocompatible and flexible have attracted extensive interest for the packaging of implantable medical devices, however realizing these devices with long-term hermeticity up to several years remains a great challenge. Here, polyimide (PI) based hermetic encapsulation was greatly improved by atomic layer deposition (ALD) of a nanoscale-thin, biocompatible sandwich stack of HfO2/Al2O3/HfO2 (ALD-3) between two polyimide layers. A thin copper film covered with a PI/ALD-3/PI barrier maintained excellent electrochemical performance over 1028 days (2.8 years) during acceleration tests at 60 °C in phosphate buffered saline solution (PBS). This stability is equivalent to approximately 14 years at 37 °C. The coatings were monitored in situ through electrochemical impedance spectroscopy (EIS), were inspected by microscope, and were further analyzed using equivalent circuit modeling. The failure mode of ALD Al2O3, ALD-3, and PI soaking in PBS is discussed. Encapsulation using ultrathin ALD-3 combined with PI for the packaging of implantable medical devices is robust at the acceleration temperature condition for more than 2.8 years, showing that it has great potential as reliable packaging for long-term implantable devices

Ultra-Thin Chip Package (UTCP) and stretchable circuit technologies for wearable ECG system

A comfortable, wearable wireless ECG monitoring system is proposed. The device is realized using the combination of two proprietary advanced technologies for electronic packaging and interconnection : the UTCP (Ultra-Thin Chip Package) technology and the SMI (Stretchable Mould Interconnect) technology for elastic and stretchable circuits. Introduction of these technologies results in small fully functional devices, exhibiting a significant increase in user comfort compared to devices fabricated with more conventional packaging and interconnection technologies

Parylene C for hermetic and flexible encapsulation of interconnects and electronic components

Flexible electronics are of a great interest for wearable and implantable medical devices due to their conformality with the body, compared to electronics made on rigid carriers. Packaging of such electronics needs to offer sufficient flexibility and in addition, has to provide good protection for the electronics inside, also in humid and harsh environments, to prevent device failure due to corrosion. Parylene C is a popular polymer due to its interesting diffusion barrier properties. Parylene C coatings are also extremely conformal, hence it offers the possibility to be used as flexibleprotecting encapsulation for electronic components and interconnects. In order to provide sufficient mechanical support for the electronic circuit, a second encapsulation in PDMS will be performed. In our work, we study the barrier properties of Parylene for long time exposure to moisture and biofluids. Since adhesion is a very important parameter to prevent corrosion, this property is studied in detail. Various substrates and various adhesion promotion treatments are evaluated. Furthermore, copper interconnects coated with parylene C are immersed in biofluids at 37 C to study corrosion. Accelerated testing is also performed at 70 C to mimic long time exposure in a harsh, humid environment. Since the Parylene barrier layers are typically 5-15 micron thick, they are highly flexible, and hence they are interesting barriers to be used in flexible/stretchable electronics. Therefore, special attention is given to the evaluation of barrier properties when Parylene is bended and stretched

Polyimide-ald-polyimide layers as hermetic encapsulant for implants

Several requirements exist for medical devices for long term implantation. Firstly, the foreign body reaction and/or inflammation occurring upon implantation should remain mild and short in time. Moreover, the device needs to be biocompatible during the total implantation duration, hence not causing reactions which decrease the patient’s health. Finally, the device needs to work properly and safe during the total period of implantation, not suffering from corrosion or chemical degradation. To meet these requirements, diffusion of body fluids into the package should be avoided as well as diffusion of toxic device materials into the body, hence a hermetic packaging method is an absolute necessity. Here, a flexible hermetic packaging is presented using alternating polyimide and atomic layer deposited (ALD) metal oxides. Good adhesion between the inorganic ALD layers and the polyimide is required to avoid the creation of lateral diffusion pads. To obtain this, surface modifications of both polyimide and ALD layers are optimized, as presented in this paper. The hermeticity is evaluated in terms of water vapor transmission rate measurements of the film stack

Accelerated hermeticity testing of biocompatible moisture barriers used for encapsulation of implantable medical devices

Acceleration protocol plays an important role on barriers reliability evaluation for encapsulation of long-term implantable medical devices. Typically, acceleration is realized by performing tests at elevated temperature: the higher the selected temperature, the higher the acceleration factor. Nevertheless, at high temperatures, reaction mechanisms might be different, resulting in false acceleration test results. Our standard barrier performance test is based on the evaluation of corrosion of copper patterns (resistivity check, Electroscopic Impedance Spectroscopy (EIS), microscopic inspection). The temperature window for accelerated testing has been investigated for our standard barrier tests. The copper patterns, protected by a barrier layer under test, are immersed in PBS (Phosphate Buffered Saline) at temperatures up to 95°C. As barriers the following material/multilayers are selected: (1) Al2O3 ALD, (2) stacked HfO2/Al2O3/HfO2 ALD (further called ALD-3), (3) polyimide, and (4) polyimide/ALD-3/polyimide. In this presentation, the results of the test protocol evaluation will be presented. As expected, the maximum applicable test temperature is dependent on the barrier under test. Furthermore, during the fine-tuning of the accelerated test protocol, we observed for some barriers a clear influence of the shape of the Cu test patterns on the barrier performance. This can be related with processing effects when fabricating the barrier on the copper patterns. This finding stresses the determination of relevant copper patterns -or test structures in general- in order to predict the barrier performance correct for each individual application

The use of ALD layers for hermetic encapsulation in the development of a flexible implantable micro electrode for neural recording and stimulation

The use of electronic microsystems as medical implants gains interests due to the combination of superior device functionality with extreme miniaturization. Electronic devices are not biocompatible and will suffer from corrosion, hence a very good hermetic device encapsulation is of utmost importance. The hermetic sealing of implantable electronics requires extremely good bi-directional barrier properties against diffusion of water, ions and gases. Moreover, extremely long biostability against body fluids and biomolecules is an important requirement for the barrier materials. In this work, an ALD multilayer of AlOx and HfOx in combination with flexible polyimide is used as a flexible hermetic encapsulation of an electronic CMOS chip which serves as an implantable probe (so called hd TIME (active high-density transverse intrafascicular microelectrode) probe) for neural recording and stimulation [1]. The main part consists of a 35μm thin CMOS chips with electrodes on top encapsulated with alternating layers of spin coated polyimide (PI2611) and biocompatible ALD layers. The total encapsulation is developed to provide excellent barrier properties. Each ALD stack (ALD-3) consists of AlOx (20 nm) capped on both sides with HfOx (8 nm) to avoid hydrolysis of AlOx. The ALD deposition temperature is 250°C. Special attention is payed to the adhesion of the ALD layers toward polyimide and vice versa. 3 to 4 PI/ALD-3 dyads are used for the total encapsulation, since long term implantation of the medical device is envisaged. Testing however is done using only a part of the total encapsulation, in order to enable to learn about the barrier properties in a reasonnable timeframe. The WVTR of a PI/ALD-3/PI film reached a value of 2.1 10-5 g/m2day (38°C and 100% RH), the total encapsulation with 3 to 4 dyads will lead to WVTR’s in the order of 10-6g/m2day. The same PI/ALD-3/PI film has been deposited on structured copper meanders and is exposed to PBS at 60°C for 3.5 years (equivalent to 17.5 years at 37°C) [2]. Up till now, no change in Cu resistivity has been observed proving the excellent barrier properties of the PI/ALD-3/PI film. [1] Rik Verplancke et al., 2020 J. Micromech. Microeng., 30, 015010 [2] Changzheng Li et al. 2019 Coatings, 9, 57

Biocompatible packaging solutions for implantable electronic systems for medical applications

Our biocompatible packaging concept for implantable electronic systems combines biocompatibility, hermeticity and extreme miniaturization. In a first phase, all chips are encapsulated in order to realize a bi-directional diffusion barrier preventing body fluids to leach into the package causing corrosion, and preventing IC materials such as Cu to diffuse into the body, causing various adverse effects. Various clean room materials are tested with respect to their suitability as encapsulation material. In a second phase of the packaging process, all chips of the final device should be electrically connected, applying a biocompatible metallization scheme using eg. gold or platinum. Device assembly is the final packaging step, during which all system components will be interconnected. To provide sufficient mechanical support, all these components are embedded using a biocompatible elastomer such as PDMS

Soft, comfortable polymer dry electrodes for high quality ECG and EEG recording

Conventional gel electrodes are widely used for biopotential measurements, despite important drawbacks such as skin irritation, long set-up time and uncomfortable removal. Recently introduced dry electrodes with rigid metal pins overcome most of these problems; however, their rigidity causes discomfort and pain. This paper presents dry electrodes offering high user comfort, since they are fabricated from EPDM rubber containing various additives for optimum conductivity, flexibility and ease of fabrication. The electrode impedance is measured on phantoms and human skin. After optimization of the polymer composition, the skin-electrode impedance is only similar to 10 times larger than that of gel electrodes. Therefore, these electrodes are directly capable of recording strong biopotential signals such as ECG while for low-amplitude signals such as EEG, the electrodes need to be coupled with an active circuit. EEG recordings using active polymer electrodes connected to a clinical EEG system show very promising results: alpha waves can be clearly observed when subjects close their eyes, and correlation and coherence analyses reveal high similarity between dry and gel electrode signals. Moreover, all subjects reported that our polymer electrodes did not cause discomfort. Hence, the polymer-based dry electrodes are promising alternatives to either rigid dry electrodes or conventional gel electrodes