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Figures 1a and 1b: Left panel—proposed cochlear implant schematic (Action project [2]); right panel—schematic of the cochlear implant inserted into a cochlea (Courtesy MED-EL).
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Figures 1a and 1b: Left panel—proposed cochlear implant schematic (Action project [2]); right panel—schematic of the cochlear implant inserted into a cochlea (Courtesy MED-EL).
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Figure 2: Moisture permeability across different materials [3].
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Figures 3a and 3b: Schematic of a packaging concept (left); sealed package with integrated VCSEL inside (middle); overall package with feedthrough (right).
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Figures 3a and 3b: Schematic of a packaging concept (left); sealed package with integrated VCSEL inside (middle); overall package with feedthrough (right).
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Figure 4: Examples of AIMD applications in which this packaging technology can make the difference.
Sometimes it can seem that the visions of science fiction come true, just with a few years’ delay. One prominent example of this is the identification tag, a microchip implant placed under people’s skin to identify them and to store data. While this is a good piece of technology, it can sound disturbing to some of us. Yet the management of many health problems could benefit from innovations using these miniaturised implants.
Our societies are growing older with an increasing population of elderly people and with fewer health care professionals available per capita. Thus, disease management and point-of-care devices are becoming more important than ever. Improving the quality of life of chronically ill patients is also a field that is attracting tremendous interest. Innovations in sensor and actuator technologies have helped to open up more and more opportunities to pursue new developments in these directions. Many new wearable and implantable devices are being developed around the world to offer the stimulation and sensing of physiological functions like blood pressure or blood glucose levels.
Active implantable medical devices (AIMDs) have different advantages and challenges than wearable devices. AIMDs enable the direct measurement of physiological parameters at the point of interest with low noise or can stimulate/actuate functions much more locally. At the same time, AIMDs need to be miniature in most cases in all the three dimensions when used for the applications discussed here and be highly biocompatible. Packaging is one of the main constraints for the miniaturization of AIMDs. The packaging technologies currently used are bulky and have changed little for decades. Further miniaturization, using only highly bio-stable and biocompatible materials [1], and new features like micro electro mechanical systems (MEMS) integration, transparency for RF, and optical signal transmission are required to enable new medical applications. A novel, long-term-implantable, miniature, and RF and optically transparent AIMDs package developed by CSEM will be presented in the following paragraphs.
One of the applications used to demonstrate this packaging technology is an optical cochlear implant. State-of-the-art cochlear implants rely on electric fields to stimulate the auditory nerves. These fields are provided by stimulating electrodes inserted into the cochlea. There are challenges associated with this approach, including electrical interference from the environment or between nearby electrodes that reduces the extent to which the stimulation can be localized and focused spatially. This can cause unwanted effects such as facial nerve twitching. Furthermore, performance is limited by the extent to which the electrical field can be controlled by the position of the electrode and by the field-shaping capabilities of the device.
Using an array of light sources to directly stimulate unmodified or optogenetically modified nerves is currently a hotly debated topic and the subject of a significant amount of research. Many of the abovementioned challenges that relate to electrical stimulation, such as interference between adjacent electrodes, could be solved using such a light-stimulation approach. An opto-acoustic (OA) single channel device being studied in the ACTION project (EU project [2]) has the potential to replace the hearing mould worn by users in therapies that use a combination of electrical stimulation and a hearing aid device, thereby freeing up the outer ear and offering the user a greater degree of comfort. This OA device can also help patients with mixed hearing loss (the mechanical conductive component in middle ear does not function), as it bypasses the middle ear by generating sound waves in the intra-cochlear fluid. The device used as a light source for optical stimulation or OA is a vertical cavity surface emitting laser (VCSEL).
The VCSEL itself is made of materials like indium, gallium, and arsenide, which are not biocompatible and cannot be implanted. A biocompatible and bio-stable package is required to encapsulate the VCSEL, precluding contact with bodily fluids or tissue. The body fluids should not ingress the package and disturb the functionality of the device during its entire lifetime. The long-term degradation of semiconductor devices due to moisture is thought to be a main potential degradation mechanism as the body is saturated with water. Just three monolayers of water are enough to start corrosion by anodic or cathodic reactions or dissolution within days or months. But a lifetime of tens of years is required for such an implant.
A packaging material with very low permeation is required if moisture ingression is not to degrade the VCSEL’s performance over the lifetime of the device. Sapphire was selected due to its well-known qualities of bio-stability, biocompatibility, and very low permeation with regard to water vapor (see Figure 2: Pure crystals), and the optical transparency necessary for the application. Besides the advantages, there are multiple challenges posed by using sapphire as the packaging material. There is no off-the-shelf solution for hermetically bonding sapphire to sapphire at low temperatures, which is required given the maximum temperature rating of the VCSEL (180°C). Another challenge is to provide the electrical connection and two feedthroughs for powering the VCSEL without compromising the hermeticity. Machining sapphire also brings additional challenges.
A new feedthrough technology was developed for two feedthroughs to drive the VCSEL. Feedthroughs were formed using a platinum/platinum-Iridium metal, which is integrated into a sapphire chip that includes a cavity (see figure 3). The platinum/platinum-Iridium metals are used in most AIMDs and have a long history of proven biocompatibility. The VCSEL is placed in the cavity using die-bonding processes and it is electrically connected to the feedthroughs using wire bonds or flip-chip bonding techniques. A novel, laser-based method is used to seal the sapphire package with a sapphire lid as shown in the schematic in figure 3.
The temperature of the sealing process is critical if the device is to work without any change in functionality. The temperature endured by the VCSEL was checked using low-melting-point materials integrated into the cavity, and was found to be lower than 120 C. The seal was very strong with 105 MPa being measured on average. Physiological saline tests carried out at 94 C for 1,000 hours showed no corrosion of the package or seal.
A non-destructive leak-testing method was developed for the package based on Fourier transform infrared spectroscopy (FTIR)—a method that yields a resolution at least 100 times better than conventional helium leak tests. The packages are exposed to a specific gas at high pressure (also called “bombing”) to accelerate the flow of gas through any leaks. The FTIR method uses the infrared transparency of the package to measure the concentration of this specific gas inside the package after the bombing step has been carried out for a specific period of time. The gas has a peak absorption at a specific wavelength, which differentiates it from other gases in the air. From the concentration of gas present inside the package after the high pressure exposure, the leak rate for moisture into the package can be calculated. The moisture concentration in the package should be far below the dew point of water, which is 6000 ppm of moisture at 0 °C. Even though it could be argued that body temperature is 37 °C, which could have a dew point at a higher moisture level, impurities on surfaces could cause local condensations and this is why 6000 ppm is taken as the reference. Considering a package cavity depth of 0.4 mm, this method can measure a leak tightness or a moisture concentration of less than 6000 ppm for ten years or more.
A sapphire package as small as 0.6 mm x 1 mm x 2 mm was developed for the optical cochlear implant using only long-term-biocompatible materials. Further miniaturization is expected to achieve a size of 0.7 mm x 0.6 mm x 1.5 mm, which would enable the package to be inserted further into the cochlea. The VCSEL was tested and shown to be functional after all the packaging steps. The leak tightness of the sealing was also tested to confirm theoretically functionality for at least 10 years in the cochlear fluid.
This packaging technology for long-term, miniature active implants has also been verified for other applications including an implantable MEMS pressure sensor. The technology has the scope to enable many more AIMD applications in various medical fields, as depicted in figure 4. Therefore the future work carried out in this direction by CSEM, will be focused on using the technology for the packaging of various novel AIMD applications.
The specific laser-based sealing method has also been be demonstrated for glass-on-glass and for glass-on-silicon sealing, opening up many more fields of application beyond that of medical implants.
[1] Zhou D.D., Greenbaum E., Implantable Neural Prostheses 2, 361 Biological and Medical Physics, Biomedical Engineering, , Springer Science+Business Media, LLC 2010, pp. 28-33.
[2] ACTive Implant for Optoacoustic Natural sound enhancement, 2014, retrieved: http://www.action-project.eu/texte/01-flyer-abstract.pdf.
[3] Tummala R.R., Rymaszewski E. J., Alan G. Microelectronics Packaging Handbook: Semiconductor Packaging, Springer science and business media, 1997, pp: 11-123.