Emerging Encapsulation Technologies for Long-Term Reliability of Microfabricated Implantable Devices

Abstract

The development of reliable long-term encapsulation technologies for implantable biomedical devices is of paramount importance for the safe and stable operation of implants in the body over a period of several decades. Conventional technologies based on titanium or ceramic packaging, however, are not suitable for encapsulating microfabricated devices due to their limited scalability, incompatibility with microfabrication processes, and difficulties with miniaturization. A variety of emerging materials have been proposed for encapsulation of microfabricated implants, including thin-film inorganic coatings of Al2O3, HfO2, SiO2, SiC, and diamond, as well as organic polymers of polyimide, parylene, liquid crystal polymer, silicone elastomer, SU-8, and cyclic olefin copolymer. While none of these materials have yet been proven to be as hermetic as conventional metal packages nor widely used in regulatory approved devices for chronic implantation, a number of studies have demonstrated promising outcomes on their long-term encapsulation performance through a multitude of fabrication and testing methodologies. The present review article aims to provide a comprehensive, up-to-date overview of the long-term encapsulation performance of these emerging materials with a specific focus on publications that have quantitatively estimated the lifetime of encapsulation technologies in aqueous environments.

Long-term Encapsulation of Platinum Metallization Using a HfO2 ALD - PDMS Bilayer for Non-hermetic Active Implants

“…Moreover, water build up under the parylene can create additional stress in the layer, leading to cracks and exposure of the metal. Adopted from the semiconductor industry, various inorganic thin-film coatings have been reported recently with high barrier properties against oxygen and water [8]. These thin coatings can be deposited in various ways with their insulation properties being greatly dependent on the deposition parameters.…”

“…The lifetime reliability of such coatings, however, does not rely only on the conformality and adhesion of the layer, but also on the stability of the layer in ionic media. For example, Al2O3 has been reported to have one of the highest barrier properties of all inorganic ALD layers, yet its low underwater stability can result in dissolution [8], [9]. Given the ultra-low thicknesses of these coatings, any degradation or dissolution can significantly affect their insulating properties.…”

Ultra-Long-Term Reliable Encapsulation Using an Atomic Layer Deposited HfO2/Al2O3/HfO2 Triple-Interlayer for Biomedical Implants

“…Researchers show that the Al 2 O 3 is a good moisture barrier for up to 24 h at room temperature, however this layer later deteriorates as it starts to dissolve in solutions [25,27]. Therefore, other ALD oxides like TiO 2 [17,22], SiO 2 [6,29], and HfO 2 [25,30] have been investigated as an alternative layer or capping layer of Al 2 O 3 to increase the stability and barrier properties in a liquid water environment. In the latter case, Al 2 O 3 is still very useful as a moisture and ion barrier for the total stack.…”

Accelerated Hermeticity Testing of Biocompatible Moisture Barriers Used for the Encapsulation of Implantable Medical Devices

“…However, these polymers do not meet the stringent barrier requirements (very limited diffusion of water, ions, and molecules) to act as barrier layers for long-term implantation [11][12][13]. Hence the polymers have to be combined with extra diffusion barriers, such as thin films of biocompatible metal oxides (Al 2 O 3 , SiO 2 and HfO 2 ) [14], deposited by atomic layer deposition (ALD).…”

“…PI has been used as a flexible substrate and carrier for implantable medical devices for a long time, and its non-(cyto)toxicity has been proven in many in vitro and in vivo studies [7,14,[34][35][36][37][38][39]. PI was selected because of its high thermal stability, making it compatible with the thermal ALD processes performed at elevated temperature at 250 • C. Verplancke et al recently developed a PI encapsulated active high-density transverse intrafascicular micro-electrode probe to interface with the peripheral nervous system [40].…”