Atomic Layer Infiltration for the Enhanced Longevity of Thin-Film Polymer Implant Encapsulation

Bioelectronic interface technology has advanced significantly since its inception, as implantable devices develop smaller form factors and better integration with the human body. Advanced implants have thousands of channels interfacing with hundreds of neurons, and often feature wireless transmission of power and data enabled by miniaturized active electronic components and multiplexing techniques. However, while the technical capabilities of implantable bioelectronics continue to increase at an astounding rate, their ability to remain functional after long periods of implanted time has not seen significant improvement, especially in the field of flexible polymer-based thin film devices. Due to their inherently poor barrier properties, polymer implants require additional hermetic encapsulation to become viable therapeutic instruments for several decades of constant use. In this regard, metal oxide, nitride, and carbide materials deposited by atomic layer deposition (ALD) represent some of the best-performing barrier materials in terms of low permeability to water and ionic species, yet they are mechanically incompatible with flexible electronic substrates: bending and stretching of the flexible substrate invariably results in cracking or delamination of the rigid barrier coating, reducing its protective capability to near zero. Here, we explore a method of increasing the mechanical compatibility of inorganic barrier coatings with flexible substrates by means of a modified ALD technique alternately known as atomic layer infiltration (ALI), vapor phase infiltration (VPI), or sequential infiltration synthesis (SIS), which results in a gradual transition between the mechanical properties of the substrate and coating, reducing the susceptibility of the interface to such catastrophic failure modes. During ALI, precursors are allowed to remain in the reaction chamber for longer times at higher pressures than ALD. This allows precursors to diffuse into the substrate and reaction products to form at depth, resulting in a gradient from pure substrate to pure coating material. This technique has been used to create flexible, nearly impermeable coatings for organic electronics applications, but to our knowledge has not been demonstrated in implantable bioelectronics. We demonstrate water vapor barrier performance of the ALI encapsulation strategy on the order of 10^-4g/m²/day, as well as high mechanical compliance (<i>i.e., </i>critical bending radius below 500μm without delamination). In addition, we fabricate test devices featuring interdigitated electrodes on flexible substrates and encapsulate them via 3-dimensional ALI using a custom stand within a domed reaction chamber, then study the effects of ALI encapsulation on device lifespan in physiological conditions by means of accelerated aging tests in phosphate buffered saline (PBS). We investigate the accelerated aging results of current bioelectronic implant encapsulation strategies and compare them with our own results. Our uncoated polyimide devices failed after 80-100 days at 37°C, while coated devices have exceeded 230 days of 37°C-equivalent aging and continue to remain below failure thresholds. Optimization of the encapsulation via material selection (<i>e.g.,</i> polyimide vs. parylene substrates, Al₂O₃ vs. TiO₂ ALI) and multilayered structure is explored. In-depth failure mode analysis of the tested thin film devices is performed, analyzing cracking, delamination, moisture permeation, and connector failures using optical and scanning electron microscopy and permeation analyzer; critical failures modes upon specific loading are discussed.