An all-diamond, hermetic electrical feedthrough array for a retinal prosthesis
Introduction
Implantable electronic devices must necessarily adhere to a very strict set of standards before they can be approved for clinical use. One of these standards is the requirement that electronically active components such as microprocessors must be encapsulated in a hermetically encapsulated [1], to protect the body from the toxicity of conventional electronic components and as well as to protect the components from the harsh environment inside the body which leads to accelerated device failure. Encapsulation materials such as titanium or ceramics have a long history of success in devices such as pace-makers and cochlear implants [1]. Such materials are impermeable to water or gasses and are very well tolerated by the body. The critical complication in most implantable devices is the need to cross the wall of the encapsulation with electrically conducting wires, commonly called a feedthrough. Very often wires are required to supply power and data to components sealed with in the hermetic capsule. In some cases, wires may also be required to carry electrical impulses to or from neural populations targeted by the device. Feedthroughs are understandably one of the most common failure points of hermetic capsules and therefore must be a carefully considered element in the design of any implantable device.
The feedthrough array discussed in this paper is designed to perform both as an electronics encapsulation and as a stimulation interface to the retinal tissue for an epi-retinal vision prosthesis. Recently there have been significant advances in the field of bionic vision [2], [3], [4], [5], [6] including the world's first commercially available implant [7]. Retinal prostheses consist of an array of stimulating electrodes that are surgically implanted against the surface of the retina. They are typically positioned on or close to the macular region of the retina, the area responsible for high acuity vision [8]. The devices are designed to treat diseases such as retinitis pigmentosa where the light detection cells within the retina are damaged but most of the inner middle retinal neurons such as ganglion and bipolar cells survive. These surviving cells can be electrically stimulated with an array of electrodes resulting in recipients perceiving an array of spots (or phosphenes) which can be used to form a crude image. Current state of the art epi-retinal devices tend to have around 60 electrodes positioned close to the retina providing the user with low resolution vision. An obvious potential precursor to achieving a higher resolution outcome for the patient is to employ higher numbers of electrodes positioned closer together. High numbers of stimulating electrodes require a large numbers of feedthroughs from the control electronics with the result that the reliability of an individual feedthrough must be extremely good. Very often arrays of stimulating electrodes are connected to the electronics capsule by a flexible leadwire. When hundreds or thousands of electrodes are called for, individual wiring of each electrode and routing wires to remote capsule becomes untenable, in particular in the eye where physical space is limited.
Fig. 1 shows an illustration of the approach currently employed by Bionic Vision Australia (BVA) to solve both the feedthrough reliability problem and the issue of connecting control electronics to a high number of stimulating electrodes. The illustration shows an array of diamond feedthroughs and electrodes. The feedthrough array (grey) is constructed from two types of diamond; a polycrystalline, electrically insulating diamond substrate containing many electrically conducting nitrogen doped ultra nano-crystalline diamond (N-UNCD) feedthroughs. On the flip side of the array (shown in Fig. 2(c)) the feedthroughs terminate in isolated N-UNCD pads which are employed as the stimulating electrodes of retinal prosthesis. We have previously shown that N-UNCD has appropriate electrochemical characteristics to act as a neural stimulation material [9], [10]. The fact that the substrate and feedthroughs are made from the same material, i.e. diamond, minimizes the possibility of feedthrough failure through materials mismatch, resulting in increased reliability. Importantly for our application the mechanical strength and low density of diamond means the capsule can be made thinner and lighter than it could be if made from a ceramic or a metal such as titanium. The approach also takes advantage of diamonds established biocompatibility [11], [12], [13] and superb biochemical stability [14] offering the prospect of a long lasting implant. Finally, the pitch and shape of the array shown in Fig. 1 has been designed to be directly flip chip bonded to a purpose built ASIC capable of delivering electrical stimulation through 256 independently controllable channels. Direct bonding of the stimulator to the array negates the need for a high count lead to the electrodes and thus the technology is easily scalable to higher numbers of electrodes. The metal tracks on the inner side diamond array (Fig. 1, right and upper edge) connect to a small number of supply power leads, forward data leads, counter electrode lead, backward data lead and power decoupling capacitors. Following is a description of the method employed to fabricate the diamond arrays and results of hermeticity, electrical testing and cytotoxicity testing.
Section snippets
Feedthrough fabrication process
Thermal management grade polycrystalline diamond wafers (TM100 grade, 10 mm × 10 mm × 0.25 mm, element six Ltd) were used as the feedthrough substrates. The diamond wafers were supplied with one smooth face (<1 nm RMS roughness) and one very rough face. The rough face was polished to <10 nm RMS surface roughness using a resin bonded wheel on a Coborn PL3 rotary polisher (Fig. 2(a)). Feedthrough arrays were fabricated within the polished diamond wafers according to the schematic shown in Fig. 2.
Imaging
SEM images of the inside of a feedthrough pit and a single feedthrough hole are shown in Fig. 3(a) and (b). In this instance the Silver-ABA fill has been removed by dissolution in 2.5 m HNO3 to show the structure of the diamond only. Fig. 3(c) shows the external face of a feedthrough array with flat 120 μm square pads of N-UNCD covering the feedthrough holes. Fig. 3(d) is a close up view of the N-UNCD surface showing the very high nano-scale roughness that occurs under our growth conditions.
Discussion
It is important to note that many of the dimensions listed for fabrication of the feedthrough array described in this paper have been chosen to fit a specific application. The provided dimensions are by no means a prescriptive set of parameters for a hermetic feedthrough array. An example is the substrate thickness of 250 μm which was chosen for reasons of mechanical strength and to allow latitude for shaping of the array face to fit the curvature of the eye. The 150 μm deep pits are a
Conclusion
A versatile scalable method has been developed to fabricate dense hermetic micro arrays of electrical feedthroughs consisting of conducting N-UNCD channels within an insulating PCD substrate. The arrays exhibited sufficiently low feedthrough resistance and sufficiently high isolation resistance to function as a retinal stimulation array. Electrochemical testing of the N-UNCD electrodes terminating the feedthroughs showed that the favourable electrochemical properties of the N-UNCD were not lost
Acknowledgements
NICTA is funded by the Australian Government as represented by the Department of Broadband, Communications and the Digital Economy and the Australian Research Council through the ICT Centre of Excellence program. The Bionics Institute acknowledges the support they receive from the Victorian Government through its Operational Infrastructure Program. This research was supported by the Australian Research Council (ARC) through its Special Research Initiative (SRI) in Bionic Vision Science and
References (30)
- et al.
Electrical stimulation of retinal ganglion cells with diamond and the development of an all diamond retinal prosthesis
Biomaterials
(2012) - et al.
Benign response to particles of diamond and SiC: bone chamber studies of new joint replacement coating materials in rabbits
Biomaterials
(1996) - et al.
Electrochemical synthesis on boron-doped diamond
Electrochim Acta
(2012) - et al.
Effects of plasma treatments on the nitrogen incorporated nanocrystalline diamond films
Diamond Relat Mater
(2008) - et al.
Biocompatibility of chemical-vapor-deposited diamond
Biomaterials
(1995) - et al.
The use of nanodiamond monolayer coatings to promote the formation of functional neuronal networks
Biomaterials
(2010) - et al.
Ordered growth of neurons on diamond
Biomaterials
(2004) - et al.
The effect of ultra-nanocrystalline diamond films on the proliferation and differentiation of neural stem cells
Biomaterials
(2009) - et al.
Influence of the surface termination of ultrananocrystalline diamond/amorphous carbon composite films on their interaction with neurons
Diamond Relat Mater
(2012) - et al.