8
Biomaterials for Bionic Implants
8.1 Introduction
Bionic implants contain the elements of biology and technology using electrical signals to replace, modify, or enhance the body or specific body part. Technology is utilized to make the implant function as the original body part functions. Many of these implants incorporate the means to stimulate cells with voltage, charge, and current; many also involve the use of sensors for feedback via biosignals [1]. A multitude of bionic implants exist for various parts of the body, including bionic eyes and bionic limbs, while aspects of bionic implants can influence tissue engineering as well. Utilizing electric fields, cells in muscle and other excitable tissue can be stimulated to repair injuries and integrate implants [1]. The biomaterials used for the various bionic implants can be divided with regard to the location and function, with an emphasis on stimulating and recording specific cells.
Recording signals and stimulating cells in the body commonly utilize metallic electrodes, such as titanium, platinum, and silver, for their electrochemical stability. Differing metallic alloys can be used according to the function of the electrode and the location of recording or stimulating. Other than metallic substances, organic conducting polymers have also been in use for more flexibility in usage with the drawback of less electrochemical stability [2]. Current studies have been looking into graphene‐based electrode materials due to their stability and malleable mechanical properties while effectively recording and stimulating electrical signals in biological systems [1]. While the materials currently still used remain metallic electrodes, further studies can bring about more usage of polymer‐based organic conductive materials, such as graphene polymers, for more effective and safer materials to record and transmit electrical signals to the biological systems.
One of the most important challenges in bionic implants consists of keeping the electronic components of the devices separate from its surroundings in the body, an essential aspect in order to pass FDA regulations [3]. In order to maintain the safety of the individual while ensuring the long‐term usage of the device, the separation, or encapsulation, of the electronic parts must be robust, especially in implants in delicate areas of the body. Some of the common encapsulation techniques incorporate titanium, ceramic, semirigid polymers, and organic coatings; however, while these materials effectively separate the biological surroundings and the electronics, they do not last long term. As such, they are not suitable for implants in delicate regions of the body where precise surgery is required as neural implants and retinal implants do [3]. Failure of long‐term encapsulation could have detrimental effects on the brain or eye. Currently, separation methods are tested with different materials to make use of their properties to make a lasting implant. One such encapsulation material is hermetic diamond encapsulation with gold active alloy brazing, making use of the longevity and bioinert natures of diamond and gold [4]. With further studies on this material, a more efficacious encapsulation material and method can be used in bionic implants to protect the body and the technology.
Other aspects of bionic implants must be considered as well, though they may vary according to the body part it is repairing or replacing: A bionic limb will require different components than a bionic retinal implant. They will need electrical components and electrodes, as well as safety measures such as encapsulation methods to protect the biological and electrical components, but ultimately have different functions and need different material properties to combat problems. Bionic limbs are used to replace limbs with controllability and possibly restore sensation. To do so, electrodes are placed on residual muscles and nerves to stimulate, in which the tissue will send the pulses to the brain and the individual will have tactile sensations [5]. However, the materials for such an implant needs to be further researched in order to prevent infection and ensure the longevity of the devices while staying resilient through stress. As can be observed, some aspects of the issue remain specific to bionic limbs, while other bionic implants may need more studies in other properties fulfilling their own respective needs for their specific functions.
8.2 Biomaterials in Bionic Eye and Neural Systems
Bionic implants in the eye require a great degree of care in structure, placement, and function. This implant was one of the main impetuses for the longer‐term encapsulation method of utilizing hermetic diamond encapsulation with gold active alloy brazing as the implant should be able to remain within the body for as long as possible before needing another complicated procedure to replace the device [3]. However, other materials go into creating the implant as well, with careful attention to the electronic components and the parts of the device touching the biological surrounding, as seen in Figure 8.1. Recently, problems have arisen with the lack of benefits opposed to the risks and adverse effects of utilizing some of the commercially available retinal devices, such as the Argus II in Figure 8.1. As such, the need to study and optimize the different components of these implants has increased to make the benefits of the device rise and for any potential risks and issues to fall [7].
Figure 8.1 (a) External and (b) implant part of the Argus II system; (c) illustration of the implantation sites of the visual cortex, epiretinal, subretinal, and supra‐choroidal prostheses.
(From Ref. [6], 2016, Elsevier).
8.2.1 Implant Package
One of the pivotal factors of eye implants, specifically the retinal implant, is in the implantation and the aspects of the device exposed to the biological surroundings after implanting; the common methods of packaging include encapsulation and hermetic encasing [8].
The implant needs to be biocompatible and resistant to erosion; as such, the materials come in either hard or soft packaging, with hard packaging consisting of metals, glass, and ceramics, while the soft packaging consists of layers of soft films [6]. The hard package can use a substrate, such as platinum or alumina, fitted with a titanium ring and cap with capabilities to host hundreds of electrode feeds. While this type of package remains the most common technique due to its ability for mass production, the need for higher‐density feedthroughs may lead to more risks of failure [6]. The soft packaging may be able to solve this problem by allowing for high‐density feedthrough. The use of liquid crystal polymer, fitted to the eye and incorporating electrode insertion, is one such soft packaging with benefits of higher‐density feedthrough, stability in the implant, and more comfort when in use [6].
Alumina, aluminum oxide, is a common encasement material due to its biocompatible nature. Although a few issues have arisen with the limitations in fabrication, new techniques of using alumina, such as using atomic layer deposition (ALD) to create thin alumina films, have started to be studied [9]. This method includes nanoscale precision and conformity in order to be used as an encapsulation material, as well as providing the electronics with its low moisture permeability [10]. Furthermore, to increase the stability of alumina in liquid environments, layers of HfO2 through ALD on the alumina films can be used to ensure the barrier [10]. HfO2 can work with other materials as well to create more conformable encapsulations.
HfO2 can be layered with SiO2 through ALD to make encapsulations so far able to withstand saline for hundreds of days without the layers dissolving [10]. SiO2, silicone dioxide, can be comparable to alumina though with higher conformability and compatibility with the biological surroundings and electronic components. Thermally grown silicone dioxide offers low water permeability with a long lifespan as an encapsulation material with high flexibility, though it would function better with a coating to ensure ions will pass through the material [11]. With multiple layers of varying materials, from alumina to the silicon dioxide, longer lasting encapsulations can be made to provide a sufficient barrier between the electronics and biological surrounding [11].
Another area of interest for long‐term encapsulation includes the use of diamonds, as mentioned with the hermetic diamond encapsulation, due to their effectiveness and longevity as barriers as well as their biocompatibility. The nonpermeable barrier is attributed to the atomic structure of diamond [12]. However, the structural integrity and barrier capabilities come with the difficulties of fabricating with it as it is a very hard and nonmalleable material [13]. Some of the methods to use and grow the material include growing microcrystalline or nanocrystalline films with hydrogen plasma in conventional deposition and growing ultra‐nanocrystalline diamond (UNCD) films using microwave plasma‐enhanced chemical vapor deposition (MPECVD) with argon plasma [14]. With a plethora of studies, diamond has been observed to remain intact when implanted in either the eye or other neural surfaces; neurons have grown and cultivated in diamond substrates as well [15]. The implications of neuronal and cell growth on diamonds add to the benefits of the material as the material allows for neurite growth, better adhesion, and longevity in functionality when implanted [13].
8.2.2 Electrodes
As previously mentioned, metal electrodes are the primary choice due to their bioinert nature, while making sure the specific metal is noncorrosive. Despite these aspects of the metals, they remain a foreign substance to the body and may lead to immune responses and scar tissue when used within the body [16]. Graphene, other carbons, and organic conducting polymer‐based electrodes have been studied to use their own properties while remaining relatively organic [1]. One proposed solution utilizes graphene and carbon nanotubes as coatings on the metallic electrode array to change the surface electrochemistry of the contact points. The coatings can be done with chemical vapor deposition (CVD) for graphene or layer‐by‐layer carbon nanotube composite coating; these have been tested with platinum electrodes to improve the impedance of the electrodes without a potential of introducing cytotoxicity [16].
Conductive polymer coatings, compared to metal and carbon‐based coats, are softer, potentially providing a better interface for neuronal contact (Figure 8.2). These conductive polymer coatings get electrochemically deposited from monomer solutions with dopant molecules, which can be functionalized by incorporating nonforeign molecules to reduce the chance of inflammatory responses [16]. While this also leads to impediments in the electrode, it introduces the idea of utilizing living cells into these bionic devices, such as the use of stem cells or embedded neural cells [17].
Figure 8.2 Schematic of tissue–electrode interfaces. (A) The electrodes of bioelectronic devices are usually implanted within 100 μm of the target tissue, and exchange of electronic signals occurs at a nanoscale electrolyte–electrode interface. (B) Application of various non‐metallic material coating technologies as electrode interface, includes bare electrode site, hydrogel, conductive polymer, carbon nanotubes, cell attachment proteins, mobile drug molecules, and embedded neural cells.
(From Ref. [17], MDPI, CC BY 4.0).
8.2.2.1 Diamond Electrodes
As explained with the encapsulation materials, diamond is another material with biocompatibility, longevity, conductivity, and ability to host cell growth. Diamond electrodes have been proven to be a viable choice for use in neural interfacing [13, 18]. Optimal electrodes have a high capacitance to be able to affect the ganglion cells within eyes while remaining safe; diamond electrodes, more specifically nitrogen‐incorporated ultra‐nanocrystalline diamond (N‐UNCD), can be coated with other substances such as platinum to bolster the ability of the electrode to evoke signals as diamond does not have very high capacitance as it is [19, 20]. The relatively low capacitance was one of the main issues, other than fabrication difficulties, of using diamond as electrodes, but ways to overcome this have been increasingly studied, with charge‐injection coating as a potential solution though with its own risks of degrading the electrode [18, 21]. The coatings, for example, platinum or iridium oxide, may be useful in the short term, but can be delaminated with the surrounding fluids [21].
One of the major topics of interest concerning visual and neural implants is acuity, sharpness in stimulation or vision [7]. Higher acuity can be linked to smaller electrode arrays, requiring smaller electrodes which in turn causes issue with capacitance. Potential solutions to this include using pillar electrodes with variances in the 3D formation of the electrode to increase the surface area of the electrode while maintaining the miniaturization, an example seen in Figure 8.3 [22, 23].
Figure 8.3 (a) Scanning electron microscopy (SEM) image of a 5 × 5 pillar array, (b) flat array with a comparable 80 μm electrode size, (c) magnification of pillars featuring an interphase of nitrogen‐doped ultrananocrystalline diamond (N‐UNCD) and polycrystalline diamond (PCD), and (d) close‐up view of the N‐UNCD structure showcasing the characteristic grain size of N‐UNCD.
(From Ref. [22], 2020, American Chemical Society).
In vivo testing of UNCD electrodes has shown promise for high acuity due to the cortical spread values, though with some trouble involving retinal trauma seen in some of the subjects due to the rigidity of the device [24].
Although issues remain with utilizing diamond electrodes, a plethora of in vivo testing also prove the robustness and longevity of the addition of the material, whether using UNCD or other diamond‐enhanced electrodes in retinal and neural settings [25, 26]. One such study compared the use of diamond‐based electrodes with the common carbon fiber electrode in detecting neurochemicals in the human brain [27]. The inclusion of diamond in these tests ranging different researchers and uses for the electrode varies in how the material is incorporated. One method, also the way to create UNCD, is with CVD to grow crystalline‐form diamonds to be used as coating [26]. These CVD diamonds can be doped with different elements, commonly boron‐doped diamond (BDD) or nitrogen‐doped diamond, to be more conductive as electrodes [26, 28]. The BDD electrode in particular was tested in vivo within a minipig to display the long‐lasting properties of the electrode after it remained usable after 3 months, indicating how the electrode lasts in biological surroundings before use in humans [29]. Recent in vivo testing in cats shows the precise production of cortical activity in the retina after stimulation with N‐UNCD, though the results relied on the aftermath of a challenging surgery with only 10 of 16 successful [24]. The size and rigidity of the device attribute to the need for a complex surgery in regard to retinal implants, necessitating improvements in the design to make further in the functionality. The study demonstrates how a small diamond electrode array, of 5 × 5 mm, can produce high acuity stimulations, yet even this size and the weight of the device remain an issue in placing without causing retinal trauma [24]. As such, many in vivo tests prove the capability of diamond electrodes, with some already in use in devices; however, the inherent problems with the rigidity and size remain areas to be studied further [26].
Even more current studies report on fabricating diamond‐based materials with reduced risk of tissue damage from the rigidity. Most of these methods incorporate BDD as the specific conductive diamond material due to its ability for various morphologies and geometries, with one group fabricating all‐diamond microfibers consisting of BDD encapsulated in nonconducting polycrystalline diamond cladding to detect neurotransmitters [30]. The microfabrication takes into account how carbon‐based materials with smaller dimensions, of around 7 microns, have been proved to reduce scarring more than silicon electrodes [31]. The findings were translated into creating the BDD all‐diamond microfiber, with batch production for wafer‐level microfabrication of the devices in the micro‐dimension level, though with slight variances [30]. The creation of devices like BDD microelectrodes with all‐diamond microfibers holds potential for future diamond‐based electrode fabrication. By leveraging the patterning capabilities of CVD diamond growth, these electrodes could be used in retinal and neural implants, minimizing the risk of scarring and trauma. [32].
Some groups have even developed flexible microelectrodes with BDD, utilizing wafer‐scale fabrication to use the growth side of the thin BDD films as the surface of the electrodes as opposed to the nucleation side. This technique allows for more flexibility while allowing for improved stability and sensitivity due to the rougher surface texture of the growth side [33]. The fabrication procedure to create this flexible BDD electrode consists of plasma etching a Parylene C substrate and anchoring the BDD with the growth side exposed. This BDD–Parylene film was then used in in vivo studies to demonstrate recording capabilities and biocompatibility as a less rigid electrode with potential for further benefits as the rougher surface can lead to reduced impedance [33]. Even with a few issues in fabrication and use, diamond proves itself to be a viable material to study and optimize further to create electrodes with longer lifespans in biological surroundings with high acuity and biocompatibility.
Other than the flexibility and rigidity aspects of diamond electrodes, other areas of improvement for the electrodes, particularly BDD, include better selectivity and sensitivity in receiving and transmitting signals. Methods to improve incorporate functionalizing the surface with nanoparticles such as carbon black and gold through self‐assembled monolayers (SAMs) [32, 34]. Other ways to apply the nanoparticles other than SAM include self‐assembled multilayer stacks, electropolymerized nanocomposites, and NP‐polymer suspension coating [32, 35]. With additions to the surface of BDD, the electrode can have differing electrochemical reactions to the biological environment, leading to higher sensitivity in obtaining signals. However, with any nanocomposite and polymer added, the reaction to the biological surroundings and the device need to be prominent as pH and other factors may affect the device or vice versa [32]. The diamond‐based electrode can be improved upon, but care must be taken into the benefits and drawbacks of how the device is improved, with considerations of what is important in the implant function.
8.2.2.2 Organic Polymer Electrode Coating
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