Silk and Silicon based Devices for Bio-Integrated and Bio-Resorbable Electronics

Abstract

This paper focuses on the use of silk and silicon along with other materials to make bio-compatible electronics. Silk acts as a platform for electronics to be built on its surface. Both biointegrated and transient bio-resorbable electronics can be developed using these devices - based on the application. For Human Machine Interfaces (HMI), where constant connection with the brain surface is required, we use non-transient biointegration, to obtain high resolution output. Similar technology can be used for electrocorticography (brain mapping for seizure patients). Special fabrication methods causes minimal stresses on the brain tissue and provides good conformal coverage. Transient electronics is bio-resorbable electronics, where in the device dissolves within the body after pre-programmed time frames

This may be used for cases, where the device need not stay in the body for long periods of time. After the time frame, these electronics are resorbed by the body, and completely disappears

Dissolvable films of silk and silicon are used for transient electronics. The application is wide and can be used for both biomedical and non-biomedical purposes

Index Terms—Bio-integration, Transient electronics, Silk Electronics, curvilinear surface electronics, Dissolvable electronics

I. INTRODUCTION

UMAN race has come a long way from bulky electronics, and slow devices to an era of sleek and high performance electronic devices. Even though so much advancements were made, some electronic technologies could not be fully utilized due to unsolvable problems, until a few years back. Although the field of bio-integration has been developing, we were not able to make a huge impact on the integration of electronics on body with ease and efficiency. The main reason is the soft, elastic and curved surfaces of the body. The integration of such a biological system with a hard rigid wafer of silicon will lead to a mismatch in mechanics and geometry [1]. If tackling this problem can be done effectively, then we may be able to make progress in bio-electronic integration that was never possible using conventional rigid silicon electronics. The existing technologies include Utah electrode arrays that act as penetrating multi-electrode arrays with dimensions similar to the cortical neurons, that they target for recording or stimulation. This technology is able to maintain a stable signal in the CNS. But these arrays penetrate and may lead to the formation of micro-hemorrhages [3]. They also lose their electrical interface stability due to unwanted biological responses towards electrodes [4]. Another technology used is the flexible multi-channel electro corticography electrode array (ECoG array) [5]. But due to their widely spaced electrode arrays, and large contact electrodes, the electrical signals on the brain surface are under-sampled [4]. So, the ECoG arrays cannot be used where high spatiotemporal map outputs are needed

The focus is to make electronics that are bio-compatible, bendy and curvy, that will act as an effective contact on uneven surfaces like brain. This can be done using materials that can bend and form complex and curvilinear shapes without compromising the performance and bio-compatibility

Although new materials may be used to achieve this property, silicon tend to act as perfect substrates for integrated electronics at reduced thicknesses. When silicon is reduced to nano-scale, the hard and rigid property of silicon will change, and silicon becomes a soft and floppy membrane [6]. Building electronics on a nano-silicon structure, and imprinting other electronic components can help to obtain a curvy and bendable form of 'Bio-Integrated electronic platform'

The future of bio-electronics is bendable and curvy, but not just limited to that. We can modify and use other materials to build 'Bio-Resorbable Transient Electronics'. Transient electronics are electronic devices that physically disappear at prescribed times and at controlled rates [2]. These transient electronics can be used for applications in implantable medical devices, and also for non-biomedical applications that will be discussed later in the paper

II. DISCUSSION

It is important to understand the possibility of using materials to make electronics that are bendable and curvy. If silicon electronics can be made bendable, then they can be laminated on the surface of an organ like brain. Thus the electrical signals can be monitored more effectively. John A Rogers and his research group have found an effective solution to this, and is relatively simple. They found that by making silicon sheets ultrathin, it can be made flexible. The reason is common for any material (Bending strain is inversely proportional to thickness) [7]. This is followed by fabricating other electronics on to the silicon substrate.

A. Bio-Integrated Electronics

Bio-Integrated Electronics is the possible next best leap in biomedical field. The process of Bio-Integration will be possible only if suitable bio-compatible efficient techniques are available. Bio-Integration can be of two major types

(i) Epidermal Electronics and (ii) Bio-Integration within the body

(i) Epidermal electronics can be implanted on the surface of the skin (on any epidermal part of the body). Much work is done on this technology and researchers have made commendable progress in a span of some years [7]. In this method they use the similar technique of using silicon nanomembranes for fabrication. The epidermal electronics is also made stretchy so as to stretch when the skin stretches. These devices will have multifunctional sensors (such as temperature, strain, and electrophysiological), microscale light-emitting diodes (LEDs) and many other active/passive circuit elements [8]. They also make use of serpentine nanoribbons that help in stretching and returning back to its non-deformed state as shown in Fig 1

Abstract doesn’t contain this figure Fig 1 Epidermal Electronics on skin in compressed and stretched form.; Fig taken from [8]

(ii) Bio-Integration within human body is possible using the same techniques as of epidermal electronics. Here also the silicon nanomembranes are used as a substrate. In some cases, materials like polyimide (PI) and polymethylmethacrylate (PMMA) are put to use as well [4]. If the purpose of the device is to monitor electrical signals on the surface of the brain, then electrodes have to be implanted on the surface of the brain. To get the best conformal contact, the thickness should be minimal as observed in Fig 2

In this method, the electrodes are coated on a carrier wafer of Silicon coated with a sacrificial layer of PMMA. On top of that a PI layer is also coated. After this, the Gold or Chromium electrode array is coated using electron beam evaporation [4]

The unwanted PI is removed using etching. PMMI is later removed, and the whole electrode array is transferred from the Si/PMMA/PI carrier wafer to a layer of silk film. An Anisotropic Contact Film (ACF) is connected as an interconnected between the arrays to the external data acquisition system and PI is coated over interconnects to prevent unwanted electrical short circuit. The device fabricated is placed on the brain surface, and the silk film is slowly dissolves using saline. As the silk is removed, the electrode array mesh (<10μm) gets high degree of conformal contact due to its reduced thickness as seen in Fig 2

Abstract doesn’t contain this figure Fig 2. The conformal contact increases with reduced thickness.; Fig taken from [4]

The observations were confirmed using experimental studies on a feline brain, and the spatiotemporal output obtained showed excellent results

Other Bio-Integration techniques include similar approaches on other internal organs. An electrophysiology mapping device was also created and implanted on a porcine heart [9]

In this case the heart is a constantly moving surface, and the substrate should be strong enough to hold the strain. For that reason, a layer of thin plastic sheet is used as a physical platform. The crystal silicon nanoribbons are fabricated on the silicon wafer and transfer printed to the polyimide plastic sheet. Unlike previous brain mapping system using simple electrodes, this electrophysiology device uses more complicated electronics, including amplifier and multiplexing transistor. The transistors are fabricated using proper chemical-vapor deposition method, photolithography and etching. The gold metal act as electrodes for the device. Fig 3(A) shows a final device structure ready to implanted on to the epicardial surface

The device fabricated was placed on the epicardial surface

The mapping device adhered on to the surface due to the conformal wrapping and adhesion energy at reduced thicknesses as seen in Fig 3(B). Data obtained is processed using custom MATLAB software. By monitoring the data obtained, the propagation of paced and unpaced cardiac depolarization wavefronts are determined [9]

Abstract doesn’t contain this figure Fig 3 (A) An electrophysiology mapping device in bent state

(B) Device conforming to the epicardial tissue due to the surface tension and adhesion forces. Both figures are taken from [9]

In another method an even improved methodology to measure the spatiotemporal cardiac measurements were taken. This method used 3D elastic membranes shaped to match the epicardium of the heart via the use of 3D printing. It also had A B a platform for deformable arrays of multifunctional sensors, electronic and optoelectronic components [10]

The future of Bio-Integrated electronics is bright and researchers are trying to improve the properties even more

Another advancement in this area of research was also developed when dissolvable electronics were developed using Silk and Silicon membranes, discussed below

B. Transient Electronics

Transient electronics are electronic devices that can dissolve and physically disappear in preset time frames. By placing these transient electronic devices within the body, they can be used for many biomedical applications for a medically useful time frame, and then removed without surgery or other incisions

The physics behind this method lies with the corrosion physics, where in Silicon membranes dissolve readily in water [11]. The Fig 4(A) shows the dissolution of a silk membrane in phosphate buffer solution (PBS) over a period of days

Abstract doesn’t contain this figure Fig 4(A) Atomic force microscope topographical images of a Si NM (initial dimensions: 3 mm×3 mm× 70 nm) at various stages of hydrolysis in PBS at 37°C (B) Transient device that includes transistors, diodes, inductors, capacitors, and resistors, with interconnects and interlayer dielectrics, all on a thin silk substrate. (C) Experimental (symbols) and theoretical (lines) results for time-dependent dissolution of Si NMs (35 nm, black; 70 nm, blue; 100 nm, red) in PBS at 37°C. Figures taken from [11]

Fig 4(C) shows the rate of dissolution with respect to the thickness. Three Silicon NanoMembranes of different thicknesses are dissolved and the rate of dissolution increases with decreased thickness. Hence the timeframe of the device can be determined and programmed by controlling the thickness

Silk acts as a wafer substrate on which the silicon electronics and other devices are built upon. Fig 4 (B) shows a final device structure that is ready to be used. It consists of resistors, diodes, capacitors, transistors and inductors. In some other applications where high performance transistors, diodes, photo detectors, solar cells, temperature sensors, strain gauges, and other semiconductor devices are needed, silicon dioxide is used along with silicon

In the figure 4 (B), the electronic devices include Magnesium for the conductors, Magnesium Oxide or Silicon Dioxide for the dielectrics, the Silicon Nanomembranes (NMs) for the semiconductors and silk for the substrate and packaging material [11]. Solution casting (Silk), physical vapor deposition (Mg, MgO, and SiO2) and transfer printing (Si NMs) are the different techniques that are used for the fabrication of this device. A layer of protective encapsulation protects the electronic devices n the circuit. In this case, the protective film is made using silk (same as the platform). The devices will only start corroding or dissolving after the protective layer is fully dissolved. By that period, the required timeframe for the biomedical application will be met. All of the components, including the inductors, capacitors, resistors, diodes, junction transistors, conductors, and interconnects along with the substrate and encapsulation, will corrode and dissolve when immersed in deionized (DI) water as shown in

Fig 5 Abstract doesn’t contain this figure

Fig 5 Images showing the time sequence of dissolution in DI water. Fig taken from [11]

The chemistry involved in the dissolution process can be explained using the following equations Silicon [ Si + 4H2O _ Si (OH)4 + H2 ] Silicon Dioxide [ SiO2 + 2H2O _ Si (OH)4 ] Magnesium [ Mg + 2H2O _ Mg (OH)2 + H2 ] Magnesium Oxide [ MgO + 2H2O _ Mg (OH)2 ] All the above equations show the corrosion chemistry [11]

The electrical properties of transient electronic components, integrated circuits, and sensors are compared with conventional silicon substrate devices, and their electrical properties match the conventional devices. Power scavenging devices like Ultrathin Si solar cells (~3 mm thick) provide fill factors of 66% and overall power conversion efficiencies of ~3%, even without proper device enhancements like reflectors, or antireflection coatings [11]. These solar cells may be considered as an option for power source. Inductors and capacitors can be used as wireless antennas for mutual inductance coupling to separately power the external primary coils

The scope for the transient electronics is huge. Transient electronics has the potential to be effective in biomedical as well as non-biomedical applications. These will be discussed in the following section

C. Applications

The applications of the bio-integrated electronics include both epidermal electronics and for integration within the body. The main focus of the epidermal electronics is to remove heavy and complicated electronics structures, and to make electronics simple and easy to work with. Existing epidermal electronics (Example: constant health monitoring systems) are inconvenient to carry around and have to be fixed on the body using straps or bands

By replacing this method with the epidermal electronics, we can integrate efficient health monitoring systems on the surface without complicated systems. MC10 is a company that works in partnership with Rogers and team to build epidermal electronics for sports related applications. One such application include these electronic devices for football or other games where there is a possibility of concussion. These devices monitor the impact of the hit during a game, and may also be used for monitoring heart rate. Similar devices may be used in hospitals thereby replacing all existing bulky and complicated systems. Since these are light and less complicated systems, they can also be used to monitor infants who are under observation

When bio-integration within the body is considered, the impact is even more. Existing devices like Utah electrode arrays that are ineffective for long term use, and electrocorticography method that has low resolution output can be replaced by this technology. As mentioned earlier, it is effective for all surfaces including heart that is dynamic and constantly beating. Due to the surface adhesive forces, and low bending stiffness, these devices will maintain intimate dynamic interfaces with the body for monitoring electrical signals from brain surface, and also equally effective in monitoring heart performance and diagnosing lethal heart diseases

Transient electronics is highly useful in the biomedical field, but not just limited to that. They also hold huge potential in consumer electronics as well as environmental sensors. One of the biomedical applications include thermal therapy to control surgical site infections[11]. These are made using inductive coils of Mg combined with resistive micro heaters of silicon nanomembranes. After the required timeframe (3 weeks to 4 weeks) in this case, the transient electronics disappear and will leave no residue. A similar example is given below, tested on a mouse

Abstract doesn’t contain this figure

Fig 6 Images of an implanted (left) and sutured (middle) demonstration platform for transient electronics located in the subdermal dorsal region of a mouse. Implant site after 3 weeks(middle). Fig taken from [11]

Fig 6 show a transient electronic device implanted subdermally in a mouse. After the time frame (3 weeks in this case), the transient device is fully dissolved. In this way, the surgical site infections can be avoided relatively easy and quick. Similarly, they can be used for drug delivery, where in it delivers drug to a specific spot for a period of time, and after that it dissolves within the body, thereby removing the possibility of another surgery to take out the device implanted

Diseases that are caused due to a nerve bundle or infection in particular are is ideal for this technology. For instance, the migraines, cluster headaches and other chronic pains that happen due to Sphenopalatine Ganglion can be cured using this technique. A small transient device can be implanted near the nerve bundle. When the migraine or chronic headaches occur, the patient can easily an electric charge that will stimulate the nerve bundle. This can be done using a remote device. Since these conditions are not long term, the transient implants can be set for a timeframe of some few years, and then they slowly resorb in the body

The non-biomedical applications include using these electronics in electronic gadgets. The commercially available gadgets including mobile phones, and computers take more than desired time to be destroyed, and sometimes they are thrown away as e-wastes. But we can reduce the problem of e-wastes by making use of transient electronics. The electronic devices will be made using transient electronics, and after the desired period of time, they dissolve away not leaving any e-wastes behind. Transient electronics can also be used for 'fieldable' environmental sensors that dissolve to eliminate the need for their retrieval [11]

D. Future

Both bio-integrated and transient electronics have a bright future. Beyond Biology, stretchable electronics have a future in engineering industry too. By using stretchable and bendable electronics, we can make stretchable sensor tapes that can be wound around big machines to monitor the mechanical and structural health [1]. In heavy machines that produce high amount of heat, power scavenging devices can be wound around, thereby harvesting thermal energy to electrical output

The future of commercial electronics using transient devices are also to be developed. Although these devices hold great potential, developing such a system that can compete with the existing technology, is an area where much research is needed

The future for bio-integration is extremely hopeful. By making more efficient HMIs, the artificial mechanical systems can be integrated with the biotic system

The power source is still a matter of concern because of the losses in wireless charging, or the impracticality in using a ~3% Power Conversion Efficient solar cell as discussed earlier. If a better power source can be made available, then the opportunity of using such systems for long-term biological uses are high

Future advancements also include the applications on human body. No trials have yet been done on human body although much work has been done on other living beings. The only bio-integration that has been experimented on humans are the epidermal electronics. Although these devices are ready to use, they may need more research and development to incorporate more electronic devices

These applications and the positive impact this technology can make on the society, provide strong motivation for continued and expanded efforts in this emerging field

III. CONCLUSION

Clearly, all the scientific advancements discussed in the paper provide a better future for stretchable, bendable electronics integrated with biology. The main advancement in the field of bio-integrated electronics was to understand the basic bending physics of materials. By reducing the thickness of silicon, the geometrical mismatch between the biotic surface and silicon can be removed. Much engineering and research needs to be done to effectively fabricate the different electronic components including resistors, capacitors, inductors and transistors. This can be achieved through proper fabrication methodologies. Material science and corrosion engineering have to be considered in the case of transient electronics for better results

The results show that there is high hope for HMI integration, and health monitoring (both epidermal and within the body)

By fabricating efficient electronic devices, we can even control and monitor the different organs. This will help us to diagnose any possibility for chronic or fatal diseases

Stretchable and bio-integrated electronics will soon be a substitute for the existing conventional methods that are both invasive and short-term solutions (Utah) and for those that have low resolution output (current electrocorticography). By making use of silk and its transient features, the biointegration can be made transient also. In the case of transient electronics, the properties are well dependent on the materials used. As a result, future research to improve material qualities would be desirable

The future applications are not just bio-medical but also for consumer electronics. Current research advancements are done by John A Rogers and his group along with other researchers, and they are trying to integrate this system for effective biomedical applications. The application potentials of wearable and implantable bio-integrated electronic systems needs the development of more mechanically compatible and electronically sufficient microcontrollers, memory, power supply, and wireless data transmission modules. This has also laid path for future advancements in biotic-abiotic interfaces and for better and more effective solutions to make biomedical advancements on biological curvilinear surfaces more easy

After much needed research and development, this technology holds the potential to be the next best bio-medical advancement

IV. ACKNOWLEDGMENT

I would like to thank Dr. Karen Cheung, The University of British Columbia, for all the help and guidance in completing this paper

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