The exponential advances in microfabrication coupled with our increased understanding of neural diseases enables the design of highly sophisticated novel devices that benefit patients suffering from neurodegenerative diseases. Most of these devices have mainly concentrated on stimulating the remaining healthy layers of the appropriate neural network. This approach works well with the central nervous system, however when dealing with the peripheral nervous system this approach does not work as well and an alternative approach must be taken. For diseases associated with the peripheral nervous system it is not enough to stimulate but also essential to record from the brain to effectively complete the feedback-loop that is absent due to disease or trauma. Alternatively, one could envision that by recording signals from the brain one could enable patients to control their environment and thereby enhancing their quality of life.
This naturally leads to an equally important aspect of neural recording: the possibility to mapping the neural code. The ability to close the feedback loop in a prosthesis that uses processed signals from the brain is essential for its success. Fortuitously from a design point of view the electrical domain is ideal for recording unlike the stimulation domain where additional challenges of preventing faradic reactions occurring at the electrode must be prevented. However, recording poses its own unique challenges and arguably the main difference is one of scale. The state of the art stimulation device, which impressively restores partial hearing, is the cochlear implant, which possesses only 21 electrodes. Neural recording devices, albeit in-vitro or tethered, in the lab on the other hand routinely record from hundreds of neurons. Furthermore, to truly be able to perform effective mapping of neural signals it would be ideal to record from a large population of cells.
This brings unique challenges to the design of the implant, and necessitates the use of microfabrication techniques. To effectively record from the brain the following requirements must be fulfilled:
1. Electrodes must be penetrating
2. Electrode material must cause minimal tissue damage upon insertion and must allow for surgical handling
3. Implant must be electrically active and incorporate an electronics platform for wireless power and data telemetry to enable un-tethered operation
4. Implant must be biocompatible to prevent adverse tissue reactions and inflammation
Each of these tasks is a formidable one and though our team possesses great expertise in VLSI circuit design, especially in implantable electronics, we believe that microfabrication and biocompatibility design of high-density neural recording array are crucial.
Designing a synthetic neural interface that is inherently biocompatible, allows for high-density neural integration and consumes very little power has been a goal for many decades. Realizing this vision would have tremendous
impact from restoring lost function and building more efficient neural prosthetic devices, to understanding neural systems and enables the design of better machines. Tremendous advances in lithography, materials and computation enabled the silicon revolution. Unfortunately, this level of success has not been enjoyed by neural interfaces. Since, the discovery of the electrical excitable cells by Galvani in the late 1800s, the primary means of stimulating neural tissue is by placing an electrode near it and passing current between two electrodes. This technique has the advantage that it uses the electronic variable that we can manipulate easily. However, it suffers from the fact that electron transport though electrolytes are accompanied by terminal redox reactions. Furthermore, the conductive nature of the electrolyte shunts most of the current away from the target neuron. These factors often results in issues with long-term stability, neural toxicity and leads to vastly lower efficiency.
We have recently established that a a small increase in extracellular potassium ion concentration can effectively stimulate neural tissue. More quantitatively the background concentration of potassium is 5mM and an increase to just 10mM can effectively stimulate neural tissue. This increase is less than 1 log unit increase in concentration that translates to a Nernst potential of 18mV and an energy dissipation of approximately 600pW assuming a 1pL volume dispensed in 30 msecs. This is 5 orders of magnitude more efficient than an electrical interface, assuming 1µA from a 10V supply. The other key advantage of using an ionic interface is that the potassium ions can be effectively sequestered from the extracellular fluid unlike other chemical interfaces that have been proposed.
Switching fabrics in data centers that rely on traditional electrical switches face
scaling issues in terms of power consumption. Fast optical switches based on a silicon
photonics platform can enable the high port speed and high interconnection density needed
while still maintaining a small footprint and low power consumption.
The interconnect fabric is taking an ever-more dominant portion of the power budget and optical
interconnects are already used today within data centers for rack to rack interconnection to overcome
the limits of electrical signaling. However, the switches interconnecting these racks have inadequate
capacity to continue to scale with complementary-metal-oxide semiconductor (CMOS)-based
technology due to fundamental limitation of on-chip interconnects and power consumption and pin
count of scheduler application-specific integrated circuits. Further, not only does the switch fabric
require significant power, but the conversion from the optical signal to electrical and back again (OEO)
adds a significant amount of power and space. We present here a unique switching technology that
allows for high radix and high bandwidth, which is not achievable in conventional electrical
interconnects, while dissipating very low power. We achieve these diametrically opposing goals by
utilizing the large-bandwidth enabled by optics fabricated in a cost-efficient CMOS platform.
Furthermore, we integrate conventional CMOS electronics within the optical platform using a chips last
integration process. (Picture provided by Martijn Heck)
There is no question that photonic systems require a diverse set of materials. Examination of the components in a typical telecom system (the most advanced example) will reveal a diversity of materials (semiconductors, glass, crystals, and polymers) even in subsystems that have components in close enough proximity that they could be integrated. Even in purely semiconductor Photonic Integrated Circuits (PICs), a number of different materials are required, with the most advanced InP PICs containing 4-5 different optical materials on the same substrate . The fundamental limits of integration with this approach are the limited wafer sizes of InP, the need for lattice matching, and the lack of a pervasive, low cost infrastructure for making these PICs.
The scaling laws that have driven the electronics industry forward do not offer the same return on investment for photonic devices. However, the close integration of photonics with electronics can lead to a new class of systems that offer superior performance to conventional macroscale integration at the board level. This disparity in scaling benefits calls for a different approach to integration than the conventional front-end of the line (FEOL) integration pursued currently by the majority of silicon electronic/photonic integration approaches. In our approach we exploit the diversity of silicon processes for the best electronics and utilize a systems in package interconnect approach developed at UCSB to enable close integration of electronics and photonics. (Picture provided by Martijn Heck)
The basic approach is as follows: a wafer containing the photonic integrated circuit (PIC) is designed to allow for the creation of a slot that will house the CMOS chip. A chip specific hole is placed in the PIC wafer via a self-aligned masking method that we have pioneered and the chip is subsequently bonded to the PIC using a spin-on-glass and planarized. The planarized die can now be electrically connected via conventional lithography and metallization allowing for high-density integration with parasitics similar to VLSI interconnects. We have successfully shown that connections > 100 wires with pitch of 50µm can be easily made with this method. This method will be extended to smaller dimensions, limited by the minimum passivation opening of the bond pads allowed by the foundry, if the same pad is to be used for electrical testing. Alternatively, custom etching can allow for opening of 5µm on a side and much higher density of wires. This process can also be extended to multichip and multilayer metallization for greater flexibility.
Self-assembly on the nanoscale is a key
property that nature relies on to generate biological membranes.
These membranes provide a structural framework
via a microenvironment and a functional framework by
incorporation of channels, receptors, and pumps. By exploiting
hydrophobic–hydrophilic interactions, these membranes
assemble into bilayers and other structures. If we hope to
mimic the principles of natural nanoarchitectures, the selfassembling
membrane is a critical component. Recently, a
plethora of polymeric systems has successfully exploited
self-assembly for the purposes of drug delivery, biomedical
coatings, virus-assisted gene delivery, and nanoreactors.
These are amphiphilic diblock or triblock copolymers that
self-assemble into micelles, worm-like micelles, tubular
structures, membranes, or vesicles in a suitable solvent.
Recently, click chemistry has found considerable use in polymer–polymer conjugation via the copper-catalyzed azide–alkyne cycloaddtion (CuAAC) reaction. Click-based conjugation
allows for the modular synthesis of block copolymer
architectures with well-defined block lengths and endgroups.
This allows for systematic investigation of the effect
of a single parameter variation on the self-assembling properties
of the macromolecular system. For example, by using
individually well-characterized hydrophobic and hydrophilic
blocks, the effect of the hydrophilic block length on the self assembling
properties could be studied. This exquisite level of control allows for sophisticated scientific exploration and
will enable the understanding of the underlying structure–function relationships that influence the physics of self assembly
in these macromolecules.