Capacitors & Coconuts! The Internet of Wearable Things

As you may have gathered from past articles, Futurama is one of my all-time favorite shows. The show follows Philip J. Fry, a 20th century loser who is frozen for 1000 years and builds a new life in the year 3000.

With one thousand years to develop, wearable technology in the Year 3000 has gotten pretty crazy. When “MomCorp” releases their new EyePhoneTM, everyone rushes down to the Mom store to get one.

Unfortunately, MomCorp engineers apparently used traditional silicon-based microchips to make their “futuristic” new phone. This makes installing an EyePhone… painful. See for yourself.

Wearable technology has become a global research initiative in the past few years. Prototypes for internet-capable sensors, smart watches, glasses with heads-up displays (HUD), shoes that store charge to power devices, GPS-capable wearables for the visually impaired, and health care devices have already hit the market.

These devices belong to the Internet of Wearable Things (IoWT). The IoWT comprises a subset of the larger “Internet of Things” craze, foretelling a new class of internet-capable devices specializing in machine-to-machine (M2M) communication [1].

Fortunately, this movement has become coupled with materials research into flexible electronics. If humanity ever actually invents an “EyePhone,” it would likely be a flexible, carbon-based implant, making it painless to install.

Before that, though, we have a lot of work to do. Wearable technology has enough potential to create completely new markets for electronics. In order to deliver on that promise, electronics components such as transistors, displays, and charge storage devices must achieve sufficient mechanical flexibility through innovations in materials science [1].

This article will first introduce you to each of these electronic components and why they are necessary. Then, we’ll discuss how present-day engineers are meeting the challenges that engineers in Futurama apparently forgot about. We’ll end with an example of how these technologies are currently being implemented, and how they could grow in the future.

Table of Contents

Organic-Ified Electronic Components: Flexible Transistors, Displays, and Power Sources

If we’re going to develop some flexible electronic devices, first we have to do some engineering. A new class of applications will become available to electronics if we can make them (i) flexible, enough to be implanted in a living organism, (ii) durable, at high temperature or strain, and (iii) scaleable, or manufacturable at low cost. Let’s discuss three major components of mainstream electronics and explore some strategies for their incorporation in flexible devices.

Transistors

The transistor comprises the fundamental unit of modern electronics. Conceptually conceived in 1926, a practical model for the transistor remained elusive until 1947, when Bardeen, Brattain, and Shockley shared a Nobel Prize for the invention.

(Consequently, Bardeen puts the “B” in the BCS theory of superconductors, and is the only scientist ever to win two Nobel Prizes).

A transistor can be defined as an electronic device containing three terminals: a source terminal, a drain terminal, and a gate terminal. In common metal-oxide semiconductor field-effect transistors (MOSFETs), the gate terminal is isolated from the other terminals via an insulating dielectric, meaning that no current flows to or from this terminal. The source and drain terminal are connected by a semiconductor.

In this set-up, a voltage applied at the gate terminal may control the current flow through the source and drain terminals. An applied voltage alters the charge distribution in the semiconductor, changing its conductivity. Thus, transistors may function as both a switch (a device that switches the flow of current on or off) and an amplifier (a device that amplifies or boosts an electric current) just by changing the voltage at the gate terminal.

This dual functionality allows for precise control over complex logic circuits. Typical silicon-based transistors are scaleable, highly functional, low-cost components. As of 2009, common microprocessors may contain as many as 3 billion MOSFETs.

However, a silicon-based MOSFET won’t fit our criteria for wearable technology. Instead, various transistor architectures using organic (carbon-based) semiconductors are employed. Particularly, organic field effect transistors (OFETs) have been fabricated for unorthodox applications in flexible electronics. OFETs use an organic molecule in place of silicon as the semiconductor connecting the source and drain terminals. Quite often, these organic semiconductors are polymers, modular substances made of many repetitions of one “monomer” subunit.

OFETs offer several advantages over traditional MOSFETs. Microfabrication processes employed to produce silicon-based transistors require a “clean room,” a costly, specialized facility designed to provide a highly controlled environment with low numbers of airborne particles [1]. In addition, microfabrication typically occurs at temperatures above 800 degrees Celsius. By contrast, the fabrication of OFETs requires less expensive equipment and may occur at room temperature. This capacity for room-temperature fabrication allows OFETs to be implanted on some unconventional substrates, from flexible polymers to paper [2].

OFETs are commonly fabricated using dip-coating, spin coating, and 3D inkjet printing methods. 3D printing, also referred to as “additive manufacturing,” is particularly attractive. Because they are made of soluble organic molecules, each successive component of an OFET can be printed via specially prepared inks [2]. Additionally, the use of computerized axial design (CAD) allows the fabrication of OFETs in any desired shape or size, which would reduce manufacturing costs and waste [2]. Given these advantages, 3D printing may be the way to go when we create logic gates for our flexible electronic devices.

It is unlikely that organic transistors will ever fully replace silicon. Low charge conductivity in organic semiconductors limits their applications in more complex integrated circuits, in which high switching speed is required.

Instead, organic transistors are ideal technologies for applications requiring durability and high area coverage [2]. That is, you wouldn’t want an OFET in your laptop, but you’d be fine with one in a device requiring less computing power – like a medical implant, or an electronic textile product. The capacity for OFETs to be fabricated at room temperature on flexible substrates could open the door for market-defining new technologies: foldable high-resolution color displays, electronics on functionally unbreakable surfaces, and highly safe, electronically capable implantable devices.

Displays

Have you ever accidentally dropped your phone and shattered your screen? I definitely have. We were all in middle school once.

Your phone screen is prone to shattering because it is made of brittle materials. A phone screen is comprised of a three layer system. The outer layer is made of aluminosilicate glass while the middle layer contains a capacitor, a device that confers touch sensitivity by detecting electrical disturbances caused by your fingers.

In the innermost layer, a complex network of light emitting diodes (LEDs) are modulated by a liquid crystal display (LCD, as in LCD-TV). LEDs are semiconductor diodes, commonly made of doped silicon, that emit light when stimulated with electrical current. The optical properties of the liquid crystal display are tunable with an electrical signal; that is, the LCD can act as an on-off switch for each of the numerous LEDs in your phone.

This sophisticated three layer network works together in concert to bring you this gif of Bender on drugs, or this exquisite visual display.

The heir apparent to this LED-based display system is the organic light emitting diode (OLED). OLEDs and LEDs work on the same general principles. As I described in my article on graphene, electrons exist only at energies permitted by quantum mechanics, known as “orbitals.” Semiconductors contain a band-gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In a semiconductor diode, the application of a voltage can excite electrons from the HOMO to the LUMO. This excitation creates “holes” in the HOMO, which may then recombine with the excited electrons. As electrons pass from the LUMO back into the HOMO, they radiate energy at a frequency that is dependent on the semiconductor’s band gap.

Organic LEDs (OLEDs) use an organic semiconductor, often a polymer or a biomolecule, instead of a metallic semiconductor. The organic semiconductors are often deposited on a very thin (and thus, flexible) metal substrate [4,5]. They are ideal for applications requiring high flexibility, biocompatibility, or durability [4,6]. Recent Samsung phones have featured an OLED-based display.

Power Sources

To make our ideal flexible circuits, we are primarily concerned with three major power source architectures:

Capacitors store energy electrostatically between two electrodes separated by a dielectric (an electrical insulator). The dielectric prevents charges from passing between the electrodes, causing a build-up of potential energy. Capacitors are great for the rapid release of a small amount of energy, but they have a low energy density. Capacitors possess a charge storage limit defined as their capacitance.

Supercapacitors form the intermediate between capacitors and batteries. They may possess capacitances up to 100,000x that of normal capacitors while charging much faster than rechargeable batteries. Supercapacitors come in many shapes and sizes. Electrical double layer capacitors (EDLCs) may store energy electrostatically (like a capacitor), while pseudocapacitors may store energy electrochemically (like a battery). Hybrid supercapacitors form a middle ground between these two.

Some scientists believe supercapacitors will eventually reach a sufficient power density to replace batteries altogether.

Batteries are still the king of the castle in terms of charge density (for now). Batteries store energy in the form of chemical potential energy. In other words, electrons in a battery move from anode to cathode in what amounts to the electron version of a ball rolling down a hill. Batteries based on reversible reactions are known as rechargeable batteries, such as the lithium ion batteries found in your cell phone. In our ball analogy, plugging your phone in to recharge is analogous to someone pushing the ball back up the hill.

These three technologies – capacitors, supercapacitors, and batteries – are used in tandem in many electronic devices. In order to make them accessible for wearable technology, electrodes and electrolytes for these components must be created from organic and polymeric materials.

Much progress has been made in this area already. Carbon nanomaterials have proven quite effective as charge storage devices due to their high conductivity and surface area. Carbon nanotubes have been employed as an electrode in common batteries [7] and in metal-oxide/carbon composites for supercapacitors [8]. Graphene-silk composites have been investigated as electrode candidates in micro-patterned electronics [9]. A technique involving graphene synthesis via laser irradiation (laser-induced graphene) has been used to fabricate supercapacitors on a diverse array of edible and wearable substrates, ranging from paper to coconuts to toast! [10].

An Eye Towards The Future

Throughout this article, I have espoused the generally accepted applications of materials innovations in the area of wearable technology. However, I feel that, even among industry professionals, their usefulness as prosthetics has been relatively unexplored.

Microfabricated bio-electronics remains a key competency at Lawrence Livermore National Lab. LLNL commonly investigates novel sensors patterned on flexible polymers. They aim to create multifunctional devices capable of interfacing directly with neural tissue. In 2009, they used this expertise to construct an artificial retina for patients suffering from retinitis pigmentosa [11].

Looking at this device, Futurama’s EyePhone doesn’t seem too far off anymore. The artificial retina project was awarded an R&D 100 award in 2010.

As LLNL continues to strive for smaller, more stable, and more biocompatible electronic interfaces, they may benefit from the same advances that will permit the wearable devices we have thus far been discussing. There is much work to be done; the materials we have created still face many challenges, including stability, performance, and cost.

The good news? In science, the future always looks bright.

I hope you enjoyed this article on the Internet of Wearable Things! I had to cut down more than I had hoped to. The IoT is a burgeoning industry, and many crazy technologies are currently in development. I’ll likely devote some more time to these stories in the future.

If you’d like to learn more about LLNL’s artificial retina, check out the video below:

Take care!

References

Note: I would also like to acknowledge Hamid Montazeri, VP of software engineering at Stanley Black & Decker, for his advising. His comments were invaluable towards building my understanding of the modern IoT industry.

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https://str.llnl.gov/OctNov09/pannu.html