Sniffing for Cancer

Nanosensors can detect odors from melanoma

3 min read
Illustration of cells.
Robert R. Johnson

Melanoma, the deadliest form of skin cancer, often causes subtle changes to the skin, such as discoloration or slightly enlarged moles. The usual detection method, a visual inspection of the skin, can overlook such signs. Instead of just looking for skin cancer, however, it might be better to sniff for it.

IEEE Member A.T. Charlie Johnson, a physics professor at the University of Pennsylvania, in Philadelphia, and his team have developed a DNA-coated nanosensor that can sense the odor from human skin cells that have turned cancerous. The team’s so-called electronic nose is expected to reach clinical settings within the next two years. As with most cancers, the survival rate for melanoma depends on how early it is detected. According to the World Health Organization, more than 65 000 people die each year from the disease.


With collaborators at the Monell Center, a research laboratory in Philadelphia focused on the senses of smell and taste, Johnson’s team was able to identify dimethyl sulfone, a volatile organic compound (VOC) specific to melanoma. The compound cannot be perceived by the human olfactory system.

“Our bodies make this compound, but we can’t smell it,” Johnson says. “In contrast, the sensor system we’ve developed using carbon nanotubes can detect dimethyl sulfone from melanoma down to concentrations of a few parts per billion.”

ANIMAL SYSTEM

A photo of a man and a dog in a white room.The e-nose aims to replicate a dog’s sense of smell. This dog is being trained at the Penn Vet Working Dog Center, in Philadelphia, to sniff out the signature compound that indicates the presence of cancer.Matt Rourke/AP Photo

A growing body of research finds that odor can be used to detect several types of diseases, Johnson notes. The spur to such research has been the canine olfactory system, including well-documented research on dogs’ ability to sense odor associated with lung cancer, he says.

Some breeds have a sense of smell estimated to be at least a million times more sensitive than that of humans. No wonder. The human nose has approximately 5 million scent receptors; a bloodhound’s has about 300 million.

Dog biology provided the blueprint for designing the electronic olfactory system, according to Johnson. His team’s e-nose aims to replicate a dog’s sense of smell with thousands of odor-detecting receptors built into the sensor. The receptors are made with single-strand DNA oligomers, or molecular complexes, coated onto a large array of carbon nanotube transistors. These transistors are then placed inside an instrument that captures the vapor released by skin, Johnson says. The vapor interacts with the DNA strands, leading to changes in the electrical characteristics of the nanotube transistors that can be used to identify a number of VOCs, not just dimethyl sulfone.

The array output will contain information from the thousands of different DNA-based receptors, which will then be combined in a manner similar to how the olfactory cortex in the brain processes messages from the olfactory receptor neurons. In a real-world application, the device could draw vapor from a lesion on the skin suspected of being melanoma. Eventually, it might also be possible to detect cancer by sniffing the VOCs in a patient’s blood, saliva, or urine, according to Johnson.

Adamant Technologies, a San Francisco company that produces chemical sensors for a range of medical applications, is working to bring the e-nose developed by Johnson’s team to clinical settings. It’s important that the rate of false positives—a positive diagnosis of a patient who is free of disease—be very low, Johnson explains. “That’s one of the difficulties of taking a new type of screening method to a mass scale,” he says. “False results could cause more harm than good.”

PERSONALIZED MEDICINE

With the new nanosensor, not only might it be possible to more easily detect early stage melanoma, but the cancer’s progress or decline might also be monitored, and that information could affect how the disease is treated.

“Physicians won’t just have one piece of information about the skin cancer but ideally several pieces of information from a compound that will provide a much more personalized look at treatment options,” Johnson says.

Results from the screenings could lead to cancer therapies specific to individual patients, he says, adding: “Working with scientists in the biomedical field, learning from them, and helping them to understand the potential of nano-enabled devices is going to let our team do many things in the area of diagnosis of disease that we couldn’t do before.

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The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
Vertical
A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay
Blue

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

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