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It’s time to give up on a childhood dream: if Robocop was really constructed in the dystopian 1980s, he would have probably sparked fitfully and fallen over. And that is simply because the human body is not that welcoming towards foreign bodies, never mind silicon chipboards and military-grade weaponry being threaded through it. If we position our goals more modestly within reality, a huge amount of research has been focused into coating implantable devices with more compatible materials to prevent implant rejection – and stop a future cyborg from falling over. A wide range of technologies can help study how the body responds to implants. The QCM-D is one of those.

Implantable devices range from the current (think artificial hips and pacemakers) to the futuristic (tiny sensing devices monitoring your health and delivering medication even as you sleep).

So, how exactly does one assess how well, or badly, the human body will respond to a coating on such a device? A good indicator of future rejection would be studying whether components of the human body (like the immune cells and proteins found in blood) actually interact with the surface. What is needed is a device that can quantify these minute interactions, such as the QCM-D.

The Quartz-Crystal Microbalance with Dissipation monitoring (shortened to QCM-D) is a state-of-the-art analytical device that provides extremely sensitive measurements that are finding extensive application in just this field of research, and many other such fundamental measurements.

The QCM-D is, essentially, an incredibly fine mass-measurement instrument that pulses ac voltage across a quartz crystal sensor, causing it to vibrate at a constant frequency. On one side of the sensor, an interacting surface is coated. If molecules attach to this surface, the tiny increases in the sensor’s mass translate into a slowing of the sensor’s vibrations. Changes in frequency are much easier to measure electronically than directly measuring the mass that a layer of molecules adds to the sensor. This is sensitive enough to reliably detect a few billionths of a gram of interacting molecules.

Using this system, one could coat the sensor surface with a molecule, A, and expose it to a solution of another molecule, B. Changes in the measured mass (frequency), or the structure of the film (dissipation), indicates interaction. Moreover, by carefully controlling the amounts of A and B participating in this experiment, we can estimate the strength of binding and even produce models of the nature of the interaction using the QCM-D.

Because of the incredible sensitivity and richness of information that this provides, QCM-D is used as a probe for a huge range of applications. Just some examples of its use are: drug discovery, modelling how certain proteins interact with other cell components or how to make better nanoparticles for specified interactions.

Nationally, there are two QCM-D in operation in South Africa. The use of the National Facility at Rhodes is available to researchers within South Africa. For more information regarding booking and operation of the QCM-D, contact Ronen Fogel at r.fogel@ru.ac.za.

Piezoelectricity:
Quartz is one of a handful of materials that are termed “piezoelectric”. When a quartz crystal is compressed in one direction, a small electrical voltage develops, which generates a current. The reverse is true – by applying an oscillating voltage (ac voltage) to this quartz, you can create constant vibration of the quartz. Quartz watches work in this manner, by counting the number of vibrations to calculate how much time passes every second.

Writer: Dr Ronen Fogel