Embedding sensors and electronics inside of 3D objects in a single build process has been a long sought after goal in 3D printing (3DP). A group at the University of Warwick, led by Simon Leigh, has now done just that. Leigh’s group developed a low-cost material they call carbomorph – a carbon black filler in a matrix of a biodegradable polyester.
In addition to being conductive, carbomorph is piezoresistive. This means that when it is bent or stressed, its resistance changes. Typically the resistance increases as the object is bent because the conductive grains are spread further apart. Piezoresistive strips of carbon nanotubes have previously been created by other groups and used in the measurement of movement, but printing them is something new.
The goal of Leigh’s group was to completely print a motion sensing glove in a single unbroken run. This required a machine with multiple heads, and their Bits from Bytes BFB3000 fit the bill.
In one head they used polylactic acid (PLA) to print the main body of the glove. The other head contained the carbomorph for the embedded sensing strips in each finger. The cross section of embedded strip was only .25 square microns, yet proved sufficient for getting a robust piezoelectric signal to compute the bend angle.
In an effort to make their work freely available they published it in the open access journal PLoS ONE. The piezoresistive measurements were done using the popular Arduino Uno interface board and captured with Processing, an open source software package for visualising and manipulating data.
The group also printed capacitive buttons of the kind used in many common touch sensors, or as mouse replacements for human interface devices (HIDs).
Capacitive measurements were also carried out with an Arduino, and implemented with the CapSense code library. The ability to print capacitive sensors potentially opens up 3DP to new areas including accurate measurement of distance, humidity, or acceleration.
When it comes to the group’s final demonstration, things start to really get interesting. Two vertical capacitive sensor strips were embedded in the wall of a 3DP mug. This “smart vessel” yielded a reliable capacitance measurement which scaled linearly with the height of the fluid in the cup. One might imagine inexpensive party cups which report and summon a refill whenever a guest’s drink falls below a certain level.
Conductive 3D printed materials, by nature of their composition, have only a fraction of metal or carbon’s electrical conductivity. Therefore at any interface with other electronics, where there will already be some unavoidable loss of signal, extra care must be taken. It is for this reason that high-end audiophiles are willing to spend the extra money for gold-plated contacts – more signal is transduced and less is absorbed or reflected back to induce ringing or other unwanted noise.
In the case of capacitive button sensors, the group got around this problem by printing high-surface-area contacts in the shape of the commonly used banana-style plug. On the smart vessel they opted to use copper pads connected with silver conductive paint instead. There is no reason why copper or other metals might someday also be printed.
For example, several cancer treatments, like cisplatin, are basically metals bonded to chemical groups which make them soluble. This allows them to pass across membranes into cells, or to be miscible with other solutions. Printing them in hydrophobic solvent which evaporates leaving behind the metal may one day be possible.
One thing yet to be done is to test the durability of the devices over time. If they are able to maintain the essential characteristics over many use cycles, and trips to the dishwasher, then these devices could find widespread application. Then again, if your product lifetime is only a couple of hours, as is the case with a plastic party cup, they would already be perfect.
Research Paper: doi:10.1371/journal.pone.0049365 – “A Simple, Low-Cost Conductive Composite Material for 3D Printing of Electronic Sensors”
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