Optimizing interconnect design: micro-tensile testing device coupled to a laser confocal scanning microscope
The group’s apparatus features an in-situ micro-tensile testing device coupled to high-resolution imaging equipment (Olympus LEXT OLS4100) and allows the researchers to examine the two main features that have been found to affect adhesion between the interconnect and polymer substrate: geometric parameters and the fabrication process itself.
Observing the interconnects under mechanical testing allows the team to focus on key parameters such as strut length and obtain quantitative information, which can be fed back into the design cycle. 3D optical profiles of interconnect geometries allow developers to monitor samples for signs of surface cracking, which can be used to optimize manufacturing processes.
In the work, the researchers observe that plasma treatment of polymers, while increasing adhesion, may promote cracking.
Semiconducting polymers are a key ingredient in organic light emitting diodes (OLEDs), organic photovoltaics (OPVs) and organic field effect transistors (OFETs) – and pave the way for future bendable electronic devices. The technology is driving advances in the design of flexible displays, conformable energy harvesters, and wearable sensors to name just a few applications. What’s more, thanks to their solubility in many organic solvents, semiconducting polymers provide device makers with a range of appealing fabrication options including ink-jet printing, spray-coating and roll-to-roll production.
Optimizing the composition of the blended film is important to balance the cost versus performance. In many cases, high electrical conductivity and high optical transmittance in the visible range of the electromagnetic spectrum are critical factors. However, other aspects such as flexibility, film formation, chemical stability and wettability can also play an important role in the choice of the material or the composition to be used, together with overall processing considerations.
Case study: semi-transparent electrodes for flexible optoelectronics
In a recent study, published in the journal Translational Materials Research (TMR), materials scientists have examined a blend comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and polyvinyl alcohol (PVA), which can be used as a flexible, semi-transparent and highly-conductive material in electronic and optoelectronic devices. The electrical conductivity and optical transmittance of spray-deposited films of various thicknesses and blend ratios were evaluated to determine the most appropriate composition for optimal device performance and cost.
Presenting their results as a series of figure of merit diagrams, the researchers observe that it should be possible to decrease the PEDOT:PSS content in the blend down to 30% (by weight) and maintain an acceptable level of electrical conductivity for many applications.
In my view, the concept of stretchable electronics that can be integrated with clothing and the human body could bring revolutionary technologies and spawn new industries. I’m a journalist working for Physics World magazine and I’ve recently produced a short film on this topic, which I’m delighted to share with TMR+. The video report takes you inside the headquarters of one of the most exciting companies in this burgeoning technology area — a start-up called MC10 based in Cambridge, Massachusetts.
One of the company co-founders is John Rogers of the University of Illinois at Urbana-Champaign, a pioneer in the field of flexible electronics. In the film Rogers talks about how his interest in the research emerged from the observation that all known forms of biology are soft, elastic and curvilinear, whereas existing forms of electronics are rigid planar and brittle. “As a result, if you want to integrate electronics with biology – with human skin or tissue – you have severe challenges in a mechanics mismatch and a geometrical form mismatch,” he says.
Rogers talks about how MC10 has overcome these mismatches by developing a printing process that allows devices to be built on rigid wafers before being lifted off in thin formats and then printed onto rubber substrates. This innovation is enabling the company to develop professional and consumer products based on integrated electronics that can flex and reshape in a range of different environments.
The film profiles one of the company’s products called the Reebok CHECKLIGHT, developed by MC10 in partnership with the sports equipment giant whose global headquarters is also in Massachusetts. It’s a type of skullcap that can be worn by athletes and provides an indication of the danger of head impacts, through the combination of an accelerometer and a gyroscope integrated into flexible materials. The product has an interesting backstory about the concerns among youth athletes and trainers about the risk of serious injuries. For example, in sports such as American football, there tends to be something of a hero culture whereby players will respond to head collisions by saying “I’m fine coach”, even if they’re really not. The Reebok CHECKLIGHT is designed to provide an objective assessment of head impact in that scenario.
We recorded the film over a couple of days at the end of last year, when I visited the MC10 headquarters with a small film crew. The experience certainly lived up to my expectations of what a dynamic start-up in this part of the world would be like. Alongside the labs and the business spaces, the office also had many classic start-up accessories such as running machines and a range of gourmet coffee on tap. Perhaps these mod cons go some way to explaining the friendly and co-operative nature of the MC10 people we interviewed. For example, one of the co-founders Roozbeh Ghaffari was more than happy to model the Reebok CHECKLIGHT for us by popping it onto his own head without a moment’s hesitation. From my experience – particularly with British academics – this type of thing normally takes a bit of persuasion!
Equally amenable was Benjamin Schlatka, the VP of Business Development who appears in the film talking about his personal interest in the products of MC10. As a parent of three children his interest was piqued by the idea of a wearable sensor that could measure the temperature and respiration of a child while it sleeps. Schlatka goes on to talk about the key business developments in the history of MC10, which was established in 2008 after John Rogers established a connection with an entrepreneur in the Boston area.
Rather than me spoiling the film anymore, I recommend you give it a watch!