From spider combs to proton exchange membranes
Even comparatively straightforward processing operations such as spooling or winding are complicated when it comes to nanofibres.
Their attraction to any surface by van der Waals forces – the adhesive forces that also enable geckos to stick to walls – makes handling them extremely difficult, and this is an issue that may well be hampering the wider adoption of nanofibres, however useful their many properties, especially in combination with nonwoven substrates for fields such as filtration.
INDEX™20 exhibitor Elmarco – the leading supplier of nanofibre manufacturing technology worldwide – is involved in an intriguing European project to find a solution to this problem, which takes its inspiration from the humble spider.
The calamistrum comb
Coordinated by the University of Linz in Austria, and involving universities in Czech Republic, Germany, Greece and Romania, the BioCombs4Nanofibers Project aims to recreate the ‘calamistrum’– a comb specific to certain spiders.
Cribellate spiders such as the feather-legged lace weaver, for example, use their calamistrum to handle and process the nanofibres they naturally produce.
They make up to 40,000 sticky nanofibres in thicknesses of 10-30nm from their cribellum – a spinning plate close to the spinnerets. To be able to handle such extremely fine nanofibres, the cribellate spiders use the calamistrum on their hindmost legs. This comb allows cribellate spiders to create a sticky capture spiral from the nanofibres to their nets – which interacts and captures the prey very effectively – without sticking to the spider itself.
Why the calamistrum is non-adhesive towards these fine nanofibres is still elusive and the clarification of these properties – as well as its technical abstraction for technical nanofibre processing – is the aim of the project.
A further goal is to transfer these bionic comb structures to technical surfaces featuring anti-adhesive properties, that will enable improved future tools for nanofibre handling. Similar nanostructures can also hinder the adhesion of nanofibrous protrusions of cells or microorganisms, which may also lead to the creation of cell-repellent or antiseptic areas on medical devices and implants.
Advanced laser-induced nanostructures are currently being employed to mimic the fingerprint-nanostructures of the cribellate spiders.
Elmarco is also involved in a second current European project that may be of major significance to the future of mobility, with fuel cell vehicles viewed as a more advantageous option to the combustion engine than electric battery vehicles in the longer term.
The €4.5 million GAIA Project, co-ordinated by the Centre National De La Recherche in Montpellier, France, brings together some of Europe’s biggest companies, including BMW Group, Freudenberg and Johnson Matthey, with research support from the Technical Universities of Berlin and Munich.
GAIA wants to develop a new membrane electrode assembly (MEA) that forms the vital component of a proton-exchange membrane (PEM) fuel cell stack.
The ambitious goal is that the new stack will be integrated into a fuel cell capable of meeting the performance, cost, and durability targets necessary for large-scale automotive fuel cell commercialisation.
With nanofibres forming the central membrane of the MEA, GAIA members are working towards achieving a step-change in performance that will largely surpass the state of the art by delivering a starting life power density of 1.8W per cubic centimetre, at 0.6V.
Further, a cost-assessment study will aim to demonstrate that the new MEA can achieve a cost target of €6 per kW, at an annual production rate of a million square metres.
In November 2020, the project reported the creation of a virtually indestructible membrane – a 14µm structure of nanofibre-reinforced perfluorosulfonic acid which has withstood an exceptional 100,000 accelerated stress test cycles of combined open circuit voltage hold and relative humidity cycling at 90°C. This considerably exceeds the target of 20,000 cycles for light duty vehicles.
The electrospinning process generally involves the formation of nanofibres from a liquid polymer jet in a longitudinal electric field. The dominant mechanism is the whipping elongation that occurs due to bending instability. Secondary splitting of the liquid polymer streams can also occur, but the final thinning process is elongation.
However, electrospinning – as the first method for the production of such very fine submicron fibres to reach industrial production scale – has a number of limitations. These include the use of often dangerous solvents but also the relatively low productivity of the process, which has prevented many developments from getting beyond the laboratory stage.
These disadvantages have motivated the development of several alternatives in recent years.
The advent of nozzle-less electrospinning has emerged as a viable new technology, largely due to its mechanical simplicity.
In this system, a self-organization of the jets occurs, and consequently, the number and spacing of the jets is optimized, even if technology variables - such as voltage, viscosity and surface tension of the solution - change. This leads to significant improvement in process stability and a consistent quality in the nanofiber layers produced.
In its simplest form, a nozzle-less electrospinning head consists of a rotating drum submerged into a bath of liquid polymer. The thin layer of polymer is carried on the drum surface and exposed to a high voltage electric field. If the voltage exceeds the critical value, several electrospinning jets are generated. One of the main advantages is that the number and location of the jets can set up naturally in the highest and most effective number of positions.
The commercially-available Nanospider systems manufactured by Elmarco are now characterised by significant improvements to this nozzle-less technology.
The Elmarco machine employs stationary string electrodes supplied with polymer solution by a proprietary moving “painting” head. This results in a dramatic decrease of solvent evaporation during the process, which has to be removed from the exhaust air released from the machine. The polymer solution concentration is also stable, enabling the system to typically run for more than 24 hours.
There are now over 200 Elmarco Nanospider nanofibre nonwoven units in commercial operation worldwide.