Microfluidic Sensing- Liquid Metal Sensors - Jason Kim
Liquid Metal Sensors
Liquid metal sensors are sensors that utilize liquid metal in their circuits. Typically liquid metal is used for properties such as flexibility, ability to be stretched, ease of application and various other magnetic, thermal, or electrochemical properties. Typical sensors use solid metal in their circuits and so have limited geometry as well as a static shape. Liquid metal sensors go a step beyond regular circuits as well as flexible ones in having conductive elements that are able to directly conform to the substrate and experience little to no strain (stretchable electronics). While liquid metal can be used in traditional sensors, appropriate utilization of the properties of liquid metals mentioned earlier can allow for some unique ways to sense various parameters, particularly in the biomedical field due to relative ease of conformation to curved surfaces.
Liquid Metal Properties
Few elemental metals are liquid at room temperature, the only common one being mercury which is rarely if at all used in circuits due to its low conductivity. In order to achieve a liquid state at room temperature or below it is necessary to use a liquid metal alloy that forms a eutectic so that the Gibbs free energy of the liquid alloy at a given temperature is the lowest. This will cause the liquid phase of the alloy to be favored at that temperature and so maintain this state. Eutectic materials have lower melting points than those of any of their pure constituents. A common example is Galinstan which has a melting point near 10 degrees Celsius and is composed of indium (MP of 156.6 degrees C), gallium (MP of 29.76 degrees C), and tin (MP of 231.9 degrees C). Liquid metals are useful in retaining metallic properties such as high electrical and thermal conductivity while remaining liquid. Liquid metals will additionally form distinct interfacial chemistry with other materials that can then be utilized for certain applications. Wetting is also possible on non-metallic surfaces allowing coatings or filling of channels to be possible.
Common liquid metal alloys utilize gallium, bismuth, lead, tin, cadmium and indium. By far the most common and well studied alloy is the commercially available Galinstan which is composed of 68.5% Ga, 21.5% In, and 10.0% Sn by weight. It has low vapor pressure, low toxicity, and high thermal and electrical conductivity. Additionally, as Galinstan is a gallium based alloy, unlike some liquid metals such as mercury, it will readily oxidize on the surface forming an oxide skin that provides additional properties to the liquid metal. The unoxidized metal will wet to the skin and when the skin contacts a non-metallic surface, this allows the alloy to 'wet' the non-metallic surface. This skin also allows it to maintain shapes and positions that would normally be impossible for a liquid. A critical stress is needed in order to break the oxide layer and allow the liquid metal access to the surrounding environment such as during fabrication when spraying or extruding the metal. The skin of the Galinstan can provide advantages such as allowing it to adhere to surfaces not normally allowed by surface tension, maintain shapes as mentioned, and 'self heal' by reforming oxide where the skin has been ruptured. The skin can also be deposited and removed through electrochemical methods. It also does not impede electrical transport to a significant degree at the surface due to how thin it is. In some cases the skin can be undesirable when it adheres to unwanted surfaces, slightly fluctuates in conductivity for sensitive measurements, or changes the rheology of the alloy.
Generally fabricating liquid metal sensors involves integration of the liquid metal, microfluidic circuit, and any solid metal component together which can present some unique challenges. The general approach to creating sensors utilizing liquid metal is the same as any other liquid metal circuit device, with the caveat that there are certain properties such as robustness, size, flexibility and others that one may wish to try to fabricate for. The four general approaches to fabricating these devices are lithography, injection, additive, and subtractive.
Lithography is a method of changing the surface properties of a substrate in order to create patterned deposition. Most lithography methods are poorly suited to fabrication with liquid metals as it tends to not form smooth layers and also remains as a flowing liquid during the early stages of lithography. Examples of this method include photolithography, imprinting, stencil lithography and selective wetting. Generally if the oxide skin is deposited in a pattern first utilizing electrochemical methods rather than simply spraying the liquid metal, there are much better results as the liquid metal will end up adhering to the substrate rather than simply forming droplets on the surface and moving across the substrate.
Injection is exactly what it sounds like, injecting liquid metal into microchannels. The only caveat is that due to the difference in viscosity as well density of Galinstan compared to traditional liquids, it can take extreme pressures to inject it into a channel and thus one is limited by the size of the channel and the strength of the material one intends to fill with Galinstan. The smallest reported channels filled with Galinstan have been as small as 150 nm in diameter.
Additive techniques are those that deposit the metal only in desired locations. Inkjet printing is a conventional additive patterning technique, yet it is difficult to use conventional inkjet printing to pattern liquid metal due to the surface oxide and large surface tension. It is, however, possible to inkjet print colloidal suspensions of liquid-metal droplets which can be sintered mechanically at room temperature by pressing on them. The presence of the surface oxide also allows the metal to be 3D printed, with features stabilized by the oxide. The smallest features using this approach are ≈100 μm. It is also possible to transfer liquid metal using microcontact printing (by contacting liquid metal with an elastomeric stamp and transferring it to a targets substrate) or by loading it in a ball-point pen. Liquid metals can be patterned additively at room temperature to form structures with metallic conductivity without any sintering or post-processing steps.
Subtractive techniques are those that selectively remove metal from a film to leave behind a pattern of metal. It is possible to laser ablate films of liquid-metal films in elastomer. It is also possible to selectively remove the metal from complex microchannels or surfaces by electrochemical reduction of the oxide.
Sensors utilizing liquid metal have not been well studied and there remains much room for exploration and applications of liquid metal sensors in various capacities. That said, there are broadly speaking two main ways that liquid metals can sense various parameters. Physical and Chemical methods.
Physical sensors are those that use the physical properties of liquid metals to sense certain parameters. Mechanical sensors are one such example which use an external force in order to sense pressure, strain, and torque through a change in resistance, capacitance or inductance due to strain. Given that in order for deformation of the sensor to occur, the material must be stretchable and durable, liquid metals are well suited for these applications being responsive while still maintaining high conductivity and sensitivity to strain. Changing positions and geometry is also possible with mechanical liquid metal sensors. Where sensors such as these are particularly useful are in the biomedical field with wearable and implantable devices. Being able to measure bodily functions on an accurate and conformable sensor is valuable. Liquid metal enables the flexibility of the device while still remaining durable and not as prone to wear. Typically in these applications, a gallium based alloy is used for its high conductivity as well as nontoxicity. This also allows such sensors to detect electrical signals and act as bioelectrodes such as in an ECG. Wireless tattoos have been made using liquid metal that can perform ECG monitoring. Somewhat related is magnetic field sensing, although most common liquid metals are nonmagnetic, adding magnetic elements such as iron particles and combining it with known principles in the liquid metal such as the relation of resistance to strain can provide accurate measurements of magnetic fields. While passive magnetic sensors are not present in pure liquid metals, induced magnetic fields are still present making them possible without an external magnetic field if a current is run through the metal in an appropriate manner. Temperature sensing is another common usage of liquid metals, while mercury is most famously known for this, gallium based alloys also display high volumetric expansion coefficients while maintaining a low melting point and have the additional benefit of being nontoxic unlike mercury.
Chemical methods are those that utilize the chemistry of the liquid metal, usually the oxide skin on the interface for sensing different parameters. Electrochemical reactions can occur on the interface, and in the center, alloying with different metals of different selectivities can be used for various types of detection. The tendency of liquid metals to form particles due to their low wettability serves as an advantage in making sensors that make use of the interface for chemical reactions. The surface can also be tailored such as through hydroxyl groups when the oxide layer is present or thiol binding when it is removed. One disadvantage with liquid metal chemical sensors is the tendency for corrosion to occur unlike in noble metals which can cause them to deteriorate more quickly. Liquid metals can also be used in composite materials to further improve the selectivity of the surface to detect certain biological substances. The surface can even customized to sense different types of gases which is very useful in biomedical devices. Challenges with chemical sensors tend to be that the full chemistry of the surface oxide layer is not fully explored and so the full potential for detecting various substances through chemical means has not been developed.
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