Tetris on the sleeve: gas-permeable material for wearable electronics

Wearable electronics has become an integral part of the life of many modern people. From fitness bracelets and smart watches to augmented reality glasses and smart shirts - the range of existing devices ranges from obviously useful to fun futuristic. Nevertheless, when it comes to something “wearable”, in addition to functionality, you need to think about comfort. Scientists from the University of North Carolina (USA) have developed a new gas-permeable material for wearable electronics, i.e. able to breathe. What techniques were used to create the new material, what are the properties of the obtained prototype, and how much more comfortable will it be to wear electronics on itself? We learn about this from the report of scientists. Go.

Study basis

Any thing that we wear on ourselves is more or less gas permeable. This is due to the need to comply with our physiology. Human skin is an important element of the excretory system of the body, providing the output of metabolic products through sweat. Therefore, blocking the execution of this function using completely “sealed” materials is not a good idea in everyday life (specialized clothing and equipment do not count).

As for wearable electronics, but with its development and transformation from ordinary bracelets to almost full-fledged wardrobe elements, scientists began to think not only about the physical properties of the materials used, which is important for performing the device’s functions directly, but also about properties that contribute to user comfort.

Most modern wearable gadgets, as the researchers themselves note, are made on the basis of solid polymer substrates, such as polydimethylsiloxane (PDMS), polyethylene terephthalate (PET) and polyimide (PI). In this work, scientists describe a new material that possesses not only good conductivity and flexibility, as in the foregoing precursors, but also good gas permeability.

There have already been attempts to create something similar, but all of them faced certain difficulties during production or limitations in use.

For example, an ultra-thin material based on polyvinyl alcohol (PVA) has been developed relatively recently. The gas permeability of this material was excellent, but the production was extremely difficult. In other words, the game is not worth the candle.

There is also a development on silver nanowires (AgNW). This option provided high electrical stability, however, bare nanowires limited the term of long-term stability.

Another unique material was PDMS (polydimethylsiloxane) sponge based on sugar matrices. The problems were the limited particle sizes of sugar, which made it difficult to obtain microporous structures. In addition, this method cannot be used for the manufacture of ultrathin films.

Recalling the prototypes described above, scientists want to say that creating really good material that combines all the necessary properties, while being easy to manufacture, is quite difficult. However, according to them, they did it.

They decided not to reinvent the wheel, but to combine the existing developments, simultaneously removing their shortcomings. As a result, a tensile (i.e., flexible) conductive film was obtained by incorporating AgNW directly below the surface of a porous thermoplastic polyurethane (TPU or TPU) film made by evaporation.

The evaporation method is a simple, efficient and scalable self-assembly process for the manufacture of porous polymer films without the need for complex steps such as photolithography, vacuum evaporation and etching.

Research results

Image No. 1

On image 1a shows a diagram of the manufacturing process of the prototype. A porous thermoplastic polyurethane (TPU) film was produced by the evaporation method, after which AgNW (silver nanowires) were introduced into the surface by thermal pressing.

In the process of evaporation, the role of the solvent was played by tetrahydrofuran (THF). In addition, a small amount of polyethylene glycol (PEG) (TPU: PEG = 10: 1 by weight) was added to the solution to facilitate the orderly assembly of water droplets.

Evaporation of the organic solvent cooled the substrate. Moisture in turn condenses on the substrate and collects on its own in droplet eliminators.

Scientists note that under normal circumstances, droplet bonding should be avoided, which can lead to the formation of disordered structures. However, in this work, the rapprochement of droplets promoted the formation of through porous structures.

As shown in 1b, pore size can be controlled by changing the concentration of the solution. A higher concentration (2 wt.% TPU + 0.2 wt.% PEG) resulted in a smaller pore size and a more regular pore structure, but a higher percentage of plugged pores (i.e. pores that do not fulfill their role due to their location). On the other hand, when the concentration was too low (1 wt.% TPU + 0.1 wt.% PEG), the resulting structure tended to be more irregular with a pore diameter of more than 100 μm. Such large pores can be seen with the naked eye and limit the resolution of the electrodes.

After several attempts, it was found that the optimal concentration of the solution is 1.5 wt.% TPU and 0.15 wt.% PEG. As a result, a uniform porous structure was obtained ( 1b and 1e) The pore shape was close to round with a diameter of ~ 40 μm, and the surface coverage coefficient was about 39% ( 1e and 1f ).

AgNWs were embedded on a porous TPU film by immersion in a solution of AgNW and water. Importantly, the pore size was much larger than the length of AgNW (~ 20 μm). Microscopy ( 1c ) of the porous AgNW / TPU film showed that AgNW were uniformly deposited on the surface of the TPU without blocking the pores.

AgNWs on the surface of TPU films are quite easily separated from it, therefore it was necessary to conduct heat treatment to solve this problem. The melting point of TPU is about 130 ° C; therefore, it was decided to use a temperature of 150 ° C for thermal pressing.

In the picture 1d2it can be seen that after the heat press, most AgNWs were embedded directly inside the TPU, and only a small part was exposed on the surface. This treatment also reduced the film thickness from 6.8 μm to 4.6 μm.


Image No. 2

Image 2a shows an optical image of a porous HP-AgNW / TPU film (HP - after heat treatment). Chart 2bshows the film resistance as a function of the number of dipping cycles (i.e., the number of AgNW incorporation cycles). Resistance decreased only after the first four cycles, after which it remained stable, reaching approximately 14.5 Ohm / sq (Ohm per square). Therefore, in the process of manufacturing the film, exactly 4 application cycles were used. The thermal press treatment further reduced the resistance, which can be explained by the improved contact in AgNW compounds caused by pressure and thermal annealing. For example, after heat treatment, the film resistance decreased to 7.3 Ohm / sq.

The porous structure of the film leads to an increase in optical transparency compared to a solid film. The optical transmittance was 72% at 550 nm for a porous TPU film ( 2c) and decreased to 63% after coating with AgNW. The transmittance was further reduced to 61% after thermal pressing due to the slightly increased TPU film width.

Next, water vapor transmission was estimated based on ASTM E96. As expected, a porous TPU film exhibits significantly improved vapor permeability compared to a film without a porous structure ( 2d ). The water vapor transmission rate was: 2 mg / cm ² h ^-1 for solid TPU film; 38 mg / cm ² h ^-1 for a porous TPU film; 36 mg / cm ² h ^-1 for porous AgNW / TPU and 23 mg / cm ² h ^-1for porous HP-AgNW / TPU.

Researchers have suggested that increased vapor permeability also improves the wear resistance of the material. To test this hypothesis, a long-term wear test was carried out when a film was worn on the skin. After 7 days of wearing on the skin of a person, there were no allergic reactions and accumulation of sweat. No difference was observed between the area of ​​the skin that was covered with the film and the area around the contact area.

It is obvious that the through porous structure allows sweat and moisture to penetrate the film, reduces the likelihood of skin irritation and improves wearing comfort and wear resistance.

Further, the films were immersed in saline to demonstrate long-term stability in contact with sweat ( 2e) After 100 hours, the resistance of the porous AgNW / TPU and HP-AgNW / TPU films increased by 60% and 15%, respectively.

Peeling tests were conducted between the film and the adhesive tape ( 2f ) and between the film and the skin. Figure 2f also shows that the AgNW / TPU film can be easily peeled off using tape (the image on the right shows AgNW transferred onto adhesive tape), while the HP-AgNW / TPU film is much more stable.

In addition, the AgNW / TPU film lost conductivity after the peeling test, while the HP-AgNW / TPU film retained conductivity.

After removing the AgNW / TPU film from the skin, some AgNWs still remained on the skin. But a similar test with an HP-Ag NW / TPU film showed that there were no AgNW particles on the skin.

It follows that heat treatment can effectively improve the conductivity and stability of the film during prolonged use.

By incorporating AgNW beneath the surface of the TPU film, the resulting HP-AgNW / TPU porous film showed significantly improved adhesion between AgNW and TPU and therefore stability, with an acceptable reduction in optical transmission and vapor permeability.

It is worth noting that the HP-AgNW / TPU film is not only conductive on the surface, but also in the thickness direction. The upper and lower sides of the film are electrically conductive, while they are also connected by silver nanowires at the edge of the pores through the thickness. Thus, the film acts as a bulk conductive material, but does not require a large number of conductive fillers, which can cause deterioration in mechanical properties.


Test with LED.

To demonstrate this property, the film was connected to an LED circuit and used as a two-sided conductor. Two drops of liquid metal were applied on two sides of the film to connect with the LED. A lit LED indicates that both sides of the film are electrically conductive and connected.


Image No. 3

Due to its physical properties, the HP-AgNW / TPU film can take a variety of forms by laser cutting. Pictures 3a show a film electrode structured into a filamentary serpentine structure with a line width of 0.5 mm. In this case, the film remains ultra-thin, which ensures close contact with the skin.


The procedure for applying HP-AgNW / TPU film to the skin.

The film is completely restored after compression, twisting and other deformations that may occur with it while it is on the skin. If necessary, the HP-AgNW / TPU film can be removed from the skin using tape and reused.


HP-AgNW / TPU Removal Procedure Using Simple Adhesive Tape.

Graph 3b shows the dynamics of resistance depending on the stretching of the film. At 5% strain on the film, the resistance doubled. When removing any voltage (deformation), the resistance dropped by 10%. In subsequent cycles, where the stretching of the film and its normal state alternated, the resistance almost always remained constant and reversible.

If the deformation was 10% and 15%, then the resistance increased by about 4 and 7 times, respectively, compared with the initial value. Despite such significant fluctuations, an interesting trend was noted - at each level of deformation, the film can be “programmed” during the first stretching, after which the resistance will change reversibly within the range determined by the first stretching. In other words, it is the first deformation cycle that plays the most important role, which sets the “rhythm” of resistance change for subsequent cycles.

As a result, after 1000 cycles of deformation (10%), the resistance increased by less than 7%. This test also showed that the film is really very flexible. So, when the film was bent to a curvature of 0.55 mm-1, the resistance increased by only 0.8% ( 3s) And after 10,000 bending cycles, the resistance increased by 0.7% ( 3d ). HP-AgNW / TPU film retains its conductivity up to 45% strain. And the destruction of the film occurs only with a deformation of 350%.

Scientists note that their development is excellent for continuous monitoring of electrophysiological signals. An ECG is commonly used to diagnose heart rhythm disturbances, while an EMG can be used to analyze levels of stimulation, muscle neuropathy, and motor behavior.

In measurements of ECG and EMG (electrocardiography and electromyography), conformal contact and low skin-electrode impedance are crucial for obtaining a high signal-to-noise ratio, i.e. to get the most accurate information.

To evaluate the contact between the porous HP-AgNW / TPU electrodes and the skin, we used artificial leather made from Exoflex, which is almost identical to human skin and has the same Young's modulus.


Image No. 4

Image 4a shows the electrode after transfer to artificial skin. Microscopy clearly shows that the ultrathin electrode formed conformal (close) contact with the skin.

The complex resistance of the initial and expanded HP-AgNW / TPU porous electrodes was only slightly higher than that of commercial Ag / AgCl gel electrodes ( 4d), and lower than that of a solid AgNW / PDMS film (0.2 mm thick). This is due to the quality of the film's contact with the skin. The smaller thickness and increased flexibility of the HP-AgNW / TPU film reduce bending stiffness, resulting in more conformal contact than the solid AgNW / PDMS film.

Further, ECG and EMG signals obtained using HP-AgNW / TPU porous electrodes were compared with signals obtained using commercial Ag / AgCl gel electrodes ( 4e and 4f ). The location of the electrodes for ECG and EMG tests is shown in 4b and 4c, respectively.

In the case of ECGs, the HP-AgNW / TPU porous electrodes provided signals comparable in quality to gel electrodes. The measured SNR (signal-to-noise ratio) for the ECG by the HP-AgNW / TPU porous electrodes was 7.0 dB, which is comparable to gel electrodes (7.1 dB).

In the case of constant movement, the signal quality deteriorated, and the SNR value fell to 6.3 dB for porous and 6.2 dB for gel electrodes.

Due to EMG, one can clearly distinguish signals corresponding to muscle contraction for different grip forces. It is noteworthy that the signal of porous HP-AgNW / TPU electrodes was weaker than that of gel electrodes, but this was due to the different arrangement of the electrodes of two types (electrodes of both types were used simultaneously). The SNR value for EMG with porous electrodes was 24.9 dB, which is comparable to the SNR for gel electrodes (25.9 dB).

It is worth noting that porous electrodes, unlike commercial gel electrodes, do not need a conductive gel. The lack of gel during data collection improves their quality because there is no such factor as gel degradation. Taking into account the gas permeability of the developed film, these experiments additionally illustrate the possibility of using porous electrodes HP-AgNW / TPU for long-term continuous monitoring of human condition.

Human skin is not the only place where developed electrodes can be placed. The second option is textiles.


Image No. 5

Image 5a shows a diagram of a capacitive touch sensor. On 5b shows the capacitance value when the sensor is touched and pressed.

In addition, the sensitivity of the touch sensor system is defined as the rate of change of the read value when a touch occurs. In this system, the sensitivity was 86%. Stability, in turn, is defined as the variance of the readings of the touch sensor, which is about 1.65. The signal-to-noise ratio was 35: 1, and the response time was less than 0.1 s.

To assemble the system of wireless touch sensors ( 5c ), a piece of HP-AgNW / TPU 50x100 mm film was integrated into a fabric sleeve and displayed as four touch buttons using laser cutting. Each of the buttons had its own function: left, down, rotation and right.


Tetris on the sleeve.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.

Epilogue

Modern technology has long been associated not only with functionality, but also with comfort in use. Wearable electronics is no exception. Most of the modern materials used for the production of wearable gadgets perfectly perform their basic functions, but are devoid of some small, but so important details. One of such details is gas permeability, which provides free perspiration in case of long-term wearing of any device on the skin.

The developed HP-AgNW / TPU film has many ordered pores. Such a design did not greatly affect the physical properties of the film, while retaining the ability to fully perform the main tasks.

During the study, several prototypes were created, demonstrating the range of applications of HP-AgNW / TPU. The first prototype was aimed at collecting important information about the user's health status. The second is the almost humorous use of HP-AgNW / TPU film to create a wireless Tetris gamepad. In both cases, the prototypes showed excellent results, and the porous film in its characteristics and performance was comparable to the currently used commercial options.

In the future, researchers intend to continue their work on gas-permeable materials, since they believe that the use of any wearable gadget should be comfortable. Well, you can’t argue with that.

Thank you for your attention, remain curious and have a good working week, guys. :)

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