Liquid Metal Coated Textiles with Autonomous Electrical Healing and Antibacterial Properties

Conductive textiles are promising for human–machine interfaces and wearable electronics. A simple way to create conductive textiles by coating fabric with liquid metal (LM) particles is reported. The coating process involves dip‐coating the fabric into a suspension of LM particles at room temperature. Despite being coated uniformly after drying, the textiles remain electrically insulating due to the native oxide that forms on the LM particles. Yet, they can be rendered conductive by compressing the textile to rupture the oxide and thereby percolate the particles. Thus, compressing the textile with a patterned mold can pattern conductive circuits on the textile. The electrical conductivity of these circuits increases by coating more particles on the textile. Notably, the conductive patterns autonomously heal when cut by forming new conductive paths along the edge of the cut. The textiles prove to be useful as circuit interconnects, Joule heaters, and flexible electrodes to measure ECG signals. Further, the LM‐coated textiles provide antimicrobial protection against Pseudomonas aeruginosa and Staphylococcus aureus. Such simple coatings provide a route to convert otherwise insulating textiles into electrical circuits with the ability to autonomously heal and provide antimicrobial properties.


Introduction
This paper describes a simple way to add functionality to textiles by coating them with particles of gallium-based liquid metals (LMs). These LMs have low melting point, metallic electrical conductivity, high thermal conductivity, effectively zero vapor pressure, low toxicity, and antimicrobial properties. [1] LMs have both fluidic and metallic properties. Hence, they show great promise in applications such as microfluidics, soft composites, sensors, thermal switches, and microelectronics. [2] In particular, the ability to form soft electronic circuits using LMs has potential applications in softrobotics, wearable electronics, and health monitoring systems. [3] One of the advantages of LM is that it can be deposited and patterned at room temperature onto surfaces in unconventional ways that are not possible with solid metals. For example, circuits based on LM can be fabricated by 3D printing, injection into capillaries, and stirring LM into polymers to form composites. [4] These developments have led to new types of devices with high thermal and electrical conductivity. In addition, some devices have self-healing abilities such that they can be mechanically compromised yet retain electronic function. [5] To date, most work on LMs has focused on depositing metal on non-porous substrates such as films and monolithic elastomers or gels that lack sufficient permeability. [6] Permeability is desirable for filters or wearables; in the latter case, permeability helps avoid skin irritation and inflammation. [7] Several examples exist of coating LM onto fiber mats to form highly permeable, electrically conductive textiles. For example, bulk LM deposited onto an electrospun fiber mat has the inherent electrical conductivity of the LM, but it blocks the pores. The coating can be rendered permeable by repeated stretching, which creates pores in the LM coatings. [4c,8] Here, we coat particles of LM directly onto textiles, which has the appeal of not blocking the pores of the textile. We show that the particles provide other benefits such as autonomous electrical healing (i.e., the textile can be completely cut and retain conductivity), mechanical patterning, and antibacterial properties.
We prepared the permeable LM-coated textile (LMT) by dipping textiles into a solution of LM particles. The particles natu- rally adhere to the textiles, but do not form a conductive pathway. Selectively applying stress to localized regions of the LMT causes the LM particles to "rupture" and merge to form percolated conductive paths. Meanwhile, the rest of the LMT remains nonconductive.
Herein, we report the relationship between the numbers of coatings and both electrical conductivity and air permeability of the LMTs. The necessary critical stress will be estimated in order to render the textile electrically conductive. In addition, we demonstrate the autonomous-healing capabilities of LMTs. This study presents multiple applications of LMTs in various capacities, including flexible circuits, heaters, and electrodes for ECG measurements. Antibacterial properties of the LMT are identified that can prevent bacterial contamination, which can degrade textiles, infect the body, and/or lead to odor issues.

Results and Discussion
We prepared LMTs by dip-coating textiles as shown in Figure 1a.
To create the LM particles, LM was sonicated in isopropanol at a volume ratio of 1:40 using probe sonication for 45 min. The textile is a commercially available non-woven textile, which is composed of 45% polyester and 55% cellulose. The average diameter of an individual fiber is 12 μm, as shown in Figure S1 (Supporting Information). The pristine textile was immersed in the LM particle solution for 5 s. After removing it, the excess solution drips off and a hot air "gun" was used to dry the textile to achieve 1 dipped (1 Dip) LMT. The same steps were repeated as shown in Figure 1a to achieve 3 dipped (3 Dips), 5 dipped (5 Dips), and 10 dipped (10 Dips) LMTs. Figure 1b shows photographs, and SEM images that compare 3 Dips, 5 Dips, and 10 Dips LMT. The "as-coated" LMTs are not conductive as shown in Figure 1b.
To a first approximation, the composition of the textile does not appear to be important; we found that LM particles adhered Figure 3. Characterization of mechanical compression process on LMTs to render conductive pathways. a) Plot of the number of dips, LM weight and sheet resistance (R s ). The sheet resistance (R s )-which was measured after mechanical compression-is calculated using the formula R s = R*(W/L) where R is measured resistance, W and L are width and length of the sample respectively. b) Plot of sheet resistance for 3, 5, and 10 Dips over seven days. c) The applied stress and conductivity as a function of normalized current (I/I max ) through the LMTs for constant voltage applied across the ends of the compressed region. I max is the maximum compliance current (1 mA). d) Plot showing the relationship bending angle with the normalized current.
to every textile that we used. [9] Three things may favor adhesion: 1) the soft nature of the particles may facilitate good particle-fiber contact, 2) capillary forces during drying can bring the particles into close contact with the fibers (although it did not cause them to sinter, as observed in other systems), [10] and 3) the thin native oxide on the surface of the particles can form van der Waals interactions with the substrate. [2a] Recently, we showed that the oxide adheres to all smooth surfaces by advancing droplets across surfaces of widely ranging chemistry and hydrophobicity. [11] It does not adhere well to rough surfaces. For example, it is difficult to adhere bulk LM to fiber mats; it requires smearing the LM back and forth multiple times over the mat, resulting in a coating that is difficult to control. Here, the particles (0.2 μm) are smaller than the fiber diameter (12 μm) and thus can make good contact. Thus, an appeal of the particle-based approach lies in its ability to produce coatings with excellent adhesion properties.
The electrically insulating LM coating can be rendered into a conductive circuit by locally compressing the LMT. By using a topographical mold to apply stress, it is possible to create conductive patterns surrounded by insulating regions, as shown in Figure 2a. The mechanical compression ruptures the thin oxide on the particles and merges the LM on the textile to form a continuous conductive path, as shown in Figure 2b. This method of creating percolation is called "mechanical sintering." [1b,12] After mechanical compression, the percolated particles rapidly reform the surface oxide layer due to exposure to air. The LM particles on 1 Dip LMT were still discontinuous after mechanical compression, indicating that 1 Dip cannot generate effective conductive paths, as shown in Figure S2 (Supporting Information). Figure 2b shows the optical and microscopic image of the boundary between the conductive and insulating regions for a 5 Dips LMT. The optical distinction between the two regions is apparent. The shinier metallic part at the compressed position shows that the particles percolate to form a continuous LM path. Figure S3 (Supporting Information) reports the diameter (≈200 nm) of the LM particles. The number of dip-coats has a significant effect on the electrical conductivity of LMTs because each dip adds more LM particles to the textile. Figure 3a shows that the weight of LM particles deposited on the LMT increases linearly with the number of dip coats (sample area: 5 cm × 5 cm). As shown in Figure 3a, after compression, the sheet resistance of the LMT decreases sharply from 3 to 5 Dips, while the decrease is gradual from 5 to 10 Dips. In addition, we did not observe any significant variation in the values for sheet resistance (sample area: 5 cm × 5 cm) over a prolonged duration, as shown in Figure 3b.
To quantify the minimum stress required to render the LMT conductive, the stress applied were varied while measuring the resistance. Electrodes were positioned at each end of the sample and a controlled stress was applied to a mold spanning between the electrodes. The stress was regulated using an Instron instrument. A constant potential of 2 V was applied while measuring the current between the electrodes with the aim of identifying the stress required to cause percolation. It should be noted that, initially, the pristine LMT exhibits high resistance, hence there is no current across the LMT at a constant potential of 2 V. With the increase in stress, LM particles rupture and form conductive pathways that decrease the resistance, and hence increase the current. Figure 3c shows the relationship between applied stress and normalized current (I/I max ), in which I max is a threshold compliance current value set by the instrument (1 mA). According to Figure 3c, the stress needed to merge LM particles decreases with more dip coating of LM particles. We note that, ≈3 MPa of stress is required to render the 10 Dips LMT conductive, which is consistent with the observations of our previous work. [13] Based on the ≈200 nm diameter of the particles and assuming a surface stress of ≈0.5 N m −1 to rupture the native oxide, [1c] we expected a stress of ≈2.5 MPa to sinter the particles. Thus, the measured stress is consistent with expectations. Bend-ing can also merge LM particles, forming conductive paths along the bending crease. However, this only occurs in a 10 Dips LMT as shown in Figure 3d. The resistance for the 10 Dips LMT starts to drop at a bending angle of 105°. For most applications, it is undesirable for normal handling (e.g., bending) to cause inadvertent conductivity. Thus, 5 Dips LMT has the best combination of conductivity, stability against bending, and weight of deposited LM.
The LM conductive path is achieved by compressing the LM particles, which raises the possibility that particles could merge inadvertently during handling. The stress needed to merge the LM particles is about 6 MPa which is a large stress (the stress exerted by an adult with a weight of 70 kg standing on both feet is about 0.02 MPa). Thus, while inadvertent particle merging is unlikely, merging may be avoided in several ways: 1) Reducing the diameter of LM particles can increase the stress needed to merge the LM particles due to the oxide skin. [12] 2) In our previous work, [3c] we proposed a breathable encapsulated LM foam. By using the porous structure or other soft substrates as a "shield," the LM-based electronics can achieve breathability and encapsulation simultaneously.
LMTs can be used for electronic textiles. Figure 4a shows a circuit patterned on a 5 Dips LMT with a LED bulb connecting to the circuit using conductive paste. The LMT-based circuits can be mounted on to a variety of curved surfaces and can be worn over joints, as further shown in Figure 4b,c. The LMT-based circuit works in both concave and convex bending states. The LMTbased circuits were stable and operational when submerged in water as shown in Figure S4 (Supporting Information). Moreover, the LMT remains conductive under cyclic stretching, as shown in Figure S5 (Supporting Information). This is due to the fluidity of the LM conductive paths (Video S2, Supporting Information).

Figure 5a
shows that the electric conductivity of LMT-based circuits can autonomously heal when cut. The cutting action provides sufficient shear stress to mechanically merge the LM particles at the border along the cut, generating new conductive paths, [5a] as demonstrated in Figure 5b. Video S1 (Supporting Information) demonstrates a complete video for autonomous healing for a 5 Dips LMT.
Another notable advantage of these particulate-based coatings is their ability to maintain porosity within the textile, allowing for air permeation. To measure the breathability, we covered bottles containing 20 mL of water with the samples with or without LM coating and weighed the remained water over 7 days at 37°C. LMTs show good water vapor transmission compared to uncoated sample, as shown in Figure 6.
The conductive paths in the LMT can be used for Joule heating, as shown in Figure 7a. Applying 3 V across a conductive path resulted in an average current of 110 mA, generating an average power of 342 mW (detailed analysis shown in Tables S1 and S2, Supporting Information). The potential was applied of 120 s while recording the temperature using an infrared (IR) camera. We observed a sharp increase in temperature in the first 20 s of heating, as shown in Figure 7b. The sample reached a maximum temperature of 52°C and a mean temperature of 32°C within the marked region (white box in Figure 7a). These temperatures are all within the range of comfort for a wearable body warmer. Video S3 (Supporting Information) demonstrates the heater during operation. After removing the potential across the LMT after 120 s, we observed a sharp drop in maximum and mean temperature in the first few seconds, as shown in Figure 7b. Using 5 V caused similar behavior, albeit with an even larger temperature change. There are no noticable changes to the LMTs after the prolonged heating and repeated use. In summary, the LMTs combine stretchability, self-healing, breathability.
We also tested the feasibility of using a LMT with a desired pattern for wearable on-body heaters powered by two AA batteries in series (3 V, 50 mA), as shown in Figure 7c. The built-in LMTbased heater took ≈15 s to reach a maximum temperature of ≈ 42°C. Figure 7c shows the highest temperature achieved during the demonstration. Further, it was observed that LMT cools down to ambient temperature in about 20 s. Video S4 (Supporting Information) shows the heating for the body-worn LMT.
LMT can also function as permeable electrodes for biopotential measurements. We demonstrated LMTs functioning as ECG electrodes and compared the data with conventional gel-based electrodes, as shown in Figure 8. Both measurements were conducted while the volunteer remained relaxed in a similar environment. Both the working electrodes were placed on the opposite arm of the subject while the reference electrode was placed on one of the elbows, as shown in Figure 8. The results were recorded at 1 kHz frequency without the use of any filters. It was observed that LMT electrodes performed equally well when compared to conventional (commercial) gel-based electrodes. The volunteer did not report any discomfort or skin irritation during or after the test due to the LMTs contact with the skin. Moreover, the permeability of LMTs overcomes the shortcomings of conventional gel-based electrodes, which do not have open pores for the skin to 'breath' for prolonged use.
The antibacterial properties of LM-coated textiles were not shown in the previous studies (Table S3, Supporting Information). This study demonstrated that the simple LM coatings can provide the protection for the textile from harmful bacterial contamination. This feature is particularly advantegeous for preventing the contamination in wearable devices by bacteria which could potentially lead to skin infection, device degradation, and unpleasant body odor. The antibacterial activity of LMTs was determined against Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus using plate counting teachnique and optical density measurements. These two bacteria are known to play major roles in causing healthcare-associated infections. [14] The antibacterial performance of the LMTs (1 Dip, 2 Dips, and 5 Dips) was compared to an uncoated textile (control). LMTs were found to be effective against both S. aureus and P. aeruginosa (Figure 9). Figure 9a,b shows that LMTs with more dips have higher inhibition percentage against both bacteria. For example, LMTs with 2 Dips and 5 Dips were found to inhibit S. aureus by 84% and 90%, respectively. In contrast, 1 Dip only inhibited S. aureus by 17%. In addition, Figure 9b demonstrates  that less bacterial colonies (i.e. less bacteria) from the surfaces of 2 and 5 Dips were found than the control and 1 Dip. Interestingly, LMTs with 2 and 5 Dips can also eradicate 50% of planktonic S. aureus and P. aeruginosa in suspension after 60 min of exposure, as shown in Figure 9c,d.
To verify the antibacterial activity of Ga particles against S. aureus and P. aeruginosa, the inhibition zone assay was adopted ( Figure S6, Supporting Information). Inhibition zone diameters were measured for the quantitative analysis. Overall, Ga particles exhibit better antibacterial activity against P. aeruginosa than S. aureus. Figure S6 (Supporting Information) shows the larger inhibition zone with higher Ga concentrations against S. aureus and P. aeruginosa. Qualitative analysis results ( Figure S6b, Supporting Information) demonstrate the antibacterial properties of Ga against S. aureus and P. aeruginosa. In the previous studies, Ga particles embedded in the coating can provide prolonged antimicrobial protection against both Gram-positive S. aureus and Gram-negative P. aeruginosa. [1g,15] Ga nanodroplets were reported to exhibit strong adhesion with bacterial cell surfaces, [1f] which may contribute to the their antimicrobial performances. Importantly, Ga exhibits low cytotoxicity and may be used safely in vivo. [1g,15,16]

Conclusion
This paper presents textiles coated with liquid metal particles-LMTs-that have unique properties. LMTs are fabricated by a simple dip-coating method that is easy to implement. Through mechanical compression, the discrete LM particles can be transformed into a conductive LM path, achieving patterns of high electrical conductivity (0.03 Ω sq −1 ). Interestingly, the LM particles in the insulating areas enable LMTs to autonomously heal in response to cuts by generating new conductive paths along cut regions. Meanwhile, the LMT has good permeability to air. The mechanism of conductivity of LMTs was analyzed using SEM, and the relationships between the conductivity and the number of coats were demonstrated. Also, the magnitude of stress required for merging LM particles was determined. The versatility of LMTs was further validated through demonstrations of several applications including flexible-circuits with air permeability, onbody heaters, and ECG electrodes. Notably, we found that LMT has the ability to inhibit bacterial proliferation to avoid bacterial contamination. In this study, Ga particles were uniformly coated across the entire fabric. However, future optimization could involve selectively coating Ga particles only in desired regions, potentially through methods such as masking the insulating regions.

Experimental Section
Materials: The LM is eutectic gallium indium, purchased from Indium Corporation. The textile is TechniCloth II Wipers, Texwipe, purchased from Avantor Inc. These textiles are described by the manufacturer as 45% cellulose and 55% polyester. Isopropanol was purchased from Fisher Scientific.
Fabrication Processes: 20 mL isopropanol and 0.5 mL EGaIn was sonicated using a probe sonicator (Qsonica Sonicator, amplitude = 45, pulse on time = 10 s, pulse off time = 10 s) for 45 min. The resulting solution of LM particles was magnetically stirred continuously at 350 rpm for 3 min. The pristine textile was dipped in this solution for 5 s and the excess solution was allowed to drip off. The resultant samples were dried using ambient air from a heat gun to achieve a coated LMT. Finally, mechanical compression was performed on the desired region by compressing the pristine LMT in a 5943 Instron using acrylic stencils ("topographical molds"). Acrylic stencils and molds were fabricated using an CO 2 laser cutter (Universal Laser Systems).
Characterization and Measurements: Microscopic imaging was done with an Olympus BX51. The antibacterial activity of LMTs was determined using counting colony-forming units (CFU) and confocal laser scanning microscopy (CLSM). Specially, two laboratory strains Gramnegative Pseudomonas aeruginosa (P. aeruginosa) ATCC 15692 and Grampositive Staphylococcus aureus (S. aureus) ATCC 25923 were used in this study. P. aeruginosa and S. aureus are two of the most common co-infecting bacteria in human infections and are commonly found on human skin. [17] Prior to the incubation, the bacterial concentration was adjusted with an optical density (OD) at 600 nm of ≈0.15. An uncoated textile served as a control, which was compared to the behavior of LMTs with different dips (1, 2, and 5 Dips). All samples were incubated in tryptic soy broth at 37°C for 1 h. At 15,30,45,and 60 min,200 μL of supernatant was collected from each sample for spectrophotometer measurement to study the inhibition of planktonic bacteria present in suspensions.
To remove the attached bacteria, the samples were further soaked in 3 mL of PBS, vortexed for 30 s, sonicated for 2 min, then vortexed for another 30 s. Colony forming units (cfu) from the sonicated suspensions were further evaluated. The inhibition of LMTs was calculated as below.

Percentage of inhibition =
( 1 − CFU (Coated textile) CFU (Uncoated textile) ) × 100 All experiments were repeated three times. The mean values were calculated with estimated standard deviations. All experiments complied with guidelines by the North Carolina State University. All subjects were volunteers and provided informed consent.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.