Mussel-Inspired Flexible, Durable, and Conductive Fibers Manufacturing for Finger- Monitoring Sensors

Here we report a bio-inspirited facile and versatile method for fabricating highly durable, washable and electrically conductive fibers and yarns. Self-polymerized dopamine plays as adherent layers for substrates and then captures Pd 2+ catalyst for subsequent metal deposition on substrates. The Pd 2+ ions are chelated and partially reduced to nanoparticles by polydopamine (PDA)-modified substrates and the catalytic performance is investigated in surface electroless deposition. Importantly, this is the first report about PDA as both ligand and enhancement in Pd catalyst system, and the mechanism of their excellent catalytic performance is studied by XPS. This approach can be extended as a general method for fabricating conductors from all kinds of substrates, and precursory research about PDA/Pd catalyst application in surface catalysis.


Introduction
In recent years, fiber-based wearable electronics such as fiber-shaped energy harvesting and storage devices, wearable displays, deformable antenna and fiber computers/processors have attracted a great deal of attention. [1][2][3][4][5][6] For realizing these devices, one critical step is the fabrication of conductive components such as interconnects on flexible and stretchable fibers/fiber assemblies. [7] Although several novel materials, including conductive polymers, carbon nanotubes (CNTs) and graphene were developed in recent decades, metal is still considered as the best coating material in terms of conductivity, stability, compatibility and cost. [8][9] Compared with other substrate materials such as plastics and elastomers, fibers and paper have the unique porous structures. Thus, the desired coating materials should penetrate into the fibrous structures. [10][11][12] Several attempts have been made to metalize fibers such as thermal evaporation, atomic layer deposition, magnetron sputtering and galvanic deposition. [13][14][15][16][17] But the fabrication via above-listed methods does not have chemical bonds or other tethering force between the metallic layers and the surface of fibers. The inherently poor adhesion between metal nanoparticles and flexible substrates limits the widespread application of conductive fibers. Additionally, for flexible substrates with the particular 3D structure, such as sponges, the electro-conductivity is poorly containable because the 3D structure, as a spatial mask, decreases the uniformity and continuity of the deposited metal films initiated by gravity. [18] Recently, Liu et al. reported the polymer-assisted metal deposition by surface-initiated atomic transfer radical polymerization (SI-ATRP). [19] The designed polymer interface introduces covalent bonds between the surface of fibers and grafted polymer brushes and viscoelastic and high-swelling intrinsic properties of polymers provide the nanometer-scale mechanical interlocking of deposited nanoparticles within brushes. Although the resultant conductive yarns are highly durable and washable, the polymerization requires an inert N 2 protection and complex steps. Moreover, the target substrates have to contain abundant hydroxyl groups. On the other hand, with the booming development of novel materials, which can be employed in wearable electronics as their variety of performances, how to develop a new surface modification method which can be used in virtually any substrate and create conductive composites is important.
According to Lee's report, [20] dopamine which mimics the adhesive chemistry of mussel plaque detachment allows the spontaneous deposition of nanoscale-thin, surface-adherent films of poly(dopamine) (PDA) on virtually all material surfaces such as polymers, ceramics, semiconductors and novel metals by simple dip-coating in an alkaline solution. More importantly, Secondary reactions can be used to produce a variety of ad-layers on the top of PDA, including metal films by electroless metallization. [21] In some reports, silver (Ag) was coated on different fibers, such as polyester (PET), meta-aramid, glass, cotton and polyurethane, via PDA-assisted electroless deposition (ELD). [22][23][24][25][26] However, in accordance with Zheng's review work, [27] silver, as a conductive coating material, is much more expensive than copper and nickel. More importantly, according to the European Commission and its non-food Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR), there are still some arguments related to the toxicity of silver nanoparticles and additional adverse effects caused by the use of silver nanoparticles should be further evaluated.
To address the challenges, we report here a simple, versatile and scalable approach for preparing highly durable, washable and electrically conductive fibers and yarns by electroless nickel (Ni) plating on fiber surfaces modified with PDA as adhesive layers. Copper (Cu) can be an alternative coating choice due to the high conductivity and low price. However, the oxidation of the Cu layer can cause the slow decrease in conductivity when the sample is placed in air. Thus, nickel, as an air-stable conductive metal, is mainly discussed in this report.
In detail, PDA that tethers one end on the surface via covalent and noncovalent bonds is firstly grown from fibers by dip-doping in tris buffer. Subsequently the grafted PDA interface anchors the catalyst in an aqueous solution. The captured catalyst final activates electroless plating metallization. The as-made conductive yarns can be demonstrated as interconnects to power light-emitting diodes (LEDs) and also can be used to track finger motions. In principle, this effective approach can be extended as a general method for producing conductors from all kinds of substrates. The PDA modification can control the coating thickness by varying deposition time and is recognized as an environmentally-friendly chemical approach. The procedure is illustrated in Scheme 1. In a typical experiment, we first immobilized the PDA interface on fiber surfaces by dip-doping in tris buffer for 24 h at room temperature. Then, the samples were washed with DI water several times and dried with a N 2 gas stream.  The successful wrapping of PDA on wool surface is also detected by FTIR (see Figure S1, Supporting Information). After PDA modification, the PDA-coated samples were immersed into a 5 mM (NH 4 ) 2 PdCl 4 aqueous solution for 2 h, where Pd 2+ moieties were anchored within the PDA interface. The captured palladium moieties/particles provide effective catalytic sides for subsequent ELD.
The proposed mechanism of dopamine polymerization has been known but when adding Pd (II) to PDA-modified substrates, the binding mechanism between PDA and Pd (II) should be explained. Thus, X-ray Photoelectron Spectroscopy (XPS) was applied to investigate the surface composition of the Pd/PDA-PET composite. The XPS survey spectrum reveals region scans for all elements detected in the catalytic sample (see Figure S2, Supporting Information).
The C 1s spectrum in Figure 2a shows three peaks assigned to C-C (284.8 eV), C-O/C-N (286.4 eV) and C=O (288.9 eV) species originating from the PDA layer. [28] From the N 1s spectrum (Figure 2b), the peak at 402.0 eV was assigned to the component of primary amine (R-NH 2 ), which is associated with dopamine. The peak at 399.7 eV was attributed to the secondary amine component (-R-NH-R), which is associated with both PDA and accepted intermediates. The peak at 398.6 eV was assigned to the tertiary/aromatic amine functionality, which was associated with5,6-dihydroxylindole (DHI) and 5,6-indole-quinone. [29] Additionally, the O 1s region in Figure 2c is fit with two oxygen signals assigned to C=O (531.9 eV) and C-O (533.2 eV) species, which were from the PDA layer. [28] In Pd 3d region and Pd (II) (binding energy 339.1 and 346.4 eV). This confirms that Pd 2+ ions were chelated by the primary amine groups of PDA and the catechol groups of PDA reduced some Pd 2+ ions into Pd nanoparticles, which were encapsulated within PDA layers. [30] It is interesting to note that in comparison with previous reports, [28,31] there is an increase of the binding energy of palladium in our report. The first reason for the increase in binding energy is due to the electronic environment around Pd catalysts. PDA, due to the characteristics of its π bond, causes the outer electrons of the palladium ions to participate in the π-metal stacking, resulting in an increase in binding energy. On the other hand, it is important that the palladium nanoparticles we reported were reduced by PDA, not further reduced by other reducing agents such as sodium borohydride. Therefore, the reduced palladium nanoparticles were nearly not agglomerated, resulting in the larger binding energy. [32] The smaller catalytic nanoparticles also contribute to the larger specific surface area of catalytic region, leading to the increased performance of the catalyst in the followed ELD. To prove that, extra NaBH 4 was added in the catalyst solution to further reduce palladium ions into palladium particles on the surface of PDA-PET and when this sample was immersed in the ELD bath, only a few weak bubbles were formed, suggesting a very low reaction rate. Therefore, on the surface of a substrate, PDA plays a role of reducing and stabilizing palladium ions with an adhesive force. Importantly, the stabilized Pd 2+ can enhance the catalytic action in ELD, causing the good performance of metal coating.

Surface Characterization
Since the surface morphology of the obtained copper or nickel coatings may affect the electrical, mechanical and optical properties of the conductive fibers that are produced, scanning electron microscopy (SEM) was employed to characterize the morphology of thin copper or nickel films in detail. The SEM photographs of the nickel coating on cotton (c, e) and PET (d, f) fibers and pure cotton (a) and PET (b) fibers are shown in Figure 3. Compared with Figure 3c,e,d,f, a significant change on the surface of raw materials (Figure 3a,b) can be observed after nickel electroless plating. And it can be clearly seen that uniform and   X-ray diffraction (XRD) is a conventional and simple technique to reveal the size and the shape of the unit cell for any crystalline compound. In previous reports, [19,33]

Electrical Conductivity and Mechanical Durability
Compared with bare textile fibers which are well-known to be insulated, the as-made Nicoated fibers are electrically conductive. In this report, four types of conductive yarns are produced, including wool, cotton, nylon and PET and the digital images of yarns are recorded (see Figure S4, Supporting Information). From Figure 6a, the electrical surface resistance of the cotton yarn decreases with increasing plating time, reaching as low as 0.05 Ω/cm at 60 min ELD. And longer plating leads to heavier samples due to the increasing amount of plated metallic nanoparticles (see Figure S5, Supporting information). However, a saturation plateau occurs at ∼60 min. This can be attributed to catalyst poisoning. Clearly, extending the depositing time resulted in a conspicuous decrease in resistance, but a more brittle and stiff film with increased thickness. This can be explained by the decreased contact resistance between plated nanoparticles when they are more tightly packed with longer depositing time.
However, no evidence is shown to prove this hypothesis. [34] Luckily, we found three different types of distributions of nickel nanoparticles on a single fiber using field emission gun scanning electron microscopy (FEG-SEM). The surface morphology of Ni nanoparticles on the cotton fiber when the ELD time is 15 min is shown in Figure 6b and it perfectly confirms the assumption. From the yellow area of the image, nickel nanoparticles were dispersed at a low rate of compact distribution, which can be defined as the first layer of nickel coating.
Based on the blue area of the photograph, the plated nickel particles were more continuous on the surface of the cotton fiber, leading to the decreased resistance. This can be defined as the second layer. When depositing time increased further, nickel nanoparticles were more densely aggregated on the second layer, leading to the further decrease in resistance and the formation of subsequent layers such as the cyan area of the FEG-SEM image. With increased ELD time, the coated nickel film on fibers was thicker and more compact, creating higher conductivity at the expense of flexibility. The electrical stability under multiple cycles of bending and washing is one of the most critical challenges of conductive yarns and fabrics. To investigate this, washing cycles using simple hand washing and squeeze-drying and mechanical bending cycles using simple hand bending with a radius of 15 mm were employed to test the rubbing and bending robustness, respectively. No obvious increase of surface resistivity on the Ni/cotton conductive yarn was observed after five washing and drying cycles or two thousand simple mechanical bending cycles (see Figure S6, Supporting Information). This was due to the chemical bonds between the surface of fibers and the coated PDA layers.

Demonstrations of Ni conductive yarns
One of the most important applications for the as-prepared conductive yarns is used as conductive wires in electronic circuits because of their high conductivity and robustness. As a demonstration, a simple circuit was built by bridging a 9 V battery and one electrical contact of a blue light emitting diode (LED) with prepared conductive yarns. When the blue LED contacted the alkaline battery, the LED turned on immediately and illuminated for more than 20 min until the contact was disconnected (see Figure S7, Supporting information). On the other hand, because of the high sensitivity to strain, these conductive yarns can be used as a strain sensor to monitor tiny motions of the finger. As a proof-of-concept, we sewed nickelcoated cotton yarns into four finger parts of a commercial glove, as shown in Figure 7a.
When the finger bent/unbent at different gestures, the conductive yarns sewn into glove were stretched/released, causing the increased/decreased contact area between conductive fibers (see Figure S8

Conclusion
In conclusion, this study demonstrates a novel, facile and versatile approach for preparing durable, flexible and electrically conductive yarns. PDA nano-films are first coated on fiber surfaces by immersing them into an alkaline aqueous solution. PDA-modified fibers then anchor the catalyst via ion chelation and finally metal nanoparticles are deposited onto the catalytic area, resulting in the formation of polymer-bridged conductive composites. This insitu plating method ensures that metallic nanoparticles are distributed on the surface of fibers continuously and uniformly. In addition, as the PDA coating serves as an adhesive layer, this leads to the superior electrical stability of as-made conductive yarns under repeated bending and washing cycles. Such manufactured conductive yarns can be used as the flexible electrical conductor in electrical circuits to power a LED and also a strain-stress sensor to monitor finger movements. The bioinspired polymerization of dopamine can be used for multifunctional coatings. Thus, PDA-assisted ELD can be a universal method for coating nickel or copper nanoparticles on virtually all material surfaces. PDA-assisted ELD is also a low-energy and green method throughout and the low price of nickel and copper makes them perfect coating materials for producing conductive yarns on a large scale. Such conductive yarns should develop a wide variety of applications in fiber-based wearable electronics, radiation and electromagnetic protection, energy, architecture and biomedical industries.
Moreover, PDA/Pd catalyst was firstly employed in surface-catalyzed reactions and show better performance than isolated Pd catalyst, and PDA as both adhesion and enhancement will show broadened application in other surface catalytic reaction.

Experimental Section
Materials: Dopamine hydrochloride, Ammonium tetrachloropalladate(II) [(NH 4 ) 2 PdCl 4 ] and all other chemicals were purchased from Sigma-Aldrich. All textile substrates were obtained from the Dye House at the University of Manchester. Each fiber substrate was ultrasonically cleaned in acetone and distilled (DI) water for 30 min, respectively, then dried with a N 2 gas stream.
Dopamine Spontaneous Polymerization on the surface of fibers: Dopamine was dissolved in 0.01 mol/L tris (hydroxymethyl) aminomethane (pH 8.5) buffer to prepare a dopamine solution (2 g/L). Cleaned substrates were dipped into the solution and the non-specific microparticle deposition on surfaces was prevented by stirring and/or vertical sample orientation.
The pH-induced oxidation of dopamine changed the color of the solution to dark brown.
After a predetermined reaction time of 24 h, the adherent PDA film coated surfaces were filtered and rinsed thoroughly with ultrapure water and dried with a N 2 gas stream.

Metallization by ELD:
The modified samples were immersed into a 5mM (NH 4 ) 2 PdCl 4 aqueous solution and placed in a dark environment for 15 min to load catalysts by chelation and reduction, followed by thorough rinsing with DI water to remove the physical absorption of catalyst inks. The Ni electroless plating was performed in an ELD bath containing 4:1 volumetric proportion of nickel-to-reductant stocks at room temperature. A nickel stock solution consisting of 10 g/L lactic acid, 20 g/L sodium citrate and 40 g/L nickel sulfate hexahydrate was prepared in advance. A fresh reductant solution containing 1 g/L dimethylamine borane (DMAB) in DI water was prepared separately. After mixing, the solution was adjusted with ammonia to pH ∼8. After ELD, all samples were washed several times and dried with compressed air. tested by Fourier transform infrared spectroscopy (NICOLET 5700 FTIR). The size and the shape of the unit cell for metallic particles on the fiber surface were determined by X-ray diffraction (PANalytical X'Pert Pro X'Celerator diffractometer).

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