Three-dimensionally deformable, highly stretchable, permeable, durable and washable fabric circuit boards

Abstract

This paper reports fabric circuit boards (FCBs), a new type of circuit boards, that are three-dimensionally deformable, highly stretchable, durable and washable ideally for wearable electronic applications. Fabricated by using computerized knitting technologies at ambient dry conditions, the resultant knitted FCBs exhibit outstanding electrical stability with less than 1% relative resistance change up to 300% strain in unidirectional tensile test or 150% membrane strain in three-dimensional ball punch test, extraordinary fatigue life of more than 1 000 000 loading cycles at 20% maximum strain, and satisfactory washing capability up to 30 times. To the best of our knowledge, the performance of new FCBs has far exceeded those of previously reported metal-coated elastomeric films or other organic materials in terms of changes in electrical resistance, stretchability, fatigue life and washing capability as well as permeability. Theoretical analysis and numerical simulation illustrate that the structural conversion of knitted fabrics is attributed to the effective mitigation of strain in the conductive metal fibres, hence the outstanding mechanical and electrical properties. Those distinctive features make the FCBs particularly suitable for next-to-skin electronic devices. This paper has further demonstrated the application potential of the knitted FCBs in smart protective apparel for in situ measurement during ballistic impact.

1. Introduction

Applications of wearable electronic devices place great demands on fibre-based devices and circuit board assemblies that are in direct contact with the soft, three-dimensional and extensible (from 3% to 55% strain) human body [1–5]. Such next-to-skin electronic assemblies require a new family of flexible and stretchable circuit boards that work in repeated large deformation in the tensile, bending and shear modes and in three dimensions [6–8]. The circuit boards should have low moduli comparable to those of human tissues such as skin and muscles; they need to be permeable, comfortable to wearers and washable [9]. Fabric circuit boards (FCBs), therefore, have enormous potential to fit such a purpose. FCBs mechanically support and electrically connect discrete electronic components by using conductive tracks or others made from electrically conductive fibrous materials of metal, conductive polymers or composites, prints or coatings, supported by dielectric fibrous structures [10,11]. FCB technologies, instead of cumbersome cables which inevitably restrict certain movements [12–17], enable the whole FCB assembly to be worn on three-dimensional human bodies in real daily activities [18], thereby opening up a large number of potential applications such as electronic skins [19,20], conformable sensor networks (or arrays) on human bodies [10,21–24] and biointegrated systems for health monitoring or therapeutic purposes [16,25–29].

Similar to printed circuit boards, FCBs can be designed with single-, double- or multi-layered structures (electronic supplementary material, figure S1). They are fabricated by computer-integrated manufacturing technologies, including weaving [30–35], knitting [36], stitching [37] or embroidering [38–40] of fine conductive fibres (e.g. stainless steel yarn [41,42], silver-plated polyamide filament [38], copper fibre [43–45]) into fabrics [2,38,46]. FCBs can also be made by printing [47–49] or coating conductive materials (e.g. conductive elastomers [50–58], carbon-filled rubbers [59], conductive carbon nanotube (CNT) or AgNW inks [60,61]) on fabrics or paper substrates instead of copper laminated polymer film substrates [62,63]. Lamination of fabrics with thin metal films is an alternative approach [63]. Of the above-mentioned three, the first approach has the potential for the real wearable FCBs that meet all the above-mentioned performance requirements.

As a family member of circuit boards, FCBs should possess excellent electrical conductivity and appropriate current carrying capacity, high thermo-electromechanical reliability and safety, electromagnetic compliance at high frequency (more than 30 MHz) as well as good connectivity for techniques such as surface mounting technology. Primarily designed for wearable applications, FCBs have additional key property requirements, that is, air permeability; three-dimensional deformability in single or mixed modes of bending, compression, extension and shear; electrical and mechanical integrity over large deformation with tensile strain as large as 60%; fatigue life of over one million loading cycles; washing capability of over 30 washing cycles.

To date, woven and printed FCBs have been studied to physically link various distributed electronic components with the functions of sensing, monitoring and information-processing, such as wearable motherboard [58,64,65], sensorized gloves [50,56,66] and intelligent knee sleeves [67,68], driving the initial step towards wearable applications coverable human bodies due to their inherently mechanical drapability and foldability. However, the woven/non-woven and embroidered/stitched FCBs have failed mechanically and electrically within 20–30% strain [31]; printed conductive tracks have suffered inconvenient cracks when subjected to bending or stretching deformation modes [69].

Alternative technologies represented by novel organic materials [19,70–77] and wavy metal films [78–82] or buckled semiconductor nanoribbons [83–93] on compliant substrates have pushed electronic components, interconnects, as well as integrated circuits towards out-of-planar possibilities where bending and stretching deformation modes may dominate [94]. Organic materials such as (well-controlled) graphene particles [19,70,71], CNT composites [72–74,95], polyaniline-conducting polymer [75] as well as PEDOT–PSS composites [76] yield different stretching levels (ranging from several per cent to more than 100%) through blending with silicone rubbers (e.g. PDMS). The conductivity for those organic elastic conductors, except MWNT/Ag composite (conductivity: 3700 S cm⁻¹) [73], is too low (conductivity: less than 1000 S cm⁻¹) to perform as electrical wirings, particularly for integrated circuits [96–98]. More crucially, most of such organic conductors, except polymeric ionic conductors [99], drop their conductivity by at least several orders of magnitude even within 10% strain [75]. In contrast, metal films (e.g. Au, Ag and Cu) with comparatively high conductivity and semiconductor (e.g. SiO₂ and Ge) nanoribbons on elastic substrates are capable of being stretched by means of mechanically optimized structural configurations. They have been evaluated from aspects of stretchability, fatigue life and relative change in electrical resistance during deformation. Net-shaped gold-coated elastomer film stretched by adjusting its struts achieved 2000 cycles under 20% strain with about 60% rise in its electrical resistance [19,20]. Microcracked gold film on the PDMS substrate, created by thermal expansion mismatch, endured 25 000 cycles at 20% strain, while increasing its electrical resistance by 300% [100]. Horseshoe copper film embedded in the PDMS substrate, with a fatigue life of 2500 cycles at 10% strain, was capable of being stretched up to 56% strain with an increment of 2.1% in its electrical resistance [101]. Controlled wrinkling semiconductor nanoribbons on the PDMS substrates, from thin silicon ribbon with a wavy shape [84,85], to a pop-up pattern [83,102,103], non-coplanar mesh [86,89], as well as non-coplanar mesh with serpentine bridges [90–92], have increased their stretchability from 10% to 140% owing to the out-of-plane deflection in thin layers, thereby accommodating strains applied in the plane. Despite the boosted stretchability by a series of elaborate mechanical optimizations, electrical integrity in the stretching process and durability for both organic and inorganic electronics are still far from the requirements in real wearable applications. What is more, next-to-skin electronics demands elastic substrates to be breathable, with the ability of transferring heat and moisture or water vapour, and washable for repeated usage [43].

In this paper, we extend our previous work on knitted electrical interconnects [36] to a knitted FCB technology, which offers a set of properties that is not effective with woven, rigid or elastomeric circuit boards. The FCB (i) is capable of being stretched beyond 300% strain with an appropriate current carrying capacity of 15 mA owing to its three-dimensionally looped configuration and the way in which neighbouring loops fit together; (ii) is soft with a low elastic modulus (approx. 350 kPa), near to that of human skin (less than 1 MPa); (iii) is porous (air permeability: from 0.003 to 0.158 kPa s m⁻¹), which is a vital concern for the comfort of the product next to the stratum corneum, the most superficial layer of the human skin; (iv) is durable with long fatigue life over 1 000 000 loading cycles at 20% maximum strain; and (v) can be washed more than 30 times. A prototype of FCB assembly in the form of a fabric sensing network, in which fabric strain sensor arrays are electrically linked and mechanically supported by the developed knitted FCB, is then demonstrated for wearable electronic applications.

2. Results

We produced FCB samples by incorporating polyurethane-coated copper fibres, together with pre-stretched (approx. 150%), elastic filament yarns into a single jersey-knitted fabric in three-dimensional hooked (or looped) configurations on a computerized knitting machine (electronic supplementary material, figure S4b). The yarn has a composite structure with highly segmented polyurethane filaments (diameter: 40 μm, 2f) in the core and textured polyamide multifilaments (diameter: 40 μm, 24f) wrapped around the core (electronic supplementary material, figure S3c). Figure 1 shows images from optical microscopy and scanning electron microscopy (SEM) of the resultant sample. Figure 1a illustrates the microstructure of knitted FCB around the nail (radius of curvature at the nail tip: approx. 1 mm) of a little finger (radius: approx. 60 mm). Figure 1b−f shows SEM images of a knitted FCB in a dry relaxed state. One typical conductive track in the FCB consists of two polyurethane-coated metal fibres (core diameter: approx. 50 μm; coating thickness: approx. 3 μm) with a series of connected three-dimensional loops in which the circular portions at the two ends are out-of-plane (height of the loop: approx. 180 μm; period of the loop: approx. 800 μm) and the straight segments, penetrating into the knitted fabric, are interlaced with the elastic textile yarns (figure 1b–d). The metal fibre loops, whose density in the knitted FCB is 10 loops cm⁻¹, are closely arranged with uniform loop length (the length of the metal fibres in a single loop, approx. 5200 μm), period (the minimum distance between two adjacent loops, approx. 850 μm) and amplitude (the maximum distance from the trough to the crest, approx. 1720 μm; figure 1b). The thickness of the single jersey-knitted fabric board is 800 μm; the mass per unit area is 160 g m⁻², and the density is 10 loops cm⁻¹ in both transverse and longitudinal directions (figure 1c). SEM micrographs in figure 1e–f show the coated metal fibre, whose surface is smooth, remains intact after being incorporated into the knitted fabric through the knitting process. The copper fibre of 50 μm core diameter is coated by a polyurethane film with 3 μm thickness. The advantages of the coating are (i) the metal fibre is insulated, avoiding short circuits when two adjacent loops contact each other [104]; (ii) the metal fibre is protected from the chemical or environmental attacks, so that it is unnecessary to further encapsulate the resulting FCB, without the risk of reducing its stretchability for practical applications [105,106]; (iii) the fine metal fibre, with reduced bending (πd⁴E/64≈7.85e(−5) N mm²) and torsional rigidities (πd⁴G/32≈5.95e(−5) N mm²) as well as enhanced mechanical robustness (breaking strain: 16%) owing to a thin and compliant protection can be deformed substantially to three-dimensionally looped configurations (electronic supplementary material, figure 3b). Apart from the fibre coating, the metal fibres are completely covered by polyamide multifilaments from the front side view of the knitted FCB (figure 1c) owing to the pre-stretched spandex filaments, which then shrink to their original states when free of external force after the manufacturing process. Such porous fibrous structures can be worn directly on human bodies, without irritating skin during long-term use. Additionally, compression and friction during wear can be mitigated if the knitted FCB has its front side towards the human body.

Figure 1.

Structure of a knitted FCB. (a) Optical image of one stretched FCB on a human finger. (b–d) Backside, front-side and cross-sectional views of the FCB. (e,f) SEM images of the coated metal fibres (core diameter: approx. 50 μm; coating thickness: approx. 3 μm). (Online version in colour.)

Figure 3.

Geometrical change of the knitted FCB in transverse and longitudinal tensile tests. (a,b) SEM images at 60% and 120% strain in the transverse direction, respectively. (c,d) SEM images for 60% and 120% in the longitudinal case. (e,f) Corresponding period and amplitude with applied strain. (Online version in colour.)

The three-dimensional-looped configuration of the metal fibres and their interlaced integration with the smooth, compliant textile yarns facilitate a large reduction in their peak strain, because of structural conversion in single jersey-knitted fabrics that leads to less than 1% even if the knitted fabric is stretched over 60% [1,107]. Because fatigue life is known to be related inversely to strain level exponentially, the resistance to fatigue of the knitted FCB during repeated deformation cycles can be greatly enhanced and its electrical integrity can be well preserved [36]. The resistance of one electrical conductive track in the resultant-knitted FCB was monitored while it was being stretched to mechanical and electrical failures. The electrical resistance of the track in the FCB of 2.5×7 cm (gauge length: 5 cm) is approximately 3.9 Ω and remains constant ((R−R₀)/R₀≈0) when it is unidirectionally stretched up to 300% strain in either transverse (course) or longitudinal (wale) direction (figure 2a). The knitted FCB has a Young's modulus of approximately 30 kPa and approximately 40 kPa in the transverse and longitudinal directions, respectively (electronic supplementary material, figure S4d). The metal fibres are first unbent in the transverse tensile test then stretched by the interlaced filament yarns. In the longitudinal direction, the metal fibres are further bent and pulled from their horizontal segments into the v-segments then stretched with applied strain [41]. As the strain further increases over 300%, the resistance starts to increase monotonically, similar to the resistance–strain curve of the free-standing polyurethane-coated metal fibre (electronic supplementary material, figure S3b). Relative change in electrical resistance and stretchability of others' work is summarized in figure 2b [115]. The stretchability has been increased by either structural conversion or using organic materials from approximately 3% (SiNW on PDMS substrate [110]) to more than approximately 400% (wrinkled graphene on PDMS substrate [71]). The relative change in electrical resistance varies from approximately 2.1% (horseshoe pattern on PDMS substrate [108]) to approximately 7300% (Ag-MWNT-SIS nanocomposite film [73]), which may hinder the use as conductive tracks in stretchable circuit boards, in particular, for low-impedance electronic components or high-precision measurements [96,97]. By contrast, the knitted FCB has exhibited an extraordinarily electrical stability with almost no change in electrical resistance up to strain of 300% in unidirectional tensile deformation.

Figure 2.

Relative resistance change as functions of strain. (a) Relative resistance change–strain relation of one conductive track in the knitted FCB in tensile and three-dimensional punch tests. (b) Comparison with others' work (2, horseshoe Cu–PDMS [108]; 3, CNT loop [74]; 4, PEDOT : PSS–PDMS [76]; 5, Ag–PDMS [109]; 6, SiNW–PDMS [110]; 7, AgNW–PDMS [111]; 8, net-shaped [20]; 9, AgNW/PEDOT : PSS [112]; 10, graphene–PDMS [71]; 11, Au–PDMS [82]; 12, Ag–silicone fibre [96]; 13, Cu-woven [2]; 14, metal/elastomer fibre [113]; 15, SWNT–rubber [72]; 16, PANI : SEBS-g-MA/SEBS [75]; 17, AgNW–PUA [114]; 18, Ag-MWNT-SIS [73]). (Online version in colour.)

Knitted fabrics can easily form a double-curvature drape owing to their excellent bending, shear and membrane stretch behaviour. To examine three-dimensional deformability, a ball punch test was conducted, where a polished stainless steel ball (diameter: approx. 25.4 mm) was punched into an FCB sample, the same as the samples used for the unidirectional tensile test, at 90° (electronic supplementary material, figure S4c) [36]. Figure 2a and electronic supplementary material, figure S4d present the relative resistance change and stress of the knitted FCB as functions of calculated average membrane strain [36]. The FCB exhibits a constant electrical resistance until being stretched over an average membrane strain of 150%, which is one half of breaking elongation of the knitted FCB. The comparatively reduced stretchability (approx. 150%) in the three-dimensional punch test may be attributed to the complex deformation mode, including fibre bending, lateral compression, torsion and axial tension as well as compression and friction between the knitted FCB and stainless steel ball. The knitted FCB, owing to its significant three-dimensional deformability, excellent electrical integrity, as well as softness and permeability, is the most feasible for application in next-to-skin electronics.

To reveal the underlying mechanism that contributes to outstanding mechanical deformability with electrical integrity, we examined the structural change of the three-dimensionally looped metal fibres in the soft-knitted FCB. SEM images in figure 3a–d show several representative geometrical configurations of the knitted FCB in various deformed states, either transversely or longitudinally. The metal fibre loops slide and adjust their geometrical parameters, i.e. period and amplitude, by means of straightening in the transverse case, whereas bending to the smaller radius of curvature in the longitudinal direction, hence accommodating the applied deformation. Figure 3e–f plots the measured period and amplitude of metal fibre loops as functions of applied strain in the transverse, longitudinal unidirectional tensile tests, as well as the ball punch test. The data were obtained from at least 10 optical images (electronic supplementary material, figure S5). In the transverse case, the period increased consistently from 0.85(±0.01) to 2.89(±0.04) mm, in approximately linear fashion proportional to applied strain; the amplitude, on the other hand, decreased to 1.07(±0.02) mm at 300% strain from an initial amplitude of 1.72(±0.04) mm. In contrast, there was a reduction in the period to 0.51(±0.01) mm and an increment in the amplitude to 2.20(±0.01) mm with applied strain up to 300% in the longitudinal direction. Unlike unidirectional tensile tests, the ball punch test shows complex adjustments in the period and amplitude, hinting that much larger fibre strain might be induced.

Further observation suggests that conductive tracks in the knitted FCB are stretched to a flat geometry as the applied tensile strain increases. The combination of out-of-plane and in-plane deformation of the three-dimensional loops accommodates the large FCB deformation in a way that avoids any significant local strain of the metal fibres, enhancing electrical integrity. Further stretching may produce larger tensile strain to the flat metal fibre loops, resulting in their breaking when the failure strain of the free-standing metal fibres is reached.

Wearable applications demand flexible and stretchable electronics to be mechanically and electrically robust with sufficient fatigue life. The Coffin–Manson law was applied for predicting fatigue life N of the knitted FCB. To derive two constants A=1.025e(−20) and B=−11.92 in equation N=Aε^B_fibre, where ε_fibre was the simulated peak strain of the metal fibre with practical elongation of the FCB (electronic supplementary material, figure S7), we examined the variation of electrical resistance when two to three specimens were subjected to a cyclic tensile test at a maximum FCB elongation of 60% (ε_fibre=0.008) and 80% (ε_fibre=0.01) in the transverse direction. According to the above-mentioned empirical equation, the knitted FCB should withstand more than 10⁷ cycles at 20% maximum strain, consistent with an average extension of the human skin [4] (figure 4a). To confirm this, we further conducted a cyclic tensile test for 1 000 000 cycles (equivalent to three months of normal wear) at a set strain of 20%. The electrical resistance remains unchanged (($R_{max}-R_{min})/R_{min}\approx 0.65$%) over 1 000 000 cycles of 20% applied strain in the transverse case, as shown in figure 4b, demonstrating the knitted FCB could pass the test without electrical failure. The SEM image (figure 4c) taken after 1 000 000 cyclically tensile tests also proves that three-dimensionally looped metal fibres in the knitted FCB are still intact, without any mechanical degradation.

Figure 4.

Fatigue life of the knitted FCB. (a) The predicted fatigue life with Coffin–Manson law. (b) Electrical resistance of the FCB plotted against number of tensile cycles (maximum FCB strain of 20%). (c) SEM image of the sample after 1 000 000 cyclical tensile test with 20% maximum FCB strain. (d) Comparison with others' published work (2, microcrack [116]; 3, PANI : SEBS [75]; 4, horseshoe [108]; 5, net-shaped [20]; 6, AgNW–PUA [114]; 7, Ag–PDMS [109]; 8, CNT loop [74]; 9, SWNT–rubber [72]; 10, Ag–MWNT-SIS [73]; 11, AgNW–PDMS [111]). (Online version in colour.)

A comparison with others' published work is shown in figure 4d [41]. The fatigue life varies from 50 cycles at 10% maximum strain (AgNW in the PDMS substrate [111]) to 25 000 cycles at 20% maximum strain with 300% relative change in electrical resistance (microcracked pattern [116]). The low reported fatigue life with substantial change in electrical resistance may prevent their use as wearable applications. By contrast, the knitted FCB maintains its electrical resistance almost constant (variation of 0.65%) over exceptionally large loading–unloading cycles of 1 000 000 cycles owing to structural conversion of knitted fabric and mitigation in strain of the metal fibres when they are interlaced with smooth and compliant multi-filament yarns in three-dimensionally looped configurations.

A more critical demand for wearable applications is washing capability in repeated usage. We washed four groups of FCB specimens (length: approx. 10 cm) in a washing machine at 40°C and then dried them at 75°C 30 times according to AATCC standard 135: dimensional changes of fabrics after home laundering (electronic supplementary material, figure S6). We found that electrical failure started to occur at the 10th wash for a specimen without the protection of a mesh bag (thickness: approx. 0.27 mm; electronic supplementary material, figure S6c) in ‘normal’ cycle (agitation speed: 179–119 spm (spm=strokes per minute); spin speed: 645 rpm), in which 84% of specimens (total number: 30) have little or no change in electrical resistance of 5.5 Ω after 30 washing–drying cycles. The resistance retention ratio, which was defined as the ratio of the number of specimens maintaining electrical integrity to the total number of specimens in the washing test, rose to 89% when the specimens were put into a protective mesh bag (figure 5a). The SEM observation after 30 washing cycles reveals that no visible fractures can be found on the specimens maintaining electrical integrity, suggesting that the polyurethane coating adheres well to the core metal fibre, so that the presence of water with detergent, mechanical agitation at elevated washing and drying temperature do not damage the metal fibres (electronic supplementary material, figure S6d). Electrical failure, on the other hand, was induced by mechanical breakage of the metal fibres (figure 5b), because the knitted FCBs were subjected to strong mechanical actions causing deformation, including stretching, bending, compression and rubbing in the washing process. Hence, the resistance retention ratio is much better using the ‘delicates’ cycle with reduced mechanical agitation (agitation speed: 119 spm; spin speed: 430 rpm), where all 60 specimens passed the 30 washing test cycles without any apparent electrical degradation (figure 5a).

Figure 5.

Washing capability of the knitted FCB. (a) Resistance retention ratio in the washing tests. (b) SEM image of one failed metal fibre in the knitted FCB after 30 times in ‘normal’ washing cycle. (Online version in colour.)

In wearable electronic applications, strain distribution of the conductive tracks plays an indispensable role in determination of deformability, relative resistance change and fatigue life [117]. We expect that local strain of the metal fibres in the knitted FCB is mitigated through the loop configuration in the knitted fabric composed of compliant and smooth multifilament yarns, which represent a very soft dielectric element to support the metal fibres at their contact regions. To reveal the underlying mechanism, we analysed the three-dimensionally knitted loops, based on Leaf's model [118], assuming that the metal fibre is a thin elastic rod [119]. Such a knitted loop is shown in figure 6a, depicting that the initial straight metal fibre has deformation simultaneously in two planes at an angle to one another; the curvature k and torsion τ of the fibre axis are given by $k=\sqrt{x^{″2}+y^{″2}+z^{″2}}$ and

$$\tau =\left|\begin{array}{ccc}{x}^{\prime }& y^{\prime }& z^{\prime }\\ x^{″}& y^{″}& z^{″}\\ x^{‴}& y^{‴}& z^{‴}\end{array}\right|/k^2,$$

respectively, where x, y and z represents the coordinates of an arbitrary point on the metal fibre, and the dashes denote differentiation with respect to the arc length (figure 6b, where 0≤ϕ≤π/2, see the electronic supplementary material). The derived geometry from Leaf's model coincides with SEM observations in the initial state (figure 1), i.e. the metal fibre is primarily bent (at the circular portion) and secondarily twisted (at the straight segment) into a three-dimensionally looped configuration. Hence, induced bending (ε_b) and shear (γ_s) strain of the metal fibre at the loop formation is ε_bmax=d_fibrek/2 and γ_s max=d_fibreτ/2, respectively, where d_fibre is the fibre diameter. The local strain of the metal fibre, represented by equivalent Von Mises strain, could then be obtained from ${\epsilon }_{\mathrm{eq}}=2/3\sqrt{3/2{\epsilon }_{\mathrm{b}\hspace{0.17em}\max }^2+3/4{\gamma }_{\mathrm{s}\hspace{0.17em}\max }^2}$. Figure 6c,d plots bending and shear strain of the metal fibre in the three-dimensionally looped configuration, such that (i) peak bending strain occurs at the crest and trough of the loop, corresponding to maximum curvature; (ii) peak shear strain is induced from the circular portion to the straight segment of the loop, corresponding to maximum torsion; (ii) both fibre diameter and loop length affect local strain of the metal fibre in the loop. Thus, it is of great importance to determine the unit loop length, before incorporation of a metal fibre with a predefined diameter into a knitted loop, in order to achieve satisfactory mechanical and electrical integrity (electronic supplementary material, figure S3b).

Figure 6.

Geometrical configuration and fibre strain at the loop formation. (a) Three-dimensionally looped configuration. (b) Corresponding curvature and torsion. (c) Fibre bending strain plotted against curvature and fibre diameter. (d) Fibre shear strain plotted against torsion and fibre diameter. (Online version in colour.)

To derive the peak strain of the metal fibres when the knitted FCB was subjected to an applied strain, three-dimensional finite-element simulation was conducted (electronic supplementary material, figure S7). Peak strain of both free-standing and interlaced metal fibres is plotted in figure 7a, with applied FCB elongation in the transverse and longitudinal directions, respectively. Within the simulation range, the peak strain of free-standing and interlaced metal fibre loops are the same until FCB elongation reaches a threshold value of 21% in the longitudinal tensile test, implying that the metal fibres are free to deform in the FCB. This threshold value is much lower, i.e. 12% in the transverse tensile test. The physical meaning of this value can be interpreted as ‘when the metal fibre is transferred from a free-standing looped state to an interlaced state with a filament yarn, its deformation is limited by contact friction and pressure’. Moreover, peak strain of the metal fibre remained the same as before the threshold values, indicating that a loose-knitted structure mitigates peak strain of the rigid metal fibre in the soft- and porous-knitted fabric, therefore enhancing mechanical and electrical integrity of the FCB. Figure 7b–e depicts the influence of three factors, i.e. unit loop length, ratio of elastic moduli as well as ratio of diameters, on the peak strain of the metal fibre interlaced with a filament yarn. The peak strain of the metal fibre can be further reduced through ways of increasing its unit loop length (figure 7b), lowering Young's modulus of the interlacing filament yarn (E_yarn/E_metal≈0.0001; figure 7c) as well as reduction in the diameter of the filament yarn (diameter_yarn/diameter_metal≈0.5, figure 7d–e) in both transverse and longitudinal directions. The results of numerical simulation are consistent with experimental observations on geometrical changes of the metal fibres in the knitted FCB (electronic supplementary material, figure S7).

Figure 7.

Peak strain of metal fibre when the knitted FCB was elongated. (a) Peak strain of the metal fibre from a free-standing state to an interlaced state. (b) Effects of unit loop length. (c) Effects of ratio of Young's moduli. (d,e) Effects of ratio of fibre diameters in the transverse and longitudinal directions, respectively. (Online version in colour.)

3. Application

To the best of our knowledge, the performance of the present knitted FCBs has far exceeded those of previously reported metal-coated elastomeric films or other organic materials in terms of changes in electrical resistance, three-dimensional deformability, fatigue life and washing capability as well as permeability [120,121]. Those distinctive features make the knitted FCBs particularly suitable for next-to-skin electronic devices. As a demonstration, we have fabricated a flexible and stretchable fabric sensing network as a knitted FCB assembly to be used for in situ measurement of strain and deformation of a smart bulletproof vest.

In the knitted FCB assembly, discrete strain sensor elements (Softceptor, from AdvanPro Limited, Hong Kong) on different locations were mechanically and electrically connected by conductive tracks of the FCB. The resistive sensor element (initial resistance: approx. 6.68 (±0.71) kΩ with a dimension of 5×10×0.5 mm) with two soft polymeric electrodes could measure strain up to 60% with different strain rates (from 0.05 to 1000 s⁻¹) [122]. Figure 8a presents a fabrication sequence for the fabric sensing network, composed of eight sensor elements in an exampled ‘eight diagram’ pattern, which was designed for in situ measurement of strain during ballistic impact, therefore opening up potential applications as smart protective apparel. The first step involves knitting an intarsia pattern on the computerized knitting machine (electronic supplementary material, figure S4b) to define wiring layout of the FCB corresponding to predetermined locations of the eight independent sensor elements. The minimum distance between two adjacent conductive ‘tracks’ in the FCB could reach approximately 1 mm and the largest ‘track’ length, in the transverse direction of the FCB, could reach approximately 2140 mm with the current computerized flat-bed knitting machine. The fabric dimensions were determined by experiments. The thickness was estimated by filament yarn diameter (approx. 800 μm), without the influence of extension and bending [123]. In the next step, eight sensor elements with two parallel soft electrodes (distance: 10 mm) were placed (adhered or sewn) on corresponding locations of the undeformed knitted FCB in a precision fashion, because the FCB was first fixed on a rigid platform. Third, the sensor electrodes went through the ‘vias’ (micropores of the knitted FCB) using a rigid needle (diameter: approx. 0.6 mm) and were physically linked by the looped metal fibres through novel ‘helical connections’ for transmission of electrical signals to outer circuits (figure 8b and electronic supplementary material, figure S8). Here, a helical geometry and subsequent semi-spherical compliant encapsulation, ‘helical connection’ for short, was for the first time proposed to enhance mechanical robustness [124] and prevent damage of the whole assembly as well as electrical shorting from surrounding environments [90] between the sensor electrodes and the metal fibre loops. The induced contact resistance was neglectable and constant ((R−R₀)/R₀×100(%)<0.5%) with applied strain in tensile and three-dimensional punch tests owing to geometrical adjustments of the helix in its in-plane radius, pitch and angle (figure 8c and electronic supplementary material, figure S9). The final step was to connect the other ends of the conductive tracks in the knitted FCB to outer circuits by means of bonding selected interposers to the metal fibre loops with a compliant silicone adhesive and then stiffening the connected region with a thin layer of woven fabric with a maximum mechanical strain of less than 20%. After fabrication, the electrical resistance of one sensor element was monitored when it was unidirectionally stretched. The two curves in the electronic supplementary material, figure S10, represent the relationships between the relative change in electrical resistance and strain of the sensor element before and after fabrication, exactly overlapped with each other, demonstrating that the electromechanical behaviour of the sensor element was not affected by the fabrication process, the knitted fabric (with a modulus: less than 1 MPa), as well as looped metal fibres and helical connections. The described FCB fabrication technology is well compatible with the existing ‘island-bridge’ approach [90,125] to fabrication methods of all the flexible and stretchable devices.

Figure 8.

Fabrication and application of a fabric sensing network integrated into a bulletproof vest. (a) Fabrication procedures of the fabric sensing network integrated into a bulletproof vest. (b) Helical connection from the sensor electrode to conductive track of the knitted FCB. (c) Relative resistance change of the helical connection as functions of tensile and punching strains. (d) Fabric sensing network integrated into kevlar fabrics for in situ ballistic impact measurement. (e) Relative resistance change of the sensor elements measured during a ballistic impact. (Online version in colour.)

The packaged fabric sensing network was then inserted between multi-plies of energy absorbing fabrics (Kevlar or UHMWPE) for smart bulletproof vests (figure 8a). To test the ballistic performance of the vest, the multi-plies with fabric sensing network was placed on a soft foundation material (clay or gelatin). Then, a bullet was fired with various speeds at the centre of the sensing network and in situ strain measurement by the sensor network was conducted during ballistic impact (figure 8d and the electronic supplementary material, table S2). The indentation cave produced on the clay had a diameter of 51(±9.1) mm and depth of 18(±6.8) mm, implying that the fabric sensing network was three-dimensionally deformed with a large strain during the impact. Figure 8e shows that the sensors closer to the impact point underwent larger strains with earlier onset of resistance change as a result of elastic strain wave propagated from the impact point to surrounding regions, and the kinetic energy of the bullet was dissipated by the energy absorbing fabrics in the propagation process. Hence, the bullet was stopped. The knitted FCBs have shown excellent reliability during the ballistic impact tests as all 35 sensor network assembly samples performed well without mechanical and electrical failure and received reliable electrical signals. The details of this work will be reported separately in the near future.

4. Conclusion

In this paper, we have reported the design, fabrication and characterization of three-dimensionally deformable, durable and washable knitted FCBs made by interlacing coated metal fibres with elastic multi-filament yarns using computerized knitting technologies. The resultant knitted FCBs exhibit outstanding electrical stability with a large strain (less than 1% relative resistance change within 300% tensile strain), extraordinary fatigue life (greater than 1 000 000 cycles at 20% maximum strain) and satisfactory washing capability (30 times with ‘delicates’ washing cycle). In combination, the knitted FCBs offer the best performance for wearable electronic applications when compared with previously reported elastomer films. Theoretical analysis and numerical simulation show that structural conversion of knitted fabrics is attributed to the effective mitigation of strain of the metal fibres. The knitted FCB structures permit easy adjustment of their geometrical configurations of the metal fibre loops (i.e. period and amplitude); in addition, they permit free slippage and transfer of the loops that are subjected to very low friction and compression from compliant and smooth interlacing multifilament yarns. We have further demonstrated the feasibility of application of the knitted FCBs in smart protective apparel for in situ measurement during ballistic impact.

Knitted FCBs have great potential in a wide range of applications, such as human/electronic interfaces, skin-mounted systems, as well as next-to-skin monitoring systems for healthcare, because the knitted FCB possesses high three-dimensional deformability, low tensile, shear and bending rigidities, and excellent breathability and durability for long-term use. In addition, the knitted FCB technologies have some similarity to and advantages over main-stream flexible printed circuit boards in terms of the computer-aided structural design and green cost–effect manufacture. First, the knitted FCBs allow patterning electronic building blocks on either the front side or back side or on multi-layers owing to ‘vias’ in the form of inherent micropores, which enable conductive fibres or leads to pass through the FCB, thereby providing more diversity, such as three-dimensional heterogeneous schemes in the placement of sophisticated electronic components. Second, the full computer-integrated manufacturing capacity directly facilitates mass production of medium- to large-area knitted FCBs with precise geometrical layouts with a spatial resolution of approximately 1 mm in a reliable and inexpensive manner. All operations are at ambient dry conditions, no chemicals are used or discharged in the manufacturing processes at all. Therefore, the combination of outstanding performance of the knitted FCBs and well-established manufacturing technologies with CAD and SIM will lead to a bright future in the development of wearable electronics.

5. Experimental set-up

(a). Structure of fabric circuit board

A variety of knitted fabrics, composed of interlaced yarns in a series of connected loops, has been considered for FCBs because they can be produced with modern precision machinery that possess full computer-aided design and integration manufacturing capacity. A single jersey-knitted structure was chosen for FCBs in this study owing to its superior resilience and relatively isotropic extensibility in transverse and longitudinal directions as well as acceptable thickness (approx. 2d, where d is the yarn diameter) as inner garments for comfort compared with other knitted structures, such as rib, interlock and purl as well as warp knitted ones (electronic supplementary material, figure S2) [126].

(b). Materials for conductive tracks

Three types of conductive fibres have been investigated for previous textile-based electrical wirings, i.e. pure metal fibres, intrinsically conductive polymers and metal-plated fibres [30,127]. Commercial copper continuous fibres of core diameter of 50 μm, coated by polyurethane with a thickness of 3 μm, were obtained from Shanghai Gold Fine Enameled Wire Co., Ltd, China. The fine metal fibres were chosen as conductive tracks for knitted FCBs owing to their superior conductivity (the resistivity of copper fibre with 20 μm core diameter is approx. 0.55 Ω cm⁻¹) and current carrying capacity of 15 mA, satisfying standard IPC-9252A: the requirements for electrical testing of unpopulated printed boards.

(c). Dielectric supporting materials

We selected textured multi-filament yarns made from polyamide fibres by considering the following: (i) soft and flexible textile materials used for intimate apparel; (ii) good dielectric constant and little moisture regain; (iii) appropriate thermal and chemical stability; (iv) smooth surface that enables less friction allowing the metal fibres to have more freedom to slide and transfer among the loops, hence accommodating a large level of mechanical FCB strain; and (v) low materials and manufacturing costs (electronic supplementary material, table S1) [128]. We also selected spandex filament yarns because of their super extensibility and elasticity (up to 400–800% strain [129] and recovery of almost 100% from 400% strain) attributed to hard and soft segments in its molecular structure [126]. The spandex filament yarns are incorporated with a pre-tension of approximately 1 cN (corresponding pre-strain of 150%) into the loop structure to make the knitted FCBs more stretchable and capable of returning to their original dimensions, hence imparting better fitting [126,129]. Textured multi-filament yarns, i.e. a highly elastic spandex filament yarn in the core (Dupont Lycra, 20dtex/2f) and polyamide multifilaments wrapped around the core (70dtex/24f), were obtained from Sun Hing, Hong Kong (electronic supplementary material, figure S3c).

(d). Fabrication methods

A circuit diagram was first designed with consideration of all electronic components and data ports. The working frequency and current carry capacity were also taken into account. Different from the weaving technology which limits the wiring layout in transverse (welf) and longitudinal (warp) directions, it is feasible to ‘knit’-stretchable FCBs with a complicated geometrical layout such as array, matrix, network and any arbitrary configurations with computerized knitting technology (electronic supplementary material, figure S4a,b). The intarsia pattern in single jersey-knitted fabrics was selected for the FCB when the stitch density and dimensions were decided by experiments beforehand. An intarsia knitting chart was constructed based on a computer program which was written to drive a computerized flat-bed knitting machine (STOLL CMS822, gauge: 14). Two copper fibres were fed into the knitting machine with a minimum tension together with the filament yarns at tension of approximately 1 cN, corresponding to approximately 150% pre-strain.

(e). Characterization

Optical microscopy observation was performed on a Leica M165C (DFC 290HD, Leica Microsystems Ltd., Hong Kong). SEM observation of the FCB samples from 0% to 60%, and to 120% strain in the transverse and longitudinal directions was conducted on a field emission scanning electron microscope (JEOL JSM-6335F, JEOL Ltd., Japan). The relative resistance change of the knitted FCBs with applied strain was investigated under different deformation modes. The unidirectional tensile tests were performed on an Instron Universal Material Tester, where the sample (2.5×7 cm) was fixed to the top and bottom clamps with gauge length of 5 cm. The tensile speed was 300 mm min⁻¹. Ball punching measurement was conducted on an Instron Universal Material Tester with a ball–burst attachment, consisting of a ring clamp (diameter: approx. 44.45 mm) to hold the test specimen with the metal fibre lines in the centre and a polished stainless steel ball (diameter: approx. 25.4 mm) attached to the movable member of the tensile tester. The ball was punched into the FCBs with a cross-head speed of 300 mm min⁻¹. The conductive tracks in the knitted FCB were connected to a high-precision digital multimeter (Keithley 2010, Keithley Instruments Inc. USA) interfaced with a personal computer. The electrical resistance was monitored when the samples were stretched unidirectionally and punched three-dimensionally to failure either electrically or mechanically. For the fatigue test, each cycle corresponded to the deformation to a certain level of maximum strain (i.e. 20%, 60% and 80%) and then returned to the original state with a tensile speed of 500 mm min⁻¹.

(f). Washability

The washing test was conducted on a vertical-axis washing machine (Whirlpool, USA) referring to AATCC standard 135: dimensional changes of fabrics after home laundering. The knitted FCBs (length: approx. 15 cm) together with ballast (total load: 1.8 kg), with a commercial non-ionic detergent (Castle super concentrate with double lemon fragrance, 66 g), was washed at approximately 40°C with ‘normal’ (agitation speed: from 179 to 119 spm; spin speed: 645 rpm, rpm=revolutions per minute) or ‘delicates’ (agitation speed: 119 spm; spin speed: 430 rpm) cycles and the knitted FCBs were then dried under 75°C with ‘automatic dry’ cycle. The whole procedure was conducted 30 consecutive times with four groups: (i) original FCB samples with ‘normal’ cycle; (ii) FCB samples in a mesh bag with ‘normal’ cycle; (iii) original FCB samples with ‘delicates’ cycle; and (iv) FCB samples in a mesh bag with ‘delicates’ cycle. The electrical resistance was monitored after each drying treatment during the cyclic washing tests.

(g). Finite-element simulations

The simulation was conducted by using the structural mechanics module of ANSYS 13.0. Half of a unit cell was investigated, owing to geometrical symmetry, consisting of a three-dimensionally looped metal fibre (element type: Beam188, diameter: 20 μm; elastic modulus: 115 GPa, Poisson's ratio: 0.32) interlaced with a filament yarn (element type: beam 188, diameter: 200 μm, elastic modulus: 2.8 GPa, Poisson's ratio: 0.37), with applied FCB elongation in the transverse and longitudinal directions, respectively. The contact model was built up by a three-dimensional line to line contact (electronic supplementary material, figure S7). Loops in free standing status and in contact mode with interlacing yarns were considered when applying unidirectional elongations in the transverse and longitudinal directions of the knitted FCB.

Funding statement

The authors acknowledge funding support from Research Grants Council of the Hong Kong SAR Government, National Science Foundation of China (grant no. PolyU503/12, PolyU5251/13, PolyU152152/14E), Hong Kong Research Institute of Textile and Apparel (grant no. ITP/007/10TP) for this research; and Nanjing University of Science and Technology for providing technical support for the ballistic experiments.