Influence of Knitting and Material Parameters on the Quality and Reliability of Knitted Conductor Tracks [original]

Citation: Rotzler, S.; Malzahn, J.;

Werft, L.; von Krshiwoblozki, M.;

Eppinger, E. Influence of Knitting

and Material Parameters on the

Quality and Reliability of Knitted

Conductor Tracks. Textiles 2022,2,

524–545. https://doi.org/10.3390/

textiles2040030

Academic Editors: Larisa A.

Tsarkova, Thomas Bahners and

Xiaomin Zhu

Received: 24 August 2022

Accepted: 29 September 2022

Published: 5 October 2022

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4.0/).

Article

Influence of Knitting and Material Parameters on the Quality

and Reliability of Knitted Conductor Tracks

Sigrid Rotzler 1,2,* , Jan Malzahn 1,3 , Lukas Werft 1, Malte von Krshiwoblozki 1and Elisabeth Eppinger 3,*

Fraunhofer Institute for Reliability and Microintegration IZM, Gustav-Meyer-Allee 25, 13355 Berlin, Germany

Microperipheric Research Center, Technical University Berlin, Gustav-Meyer-Allee 25, 13355 Berlin, Germany

3School of Culture and Design, University of Applied Sciences for Technology and Economics Berlin,

Wilhelminenhofstraße 75A, 12459 Berlin, Germany

*Correspondence: [email protected].de (S.R.); [email protected] (E.E.)

Abstract:

Many electronic textile (e-textile) applications require a stretchable basis, best achieved

through knitted textiles. Ideally, conductive structures can be directly integrated during the knitting

process. This study evaluates the influence of several knitting and material parameters on the

resistance of knitted conductive tracks after the knitting process and after durability testing. The

knitting speed proves to be of little influence, while the type of conductive thread used, as well as

the knitting pattern both impact the resistance of the knitted threads and their subsequent reliability

considerably. The presented research provides novel insights into the knitting process for conductive

yarns and possible applications and shows that choosing suitable material and processing methods

can improve the quality and robustness of knitted e-textiles.

Keywords: e-textiles; knitting; reliability; smart textiles; conductive textiles; knitted conductors

1. Introduction

Electronic textiles (e-textiles) represent a growing group of hybrid textile products

characterized by integrated electronic and conductive components. Main application

areas for wearable e-textiles are sports, medical, personal protective equipment (PPE) and

therapeutical purposes with the aim of measuring body functions and activities of the

users [

]. Despite a wide range of application scenarios, challenges including comfort,

stability against mechanical wear occurring during use (such as cyclic stretching and

bending or abrasion), as well as washability have so far prevented e-textiles from reaching

a wider market [2].

E-textiles intended for body monitoring, where continuously gathered data are used

to improve training or therapy results, prevent strain or accidents, for diagnostics or to

regulate integrated actuators, need to be especially flexible, stretchable, comfortable and

washable [

]. The integrated sensor systems often need to be in direct contact to the user’s

skin and should not impact the movement and the breathability of the garment. Compared

to woven textiles, knits possess a much higher structural elasticity [

], making them ideal

substrates for such form-fitting e-textile applications [

]. Yet, integrating circuits into a

stretchable base—without impacting the stretchability of the fabric—is especially challeng-

ing. A promising approach to produce adequately elastic circuits is to knit them directly

into the textile during production. This way, the circuit paths will have a comparable

stretchability to the textile substrate. The advantage of this approach is a minimal impact

on textile properties, making the e-textile more comfortable for users. No additional inte-

gration resources and processing steps are necessary, and almost no waste of conductive

material occurs, making the knitting of conductive elements into a textile substrate fabric

both sustainable and cost effective. Apart from textile circuits, conductive yarns knitted

into textiles can also function as strain or touch sensors, electrodes, heating elements or

induction coils [6–8].

Textiles 2022,2, 524–545. https://doi.org/10.3390/textiles2040030 https://www.mdpi.com/journal/textiles

Textiles 2022,2525

During the knitting process and the subsequent use phase, the conductive yarns in

such knitted e-textiles will experience a range of mechanical stresses, as well as strain from

washing cycles. Research into the effects of knitting parameters on textile properties has

been done extensively for non-conductive knitted textiles. Gosh et al. investigated the

influence of yarn count and tension, speed and loop length on the comfort of jersey and

rib knits [

]. Research by Ramachandran et al. shows that thermal properties of knits

is dependent on the yarn type and different knitting parameters [

], while Nazir et al.

show that tighter knitted interlock fabrics and smaller stitch lengths lead to better moisture

management properties [

]. Other research looks into the influence of processing and

material parameters of jersey textiles [12], just to name a few examples.

Yet, there is less insight for conductive yarns, as e-textile research and product develop-

ment thus far was focused predominantly on creating circuits through embroidery, printing

or laminating. Euler et al. investigated how different factors affect the contact impedance of

knitted electrodes. Electrode size and shape, knit construction and the yarn density of the

conductive yarn were varied, with size and construction showing the biggest influence [

The research of Atalay and Kennon shows that when knitting strain sensors, the input

tension for the conductive yarns, but also the properties of the non-conductive base yarn

used, will influence sensing capability. The number of parallel conductive lines also affects

the sensor characteristics, but to a lesser extent [

]. The type of yarn and the density of

conductive lines influences the suitability of the resulting fabric for heating application,

as Repon et al. show [

]. Ullah et al. researched the suitability of different conductive

yarns and knit structures for knitted pressure sensors, but did not evaluate the effect that

different knitting parameters have on the integrity of the yarns and their resistance [

Overall, findings into the influence of knitting and material parameters when knitting with

conductive yarns are limited, especially in combination with reliability considerations.

This research aims at bridging this gap in insight by evaluating how knitting parame-

ters and different conductive yarns influence the resistance of knitted circuits during the

production of the textiles and after a series of simulated wear scenarios. A higher knitting

speed is suspected to lead to a higher mechanical strain during the knitting process, as

other studies have shown a pronounced influence of processing speed when using em-

broidery to create textile circuits [

]. The second assumption tested in this research is

that the knit structure influences the resistance of the finished knitted circuit, both due to

different amounts of mechanical strain during the production process as well as a different

amount of contact points in the conductive thread within the textile. The conductive yarn

is also suspected to influence the quality of a knitted circuit, as different yarns will behave

differently during production and the use phase. To simulate the use phase, the knitted

textiles with integrated conductive tracks were subjected to cyclic stretch testing, wash

cycles and a tensile test.

2. Materials and Methods

2.1. Main Parameter Test

The parameters knitting speed, conductive yarn type and knit construction type were

analyzed to determine their effects on the resistance of knitted textile conductors directly

after the knitting process.

Conductive yarns: The tested yarns were chosen from commercially available con-

ductive yarns marketed as suitable for knitting. Four different silver (Ag) coated, nylon

(PA) based materials were chosen from two suppliers: Elitex (ELITEX) (see Figure 1a,e)

from IMBUT and Shieldex 33/10 (SHLD33) (see Figure 1b,f), Shieldex 44/10 (SHLD44)

(see Figure 1c,g) and Madeira HC40 (MDR40) (see Figure 1d,h) from Statex. The Shieldex

yarns SHLD33 and SHLD44 are comparable in all aspects except the yarn count. Both

Shieldex yarns are too thin to be knitted as a single thread, so they were tripled during

specimen production by leading the threads of three cones into a single yarn feeder of the

Kniterate machine. To assess if twined yarn shows better results for knitted conductors

than doubling or tripling 1-ply yarn, MDR40—a twine made of Ag-coated PA, also manu-

Textiles 2022,2526

factured by Shieldex (but marketed as an embroidery thread)—is tested as well. Compared

to the Shieldex materials, ELITEX yarn from IMBUT has a thicker silver coating due to an

additional, second coating step.

Figure 1.

Scanning electron microscopy (SEM) and cross-section images of the conductive and

non-conductive yarns prior to the knitting process. Yarns: (

) ELITEX, (

) SHLD33, (

) SHLD44,

(

) MDR40, (

) STEEL, (

) PA base yarn; Cross sections: (

) ELITEX, (

) SHLD33, (

) SHLD44,

(h) MDR40, (j) STEEL.

Silver coated yarns are widely used in knitted, woven or embroidered e-textiles due

to their textile-compatible properties, but often show unsatisfactory reliability results [

To compare a differently constructed yarn to the Ag-coated PA yarns, Steel-tech from

supplier Amann was chosen as the fifth material, a yarn consisting of solid stainless steel

wires twined with PA filaments (STEEL) (see Figure 1i,j). Table 1gives an overview of the

5 different materials

used for the test as well as the PA yarn used as a non-conductive base

yarn for all test specimen (see Figure 1k). The electrical properties of the conductive yarns

are also provided in the table.

Textiles 2022,2527

Table 1. Properties of utilized yarns.

Yarn &

Manufacturer ID Construction

Yarn

Count *

[dtex]

Material

Ag-Layer

Thickness

[µm]

Linear

Resistance

[Ω/m]

Non-conductive base yarn

PA (OTEX) PA 184 filaments Z-yarn 356.4 ±15 PA – –

Conductive yarns

Elitex (Inbut) ELITEX 17 filaments 2ply

Z-twine 400 ±20

PA with Ag

(>99%)

coating

ca.

1 */0.43–2.08

#<50 *

Shieldex® 33/10

(Statex) SHLD33 10 filaments Z-yarn 40/10 ±2

PA with Ag

(99.9%)

coating

0.43 #<4000 */650 #+

Shieldex® 44/10

(Statex) SHLD44 10 filaments Z-yarn 54/10 ±4

PA with Ag

(99.9%)

coating

0.5 #<4000 */381 #+

Madeira HC40

(Statex) MDR40 multifilament 2ply

twine 290 ±6

PA with Ag

(99.9%)

coating

0.24–0.45 #<300 *

Steel-Tech (Amann) STEEL

8 steel filaments,

polymer

multi-filament core

930 stainless steel

PA core – 90 *

* manufacturers’ data, #own measurement, +tripled yarn.

Knitting speed: One of the assumptions motivating the presented research is that

higher knitting speeds lead to increased mechanical strain (and subsequently to higher

damages) on the yarn. The Kniterate machine used to produce the specimen is capable

of knitting speeds up to 800 mm/s [

], which was the speed chosen as the maximum

testing speed. For industrial production, the lowest possible knitting speed of 10 mm/s

is much too slow and thus not feasible, so the lowest testing speed is set slightly higher

at 100 mm/s. The default setting for the knitting speed when creating new designs with

the Kniterate software is 300 mm/s [

], which is why two more testing speeds were set at

250 mm/s and 400 mm/s, respectively, slightly below and above this default value. The

knitting speed for the non-conductive rows knitted from the black PA yarn is kept at a

constant 400 mm/s.

Knit structure: In knitting, a distinction is made between four basic weft knits: single

jersey (SJ), rib, interlock (IL) and purl (PU) [

]. Since all structures are formed differently,

it is likely that the mechanical stress on the yarn also differs during their construction.

Single jersey (SJ): SJ consists of right loops that are knitted using only one needle

bed [

]. As is illustrated in Figure 2a, SJ shows stitch legs and thus knit stitches on the

face side and stitch heads and stitch feet and therefore purl stitches on the reverse side [

Single jersey has moderate stretchability (10–20%) in the longitudinal direction and high

stretchability (30–50%) in the transverse direction [

]. SJ has the densest stitch structure

of all four basic knitting types, since only one needle bed is used in production and no

additional yarn length is generated during transfer between the needle beds, which is why

the contact points of the conductive yarns are also the densest. Due to this property, the

path of the lowest electrical resistance through the knitted fabric should be the shortest

compared to the other three basic weft knits. Therefore, the electrical conductivity should

be the highest, which makes SJ a relevant structure for e-textiles.

Textiles 2022,2528

(a)SJ front & back (b)RIB front & back

Front Back

(c)IL front & back

Front Back

(d)PU front & back

Figure 2. Overview of knit structures, own representation based on [21].

Rib: The simplest version of the rib structure is the 1:1-rib (RIB) [

]. In the RIB

structure, alternating wales of knit stitches on the front bed and knit stitches on the back

bed (purl stitches on the right side of the fabric) are produced [

]. RIB has moderate

stretchability (10–20%) in the longitudinal direction and very high stretchability (50–100%)

in the transverse direction [

]. Due to the particularly high transverse elasticity and the

high volume, RIB is suitable for use in strain sensors and pressure sensors and is therefore

an interesting structure for e-textiles [

]. Both sides of the RIB show knit stitches, as can

be seen in Figure 2b [4].

Interlock (IL): While some sources list IL as distinct type of basic weft-knit [

], other

sources only include it as a sub-type of rib knit [

]. Since IL has a dimensionally stable

structure due to its manufacturing method and is therefore interesting for the research

and development of electronic circuit paths, in which few unwanted changes in resistance

should occur due to changes in strain, it is treated and examined in this work as a basic

type of knitting. Each row of IL requires two rows of stitches knit on separate alternating

needles, creating two rows of half-gauge ribbing whose sinker loops cross over. Thus, odd

feeders alternately knit wales on each side and even feeders knit the other wales [

]. IL has

a moderate stretchability both lengthwise and crosswise [

]. IL has the same appearance on

both sides, but its surface cannot be stretched to reveal the wales of the underlying stitch

because the wales on each side are diametrically opposed and connected [

]. This means

that knit stitches are visible on both sides of the fabric (Figure 2c) [4].

Purl (PU): PU is the fourth basic weft knitting structure. Both sides show purl stitches,

as depicted in Figure 2d. During its production, a stitch transfer between the beds is carried

out after each row of stitches [

]. Knitting PU samples on the Kniterate machine caused

persistent problems and defects. For this reason, the PU structure was excluded from

this research.

Table 2gives an overview of all the parameters varied in the main parameter tests

and the tested values. All possible combinations of knitting speed, knit structure and

conductive yarns were tested. After the resistance measurement, the conductive tracks

were evaluated for damages and defects using SEM microscopy.

Table 2. Varied parameters for the parameter test.

Parameter Tested Values

Knitting speed [mm/s] 100, 250, 400, 800

Knit structure SJ, RIB, IL

Conductive yarns ELITEX, SHLD33, SHLD44, MDR40, STEEL

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