Aachen Technology Overview of 3D Textile Materials and Recent Innovation and Applications

3.1

3D Large Circular Knitting

The ITA 3D-Knit Technology was developed by Dr. Kristina Fuhrmann at the Institut für Textiltechnik of RWTH Aachen University (ITA). The basic principle was presented at the 8th World Conference on 3D Fabrics and their Applications in Manchester, UK on the 28-29th of March 2017. The ITA 3D-Knit principle enables the production of 3D shaped knitted fabrics while still using the high productivity of the continuous needle movement of large circular knitting machines. There are no machine modifications needed and the productivity of large circular knitting machines can be enhanced with the new technology. As it is generally the case with 3D knitting, the prerequisite for this application is individual needle control, which means that a jacquard machine is required [7, 15,16,17,18].

To enable the production of 3D-knitted fabrics on large circular knitting machines, a new knitting pattern was developed. With this pattern, the implementation of a reduction of the surface, which is typically made by confection during the cut & sew process, is enabled. This “sewing-like” knitting pattern (or darts) can be integrated into the knitted fabric to achieve the desired three-dimensional form. The knitting pattern consists of floats and stitches that alternate horizontally over the area to be reduced. Due to the continuous stitch wales of floats, the corresponding needles are not moved in this area and thus hold the knitted fabric in position. The other needles continue to form loops, but produce these on the backside of the knitted fabric. In this way, the continuous movement of the circular knitting machines can be maintained and the effective surface area can be reduced [15,16,17,18].

Although the ITA 3D-Knit Technology enables the production of 3D knitted fabrics, it is not possible to achieve the same high flexibility of flat knitting machines. It is still necessary to cut the fabric out of the knitted tube and the reduced fabric remains on the backside, and therefore does not reduce the amount of material used [15,16,17,18].

3.2

3D Braiding in Medical Applications

Current applications of the 3D hexagonal braiding technique include synthetic ligaments as well as complex stent structures. Synthetic ligaments show deficits regarding mechanical long-term performance. Therefore, the current gold standard are autologous grafts. These have limited availability and increase the risk of donor-site morbidity. By using degradable polymeric yarns tissue engineering scaffolds, tendon regeneration and augmentation can be achieved. 3D braids offer improved mechanical properties i.e. maximal tensile force and stiffness, due to the interconnection of the different braid layers. The dense packing of the machine bed enables a particularly gentle fiber processing of degradable and fine polymer materials. The fibers can serve as a guide structure for cells due to three-dimensional deposition.

Tumors in bifurcations in e.g. the respiratory tract have to be treated extensively, for example by double stenting. Self-expanding bifurcation stents made of Nitinol allow a simplified treatment. By 3D hexagonal braiding these stents can be manufactured in one process step. The braiding angle, number of filaments and bifurcation angle can be adjusted individually. By switching between different braid geometries e.g. round braid and flat braid, holes can be introduced into the structure to accommodate the patient's individual anatomy. Further applications can be occlusion devices and blood filters for the venous system (Fig. 13).

Examples of 3D Hexagonal Braiding; (a) bifurcated Nitinol stent, (b) synthetic ligament [4]

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3.3

3D Warp Knitting in Medical Applications

Warp knitting technology is particularly suitable for the production of 3-dimensional structures for use in medicine. Mechanical and geometrical properties can be precisely adjusted via material selection, combination of different materials or yarn counts and by different process parameters.

The current research at the Institut für Textiltechnik of RWTH Aachen University aims at avoiding compliance mismatches between the artery and the graft (Fig. 14). A compliance mismatch is a difference in the stress–strain behavior of the artery and the graft. Especially in small caliber vascular grafts the compliance mismatch leads to insufficient patency rates. The current research aims for the development of a textile-based vascular graft with physiological compliance properties. To achieve this goal, a biomimetic approach is chosen: by combining elastic and non-elastic fibers in a tubular warp-knitted structure, a combination of material elasticity and structural elasticity in grafts is aspired.

Synthetic vascular graft with reduced compliance mismatch [4]

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Furthermore, warp knitting can be used as a platform technology for the production of textile reinforcement structures for hydrogels in tissue engineering applications. Due to their high-water content and their cell adhesion motifs, hydrogels offer a good biological environment for cell growth. Additionally, the structure of hydrogels resembles the structure of the extracellular matrix in the human body. However, hydrogels show poor mechanical properties. Warp knitted spacer textiles can be used to compensate for these disadvantages. The pore size and porosity as well as the mechanical properties of warp knitted spacer fabrics are well adjustable. In contrast to the hydrogels, however, they do not create a favorable biological environment for the cells. By filling the spacer fabrics with a hydrogel, the advantages of both materials can be combined resulting in a hybrid scaffold with favorable biological properties, as well as adjustable mechanical properties. By adjusting the mechanical properties of the hybrid scaffold, a variety of tissue engineering applications for different human body tissues can be addressed.

3.4

Patient Individualized Implants Using 3D Fabrics

Clinically established implants are usually off the shelf products available in certain standard sizes. Medical-technological progress, especially in the field of medical imaging and emerging production technologies as e.g. textile technologies have recently enabled the realization of custom-made solutions. The challenge is to translate the medical image data from CT or MRI scans into the patient specific implant design. This design may include geometrical as well as functional individualization parameters. This process requires the construction and development of robust and highly flexible parameter-based CAD models. In a second step, these CAD models need to be translated into a textile machine code, to enable a fast and effective production of the implants. The decisive factor in choosing a suitable textile technology is the digital interface on the machine side. Textile technologies which allow for a high adaptability and variability in the producible structures are predesignated for such a task. Examples are narrow weaving, flat bed weft knitting and warp knitting, as these technologies are capable of producing tubular fabrics and three-dimensional fabrics. It is the holistic approach to combine the prediction of textile behavior and the extraction of geometrical design parameters from the medical image data into robust models that leads to the vision of cost-efficient and fast delivery of patient individualized implants.

This is the aim of PerAGraft, a spin-off of the Institut für Textiltechnik of RWTH Aachen University. The team develops patient-individualized textile implants for cardiovascular applications. The implementation of an end-to-end digital process chain is at the core of its innovation to produce textile-based patient specific implants.

3.5

Tubular Weaving in Medical Applications

The most popular 3D woven textile for medical applications is the tubular woven conduit. Woven conduits are preferably used for vascular implants. Due to their adjustable pore size and porosity, which can achieve very small values, they can be woven as water- and blood-tight structures without additional coatings. Furthermore, woven conduits show an excellent high tensile strength behavior in all directions which gives the implants a durable shelf life. However, they lack elastic behavior to give a necessary flexibility to the implants. Also, woven conduits have a low kink-resistance due to their comparatively stiff behavior. A way to counteract the disadvantages is to pleat the weaves. This accordion-like shaping of the fabric gives the woven conduit the necessary elastic and kink-resistant characteristic for the use as vascular grafts. In case of stent grafts, metal stent structures are sewn on the woven conduits to keep the hollow lumen open while bending.

At the Institut für Textiltechnik of RWTH Aachen University, a currently ongoing research project broaches the issue of ultra-low-profile woven stent grafts for treating abdominal aortic aneurysms in patients with strongly angulated and narrowed access vessels. The aim of the project is to reduce the folded profile of the woven stent graft by using thin-walled woven conduits. Hence, a better access through the complicated femoral vessels can be enabled. For this purpose, ultra-fine medical-grade multifilament yarns (≤ 20 dtex) are used for weaving the thin-walled woven conduit. To guarantee the impermeability against blood leakage, an extremely tight woven structure with high yarn densities is required. This requirement leads to several difficulties such as yarn clamping during the shedding and yarn breakage in the weaving process which this project is going to address.

3.6

Testing Equipment and Methods for Spacer Fabrics

Spacer fabrics are three-dimensional textile structures consisting of two textile cover surfaces and a spacer thread. The cover surfaces are kept apart at a distance by the spacer thread. This structure gives spacer fabrics special properties that make them a suitable substitute for other, non-textile materials. Applications can be found wherever increased air circulation, spacing between cover surfaces or a comfort effect due to the cushioning structure is desired. Due to their special properties, spacer fabrics differ from conventional, flat textiles. Up to now, spacer fabrics have been tested according to standards for flat textiles or other non-textile materials. However, these standards do not take into account the special requirements for testing spacer fabrics. Therefore, objective comparisons among spacer fabrics or between spacer fabrics and conventional flat textiles or non-textile materials cannot be made in all areas of application. In a publicly funded joint project, the Institut für Textiltechnik of RWTH University (ITA) has developed testing devices and methods in cooperation with industrial partners and has transposed them into standardization documents in cooperation with the German Institute for Standardization (DIN e.V.). The material properties taken into account are for maximum force, thickness, compression, mass per unit area, definition of terms and sample preparation. Within the new testing standards, the special properties of spacer fabrics are given special consideration. This enables standardized testing of these properties and thus objective comparisons not only between spacer fabrics themselves but also, with conventional flat textiles and with non-textile materials.

In order to enable standardized testing of spacer fabrics and thus objective comparisons of spacer fabrics with each other, with conventional flat textiles and non-textile materials, further research is needed. It will be necessary to develop and validate test equipment and procedures for the determination of further properties of spacer fabrics, e.g. compression hardness, compression set, tilt stability, pressure point distribution, abrasion test and maximum tensile strength. Furthermore, since these results are mostly developed to fit the requirements of warp knitted spacer fabrics, the research results should be evaluated regarding weft knitted and woven spacer textiles.

3.7

Multiaxial Fabrics for Higher Fatigue Strength of Composites

Glass fibre-reinforced plastics (GFRP) are an ideal material for leaf springs due to their very high energy absorption capacity. GFRP leaf springs are used in both machinery and automobiles [19]. The use of GFRP as a spring material imposes a dynamic load on the material, making fatigue strength a key design criterion [20]. A GFRP leaf spring consists of individual glass fibre layers. Most of the layers are oriented in the longitudinal direction of the spring (0°) and serve as load-bearing elements. However, for manufacturing reasons, also layers in the transverse direction of the spring (90°) are present in the laminate. The transition between the 0° and 90° glass fibre layers is one of the places in the laminate where fatigue fractures occur. The fatigue fractures appear as delamination between the 0° and 90° layers, which in turn can occur because the 0° and 90° layers are only connected in the thickness direction by the relatively weak polymer matrix.

Within the framework of ongoing research projects at ITA (Fig. 15), it is being investigated how fatigue fractures at the layer transition can be prevented using hybrid multiaxial fabrics. For this purpose, the previous individual layers in the 0° and 90° directions are combined in a multiaxial orthogonal fabric. Within one fabric layer, three fibre directions are combined: 0°, 90° and 0°. The orthogonal fabrics are manufactured in such a way that the glass fibres are almost free of undulation. They are connected by an additional thread system composed of aramid fibres. The use of a multiaxial fabrics enables a connection of 0° and 90° layers with load-bearing fibres in thickness direction, thus counteracting the occurrence of delamination.

However, fatigue fractures are also a result of the accumulation of previous micro damages. One of the critical damage mechanisms are fibre breaks. Therefore, additional investigations are being carried out to determine how the formation of fibre-break clusters can be prevented. The approach is to hybridise individual laminate layers: Glass fibre rovings are partially replaced by crack arresting fibres. The crack arresting fibres are characterized by a high elongation at break or a high fracture toughness, which prevents the formation of fibre fracture clusters. Polyamide-, UHMWPE-, Aramid- and Polyester- fibres are investigated for this purpose.

Prevention of delamination through multiaxial fabrics

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3.8

3D Warp Knitted Structures for Space Applications

Knitted metal meshes are a key component of Large Deployable Reflectors (LDR) for telecommunication antenna satellites. Both weft and warp knitting methods, are currently used to produced mesh reflector surfaces. In the project Space-R-eflector, a new innovative reflector surface is developed by Large Space Structures GmbH, LSS (concept) and ITA (production technology).

Metal mesh-based reflector surfaces are produced as weft or warp knitted fabrics. Currently, knitting machines with a gauge of E24 or higher are generally used to produce warp knitted metal mesh reflecting surfaces. Tungsten or molybdenum is used as yarn material. [21]. Warp knitted meshes have been previously developed at ITA for use in the surface of large deployable reflector antennae. Currently ITA is investigating warp knitted spacer fabric technology to develop reflector surfaces [22, 23]. This is the first time space fabrics have been considered for use in a reflector surface. The key benefit of the spacer fabric technology is the combination of drapeability (mesh surfaces) and bending stiffness (pile yarn). Thanks to these two initially contradictory capabilities, spacer fabrics are an innovative solution for applications not only in space but in other fields of research as well.

3.9

Spacer Warp-knitted Non-crimp Fabrics for Construction

3D spacer fabrics consisting of two textile cover surfaces kept apart by pile yarns are state of the art, e.g. in the field of upholstery. In the z-direction and using predominantly polymer fibers, they retain excellent compression force distribution over thousands of cycles. Their open, sandwich-like structure offers elasticity, insulating properties and has acoustic dampening as well as filtering potential. Technical applications are currently in the field of mattress pads, upholstery, filtration systems and water harvesting methods using so-called fog traps.

For construction applications, spacer warp-knitted non-crimp fabrics are introduced at the Institut für Textiltechnik of RWTH Aachen University (ITA) via a machine development, which can absorb occurring tensile forces in x- and y-direction, especially in concrete structural elements. The 3D-structure consist of two top surfaces made out of pillar (0°) threads, (90°) weft threads, and warp-knitting yarns, which fix the pillar and weft threads together. Pile threads from monofilaments connect both surfaces to each other (Fig. 16). Typical reinforcement materials used are glass, cellulose, aramid, polymer or basalt fibers. Double-needle bar Raschel machines are used for the production of spacer warp-knitted non-crimp fabrics. The warp-knitting elements of the machine are designed symmetrically.

Spacer warp-knitted non-crimp fabric structure and its use in concrete matrix

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The distance between the two top surfaces can be adjusted by changing the distance between the needle bars. Double Raschel machines consisting of two 90° weft thread transport systems supply the weft threads to the knitting elements. Special yarn insertion systems allow weft threads to be integrated into the structure in regular or non-regular patterns. Knitting yarns and 0° yarns are fed to the two knitting units of guide bars. With up to six computer-controlled guide bars currently available, the pattern possibilities offered by this technique are very extensive.

These 3D reinforcement structures have been successfully used as concrete reinforcement in recent years (e.g. from V. Fraas Solutions in Textile GmbH, Helmbrechts, Germany). 3D warp-knitted spacer fabrics offer the advantage that two layers of reinforcement can be integrated into one textile and can be designed variably. Thus, the degree of reinforcement can be freely adjusted via the size of the mesh. The high stiffness of the spacer yarn enables the reinforcement layers to be fixed to each other in a precise position, which is essential for reinforcing concrete. Even a slight deviation of a few millimeters in the reinforcement position leads to unreliable mechanical properties in a concrete component, e.g. a facade panel.

Further applications for 3D spacer fabrics in the construction sector are being researched in the current research project "6d-TEX", funded by the Bundesamt für Bauwesen und Raumordnung, Germany, at the Frankfurt University of Applied Sciences and at the ITA. By combining 3D spacer fabrics with 3D printing, convertible, functional lightweight construction elements are to be realized, which will find their use, for example, as a textile facade shell for shading and insulation.

3.10

FreePreg

Prepreg components with load-path optimization can currently only be manufactured to a limited extend. Local reinforcements are done by manual layer build-up, resulting in a lot of waste of impregnated semi-finished products. On the other hand, Tailored Fiber Placement (TFP) technology is a key to load-path optimized composite production. TFP is limited to small structures and further limited to dry fiber processing. Using TowPregs has proven impractical due to increased contamination of high-speed TFP embroidery machines.

The FreePreg approach (see Fig. 17) combines the strengths of both prepreg and TFP technologies. The innovation consists in the fact that the novel partially impregnated TFP structures can be used in combination with classic prepregs for the first time. Large-area component sections are built up from prepreg layers at high throughput. TFP reinforcement structures, so-called FreePregs, hence the name, already partially impregnated with epoxy resin, are then applied to the prepreg layers. Consolidation is carried out in the same way as in prepreg processing. This offers advantages over existing process chains. The new step is an impregnation of TFP structures on a roll to roll coating line, fully automated with high throughput.

Combination of process chains for the production of load-path optimized prepregs

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The FreePreg process therefore separates the impregnation from the embroidery process. By combining large-area prepregs and local TFP reinforcements, large components in particular can be produced with a single-sided mold, resulting in economic advantages over cost-intensive RTM molds. This enables savings in material, handling and manufacturing costs and allows TFP technology to be used in combination with prepreg processing. The implementation in a process chain for the economic production of high-performance fiber composite structures in small to medium series from batch size 1 to 5,000 units/year is targeted. This will enable a reduction in unit costs (up to €8/kg) and investment costs (up to €50,000 per series) compared to established processes. Furthermore, it opens up the possibility for established TFP embroideries to develop further market segments in the CFRP market (up to 8% of the total CFRP market). It also opens up applicability for TFP in prepreg processing, the most frequently used CFRP manufacturing process (approx. 45% of the worldwide production volume). Finally, one-sided partial impregnation with solvent-free systems makes it possible for out-of-autoclave-technology and high component quality.

3.11

Ceramic Matrix Composite 3D-Braiding

The integration of fiber reinforcement in ceramics can increase the mechanical properties compared to monolithic ceramics. However, the use of the 2D fiber-reinforced CMC material class remains limited to predominantly thermal applications due to their high shear susceptibility. For example, 2D fiber-reinforced CMCs exhibit a shear strength of 25 MPa, which corresponds to only 6% of the flexural strength (400 MPa) [24]. A significant increase in the mechanical properties of fiber-reinforced ceramics is provided by the 3D-Braiding technology, which was developed at the ITA. Both the machine technology with its components and the process itself have been further modified, so that the handling of brittle ceramic fibers is now possible and allows the production of semi-finished products that are reinforced in all three spatial directions. The 3D-braiding technology adds a reinforcement in the z-direction (see Fig. 18) compared to the 2D-braiding technology, so that interlaminar interfaces inside the braided cross-sections are completely avoided. Consequently, individual layers do not delaminate in the event of damage, as is the case with other reinforcing semi-finished products. This results in an increase of the damage tolerance values for impact stresses and the energies that can be absorbed by the structure (work absorption capacity, vibration damping). Thus, the produced 3D-braided CMC do fulfill the mechanical requirements for structural components in addition to the thermal stress this material can resist. Of particular interest is the use of such 3D-braided components in aircraft turbines. Here, the process efficiency can be increased by means of higher combustion temperatures. Further applications of 3D-braided CMC can be media lines, solar thermal power plants or solid oxide fuel cells.

CT-Scan of a 3D braided T-profile

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3.12

3D Printing On Pre-stressed Textiles

4D textiles are textiles and textile products that can change their shape or function over time due to the influence of an external stimulus. External stimuli are mainly force or heat [25]. 4D textiles can appear along the whole textile chain, from shape memory fibers, to self-wrinkling weaves and self-folding hybrid materials [26].

At the Institut für Textiltechnik of RWTH Aachen University (ITA) self-folding and self-bending textiles hybrid materials are produced by 3D printing. Mainly fusion deposition modelling (FDM) on prestressed textiles is investigated. Therefore, a filament is extruded on the textile, infiltrates the pores and creates an adhesion-based bond between the materials. The most common filament materials are thermoplastic elastomers such as TPU 95 and thermoplastic PLA. The textile serves as an energy storage medium either through structural elastic properties that can be found in warp and weft knitted fabrics or through material elastic properties to be found in elastane [25].

A shape change from a two-dimensional to a three-dimensional shape appears after releasing the stress from the textile and creates double curved surfaces which represent the lowest possible energy level. More than one equilibrium state is created which allows the hybrid material to shape change between these states after the release of the stress [25, 27]. The change of shape is activated e.g., by heat which softens the polymer and allows the textile to move.

Applications of 4D textiles in architecture for adaptive shading or acoustic element, as well as interactive car interior are the current focus of research.