Extrusion of High-Resolution Polymer Structures with Direct Ink Writing Techniques

Abstract

This chapter explores 3D printing, often called additive manufacturing, with a focus on printing structures with high-detail resolution. There are mainly two techniques that allow for 3D printing in the μm to sub-μm range: two-photon printing and direct ink writing (DIW). The two-photon technique is briefly explained, while the focus here is on DIW since this technique gives an opportunity to print a wide variety of materials. To exemplify high-detail resolution polymer 3D printing, biocompatible cellulose acetate (CA) is selected. Printability and the possibility of printing μm feature-size structures with inks containing different amounts and molecular weights of cellulose acetate are presented. Results indicate that by optimizing inks and printing parameters such as the internal and external diameter of the nozzle, strands down to sub-μm can be printed with high placement control. Various challenges as clogging and low printing speed are also discussed.

1. Introduction

Three-dimensional (3D) structures can be built through layer-by-layer deposition of materials on a suitable substrate. This method called 3D printing or additive manufacturing (AM) allows the fabrication of complex 3D structures from computer-aided design models [1]. 3D printing has several advantages in comparison with conventional manufacturing methods. Some of the important benefits of 3D printing are the freedom of design, shorter lead times, and less waste of material [2]. Creating structures with smaller dimensions, complicated geometries, and much higher resolution is also possible by 3D printing.

The present high-detail resolution AM technologies can be divided into two main categories: (1) light-based 3D printing and (2) ink-based 3D printing. Two-photon polymerization (2PP), the light-based printing technique with true sub-μm detail resolution, has many advantages [3]. Some basic facts regarding this technique will be given for comparison reasons. If we focus on high-detail resolution ink-based techniques, these can be further subcategorized with respect to, for instance, the particular mechanisms for how the ink is extruded from the nozzle [2]. The simple direct ink writing (DIW) technique has become widely interesting for diverse applications due to reasons such as the availability of various materials, the detail resolution, and the scalability [4, 5]. Typical envisioned applications are additive manufacturing of membranes for, for example, waste water, medical devices, and bio-separation. In polymer DIW, precise control over the positioning of the printed fibers is key, and by extruding extremely fine fibers, it is possible to reach μm or even sub-μm detail resolution. In this chapter, we will examine examples of DIW structures using ink containing cellulose acetate, which is the naturally derived polymer [6, 7], and in particular discuss methods to create high-detail resolution structures.

2. Light-based printing

2.1 Method

Two-photon polymerization (2PP) is one of the more powerful light-based printing techniques that enables 3D printing of complex structures with high resolution [8]. A 2PP printer is similar to a stereolithographic printer, but it typically uses a femtosecond laser and a high-resolution specimen table in combination with galvomirrors to scan the laser beam fast [9]. From a printing point of view, the major difference is that it is possible to print at a point inside the photocurable resin due to the so-called two-photon process. The high laser intensity in the focal spot allows two low-energy photons to simultaneous activate local photopolymerization, while there is no photopolymerization outside the focal spot since the photon energy from single electrons is not sufficient there [10]. With a focal spot that has the necessary intensity within a fraction of a μm, it is possible to reach sub-μm detail resolution [11].

2.2 Printing parameters

It is essential to realize the importance of the printing parameters that affect the polymerization process. This involves exploring the relationships among, for instance, the resin formulation, scanning speed, and laser power [12]. Depending on printing equipment and printing strategy, the print speed will vary, but as all other high-resolution techniques, it could take hours to print solid object of mm size.

2.3 Printed structures

We have printed structure with holes ranging from 4 μm to 250 nm to exemplify what can be done with a reasonable amount of tuning. As shown in Figure 1, it is possible to print holes down to 500 nm with the 2PP technique, and with some tuning, it is possible to go somewhat further. Figure 2(a,b) illustrates a successfully printed complex structure comprising two nozzles with 4 μm diameter in the nozzle opening.

Figure 1.

Printed structure with holes ranging from 4 μm to 500 nm.

Figure 2.

(a) Two-nozzle printed structure, (b) opening part of the nozzle with 4 μm diameter.

The example in Figure 3 demonstrates the ability to print free-standing structures within a broad size range, from micrometers to nanometers, and in this case, the finest pin diameter is 0.5 μm. This demonstrates that the 2PP printing technology is suitable to produce structures with detail resolution down to sub-μm, and the examples in Figures 1–3 suggest that it is possible to design structures for diverse applications.

Figure 3.

Printed structure showing pins of various lengths with diameters from 4 to 0.5 μm.

2.4 Benefits and limitations

Due to the possibility to print truly complex structures, for instance, with overhangs, from computer-generated models, combined with the high-detail resolution in several commercially available printers, 2PP has attracted wide attention [13]. However, one of the main factors that limited the progress of 2PP is the challenge of producing components from a broader range of the materials [8]. It should be mentioned that still there are much to explore in the 2PP technique, to fully utilize the capabilities.

3. Direct ink writing

3.1 Method

In the DIW technique, which is an extrusion-based method, ink is deposited through a nozzle on a substrate. After extruding the liquid ink, different parameters such as extrusion rate, print speed, solvent evaporation, and absorption control the solidification of the ink [14]. Figure 4 shows the printing setup designed for fabricating high-detail resolution structures [15].

Figure 4.

A direct ink writing setup for high-detail resolution 3D printing.

The setup includes different components such as a syringe pump (motor to drive the syringe piston, the syringe pump uses a LEGS motor from Piezomotor allowing nm movement control), the head part holding the motor that has a force cell to measure force between head and piston, a syringe that should be filled with the desired ink, a glass capillary with nozzle (various nozzles can be attached to the syringe), and a 3D table that moves the substrate [16]. The 3D table should allow for the desired positioning control, and for high-detail resolution, we usually demand sub-μm repeatability. Not shown in Figure 4 is a microscope that makes it possible to both observe the printing and place the nozzle of the capillary with the optimal spacing, often in the μm range, from the substrate. The capillaries are typically made long and slender to handle occasional contact with the substrate without braking. The choice of slenderness is delicate since a too slender capillary will result in reduced placement control due to various viscous and friction forces during printing.

3.2 Printing parameters

In the direct ink writing technique, it is important to evaluate the parameters that impact the printing resolution. One of the critical factors is the nozzle size, which can be adjusted based on the desired resolution and application requirements. However, it should be noticed that a reduction of the nozzle size may increase the risk of nozzle clogging. Moreover, shear thinning behavior of the ink in DIW plays an important role in achieving high-resolution structures since this allows for extrusion through narrower nozzles and faster solidification after extrusion. The viscosity of the ink will affect the printing resolution as excessive viscosity hinders the ink extrusion, while insufficient viscosity could cause ink spreading that reduces the lateral resolution of the structure [17].

Printing speed is another parameter that influences the final resolution of the printed structure [6]. Printing speed is typically controlled by the movement speed of the substrate in the X and Y directions. Higher speeds present challenges in the resolution, particularly when there will be high accelerations or retardations as in the corners. It is theoretically possible to adjust the ink flow to compensate for speed changes, but this is a great challenge when ink flow gets close to pl./s range. This flow range corresponds to nozzle diameters in the micron range and speeds of few mm/s, which is often used to get reasonable printing times for micrometer detail resolution structures. Additionally, the applied pressure in the syringe pump could impact the printing results. The compression of the ink and/or gas bubbles will make it difficult to adjust the ink flow, particularly when larger volume inks are under pressure. The pressure should therefore be carefully calibrated in relation to printing speed and ink viscosity. For instance, high pressure could lead to an increased extrusion rate for an ink with low viscosity at low printing speeds, which compromises detail resolution.

3.3 Materials

The DIW method gives flexibility in producing structures for various applications due to the ability of using diverse printable materials. In this chapter, we will focus on polymers since this best illustrates the possibilities to extrude high-detail resolution 3D structures. Among different polymers that can be used in 3D printing, cellulose, which constitutes the main part of a plant’s cell walls, has attracted attention due to several interesting properties. Cellulose is biocompatible, and it can be used in various applications like biomedical, pharmaceutical, construction, packaging, clothing, thermal insulation, and so forth. Further, cellulose is a mechanically robust as well as chemically versatile material. Cellulose is insoluble in many solvents; to increase the solubility, derivatives of cellulose can be used. Here, Cellulose acetate (CA) has been selected for its high solubility [18].

3.4 Printed structures

To exemplify the possibilities with direct ink writing, various membrane structures will be discussed. Typically, membrane structures are characterized in terms of physical properties such as pore diameter, pore size distribution, porosity, and tortuosity, which all are dependent on the fabrication method. The placement of the printed strands as well as the integrity of the strand after placement forming the fibers will be of great importance for the membrane properties.

As mentioned in the introduction, the DIW technique allows controlled positioning of printed fibers. In other membrane fabrication techniques, such as electrospinning, positioning of the fibers is nearly random and lacks placement precision. Although it is possible to achieve pore size and fiber diameter in the range of a few nanometers, there is an inaccuracy within the structure, ranging from hundreds of nanometers to micrometers. Figure 5 illustrates nanometer diameter electrospun fibers as well as presence of clusters, contributing to uneven structures and unpredicted pore sizes.

Figure 5.

Electrospun structure with uneven fibers and pore sizes.

The direct ink writing setup in Figure 4 allows us to move the substrate relative to the nozzle with sub-micrometer control. Various parameters could influence the size of the printed structures. To print 400 × 400 μm square structures, in Figure 6(a,b), a nozzle with inner diameter of 6 μm and an outer diameter of 40 μm was used. In both structures, cellulose acetate (CA) was used as the ink, but the rheological properties of the ink differed. For Figures 6(a), 5% CA with a molecular weight of 50 kDa was utilized, while Figure 6(b) employed 12.5% CA with the same molecular weight. All other printing parameters remained constant for both cases. The speed in the X and Y directions was set at 0.5 mm/s, and the ink flow remained constant at 3000 pl./s (calculated from syringe piston movement). The results illustrate that by using constant parameters and altering the ink concentration, significantly different outcomes can be achieved. In Figure 6(a), controlling the ink flow through the nozzle was challenging, resulting in a printed line much larger than the nozzle’s outer diameter. By increasing the amount of CA, properties such as the viscosity of the ink improved, which led to precise control over the printed fibers and resulted in the desired resolution, Figure 6(b,c). In a structure with more than 10 layers printed, as shown in Figure 6(c), the size of the printed fibers in the upper layers is close to the inner diameter of the nozzle, while the fibers size in the first layer is almost equal to the outer diameter of the nozzle. The larger ink spreading on the substrate in comparison with that on already printed material depends on several factors such as differences in wetting, solvent absorption, and solvent evaporation. The strand width is often controlled by the outer diameter of the nozzle in the first layer on non-absorbing substrates, while it is controlled by the inner diameter in the upper layers as observed in Figure 6(c).

Figure 6.

Structures printed using inks containing 5% CA (a) and 12.5% CA (b, c). Sizes of the printed fiber in the first and upper layers are 36 μm and 5 μm, respectively (c).

Optimizing parameters allows for the reduction of pore size as well as a reduction of fiber diameter giving larger porosity. To exemplify membrane structures, Figure 7 demonstrates what occurs in the different layers during printing. An ink with 12.5% CA and a capillary with inner and outer nozzle diameters of 3 and 9 μm, respectively, were utilized. All other parameters were identical to those for Figure 6(b). The images demonstrate the capability to achieve controlled fiber spacings that would allow for varied pore sizes as designed for the structure. In the bottom layers close to the substrate, the ink typically spreads laterally, and this layer will typically be used to support the upper membrane layers. Moving from right to left in Figure 7(a), the inter-fiber distance decreases from 100 to 50 μm, 30 μm, 15 μm, and finally 10 μm. As can be noted, when the inter-fiber distance is reduced to 10 μm, fibers become attached to each other in these first layers. Figure 7(b) is the same structure with more than 15 layers, magnified to highlight the 6 μm inter-fiber distance between free-hanging printed fibers. The fibers in these upper layers are about 1 μm, allowing for large and controlled porosity. As exemplified in Figure 7(c), where the pattern has a pitch of 15 μm in the designed structure, strands positioning is well controlled, which demonstrates placement control in the μm range in this DIW experiment. The deviations from the printing pattern are typically due to the capillary bending, resulting from a long and slender capillary to reduce risk of breakage.

Figure 7.

Printed structures with 12.5%, (a) four layers structure with varying pore sizes, (b) inter-fiber distance of 6 μm when the pitch is 10 μm, (c) positioning of the printed strands with 15 μm pitch.

3.5 Benefits and limitations

DIW has notable benefits for high-detail resolution 3D printing and, in particular, the ability to print micron-sized fibers with micrometer placement control. Membranes can therefore be made with well controlled pore size and size distribution. It is interesting to note that free-hanging strands can be printed rather large distances allowing advanced geometrical arrangements. The possibility of using a diverse range of materials makes it versatile and suitable for different purposes. However, these advantages come with some limitations, and for extrusion, it is necessary to prepare the material as an ink prior to printing. To achieve micrometer detail resolution, it is often necessary to prepare a capillary before each printing session, typically using a nozzle diameter within the range of a few micrometers. Typical challenges are frequent nozzle breakage and clogging, as shown in Figure 8. The ink therefore has to be well dispersed with no agglomerations larger than a fraction of the nozzle diameter. Furthermore, DIW has relatively low printing speeds, which makes it time-consuming to print large structures, and it is not yet optimal for industrial applications.

Figure 8.

(a) Nozzle with inner diameter of 4 μm, (b) nozzle with breakage, (c) nozzle with clogging.

4. Conclusion

The chapter examines the 2PP and DIW techniques, both offering the potential for high-resolution 3D printing. 2PP technique gives the possibility of printing high-resolution structures; however, it demands photocurable resins, which today gives a rather limited material choice.

DIW appears to be a more versatile technique since there is a wide selection of printable materials. This versatility opens up new possibilities for applications in various industries, allowing for customization based on specific application requirements. The availability of a broad range of materials in DIW enhances its adaptability and makes it a compelling option for printing projects. Various parameters in DIW affect the final structure. Parameters such as nozzle size, ink rheology, printing speed, and ink flow have impact on the printing resolution. Balancing these factors is essential for achieving optimal resolution and desired printing outcome.

Acknowledgments

This work is conducted within the Additive Manufacturing for the Life Sciences Competence Center (AM4Life). The authors gratefully acknowledge financial support from Sweden’s Innovation Agency VINNOVA (Grant no: 2019-00029). They acknowledge Myfab Uppsala for providing facilities and experimental support, in particular Victoria Sternhagen for some of the 2PP SEM images. Myfab is funded by the Swedish Research Council (Grant no: 2019-00207) as a national research infrastructure.

Conflict of interest

The authors declare no conflict of interest.