Round hollow ber membranes are long-established in applications such as gas separation, ultra ltration and blood dialysis. Yet, it is well known that geometrical topologies can introduce secondary ow patterns counteracting mass transport limitations, stemming from di usion resistances and fouling. We present a new systematic method- ology to fabricate novel membrane architectures. We use the freedom of design by 3D-printing spinnerets, having multiple bore channels of any geometry. First, such spinnerets are stationary to fabricate straight bore channels inside a monolithic membrane. Second, in an even more complex design, a new mechanical system enables rotating the spinneret. Such rotating multibore spinnerets enable (A) the preparation of twisted channels inside a porous monolithic membrane as well as (B) a helical twist of the outside geometry. The spun material systems comprise classical polymer solutions as well as metal-polymer slurries resulting in solid porous metallic monolithic membrane after thermal post-processing. It is known that twisted spiral-type bore channel geometries are potentially superior over straight channels with respect to mass and heat polarization phenomena, however their fabrication was cumber- some in the past. Now, the described methodology enables membrane fabrication to tailor the membrane geometry to the needs of the membrane process. Keywords: Additive manufacturing, Spinneret design, Twisted hollow ber membrane, Helical membrane, Multibore 1. Introduction In membrane ltration processes, round membranes in the shape of hollow bers as well as at sheets are well-established. Yet, the challenge remains to improve mass transfer at the membrane surface. In membrane processes with high permeation rates, the retained component accumulates at the membrane surface. If back di usion or mixing is limited, the retained component builds up and an additional mass transfer resistance emerges and the 1 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 2 e ective driving potential across the membrane is lowered. Additionally, in case of ultra l- tration, retained components increase the viscosity. This also acts as an additional transport resistance and hence lowers the mass transfer. Finally the overall performance of the process decreases. Measures to counteract these concentration polarization and fouling phenomena are manyfold. One of them being the induction of secondary ows normal to the membrane surface back into the feed channel preventing the establishment of a concentration polariza- tion or fouling layer. These secondary ows emerge when the surface of a membrane causes the owing feed to absorb a momentum, di erent then the main ow direction. It is highly desirable to have at hand a freedom of design for membrane extrusion processes through the tailored design of a spinneret. This would then result in the desired membrane geometry. In the scope of our publication we aim to overcome the previously described issues concern- ing mass transport resistance by varying the geometry of the hollow ber membrane. This paper describes a methodology to freeform fabricate spinnerets with any channel geometry by additive manufacturing, or 3D printing, combined with a superimposed rotation of the spinneret and the fabrication of polymeric and sintered hollow ber membranes. 2. Background 2.1. Membrane geometries Two membrane geometries are used in general: at sheet and hollow ber membranes. To enable membrane operation, the membrane needs to be implemented into a module to de ne ow conditions. For at sheet membranes spiral-wound and stagged at modules are most widely used. Both the feed and permeate channel are equipped with spacers. These spacers de ne the channel height and additionally act as mixing devices by periodical redirection of the ow eld [1, 2]. Investigation and optimization of spacer geometry and orientation in spiral wound modules has been subject to many scienti c publications, as reviewed by Schwinge et al. [3{12]. For hollow ber or extruded ori ce membrane geometries, only the membrane itself and its arrangements in a module without any spacer de ne the ow con guration. The hollow 2 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 3 ber geometry simply has a round cross-section and a linear axial orientation. The small dimension of the bers and the absence of spacers results in high speci c surface areas as compared to at membrane module types. The mixing of feed ow, showing concentration polarization, and the reduction of this in hollow ber and tubular membranes is subject of scienti c research since almost 20 years [13{20]. Especially, concentration polarization or deposition layers are predominant on the membrane surface in inside-out micro- and ultra- ltration. In contrast to the virgin membrane's hydraulic resistance, better lumen mixing o ers signi cant performance improvement [21]. While in at sheet membrane modules the ow channels are formed by two di erent membranes with spacers inserted during module fabrication, the situation is di erent in hollow ber membranes. Here, the ow channel is not accessible after membrane formation. Further, the placement of passive mixing elements such as turbulence promotors is possible [22] however not feasible in small size hollow ber geometries. For the purpose of increasing surface area, non-circular structured hollow ber mem- branes with cross-sectional geometries other then round are described in [23] for outside structures and in [24] for inside structures. Outside structures can also have bene cial transport properties when twisted and used in aerated submerged ltration processes [25]. However, integrated mixing functionalities on the inside are not established well due to di- culties in fabrication. To address this fabrication challenge, recent developments utilize the shape of the membrane itself to induce secondary ow, mixing up the di usion boundary layer. The ow eld is designed to transport retained components back to the bulk ow and thus decrease concentration polarization. A method to fabricate circular curled hollow bers [26] via the liquid rope coil e ect has been presented by Luelf et al., as well as fab- rication of hollow bers with axially sine-shaped but radially circular lumen channels [27]. Such passive mixing in single bore hollow ber applications has been subject to di erent studies on mass transfer. Especially, spiral shaped structures are of interest due to their speci c ow eld. Moulin et al. investigated its potential application in ultra ltration and found curled orientation of hollow bers to potentially o er increased uxes [28]. Further- more, computational uid dynamics have been applied to show the mixing potential in spiral 3 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 4 shaped membrane channels [15, 16]. Pentair recently developed tubular membranes with increased mixing performance, called X-Flow Helix technology. The X-Flow tubular mem- branes were evaluated by Wiese et al. using NMR and ltration measurements [29]. Here, a small spiralling ridge or corrugation is directly introduced into the bore channel during membrane fabrication, improving mass transport signi cantly at turbulent ow conditions. 2.2. Spinneret design Hollow ber membranes are produced via co-extrusion of a bore uid and a polymer dope solution through a spinneret. Multilayer co-extruded hollow bers are also known for di erent applications [30, 31]. Traditional manufacturing techniques for spinnerets produce round geometries of the main polymer outlet and the bore channel only. As dimensions of the spinnerets are in the range of the produced membrane geometry, sophisticated design of spinnerets would lead to excessive manufacturing costs. If additional heating channels have to be provided, limitations in manufacturing become predominant. In general, manufactur- ing limitations nowadays dictate the design of spinnerets. It would be highly desirable to become independent of current spinneret manufacturing processes and move towards quick, cheap and exible design methods enabled by 3D-printing. Membranes with multiple bore channels are well known for monolithic ceramic mem- branes. Also hollow ber membranes with multiple bore channels are available from GE or BASF Inge GmbH or, known as Multibore®membranes for ultra ltration [32]. Researchers also employ this geometry experimenting with di erent channel amounts in a single ber [33{37]. There have been attempts to improve the membrane geometries with multiple bore channels with regard to wall thickness distribution [34{36]. All of these studies approach spinneret design from a mainly qualitative perspective. However, manufacturing of these multi-channel spinnerets with traditional manufacturing techniques is complex and expen- sive. Hence, iterative geometry optimization of spinneret design without rapid prototyping technique is currently not feasible. Bonyadi and Mackley fabricated at membranes with multiple bore channels, named micro-capillary lm membranes and reported the e ect of spinning conditions, such as take 4 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 5 up speed and air gap distance on the macroscopic membrane formation [38]. 2.3. Spiralling ow channels Spiralling channels are known to enhance mixing and transport limited processes [39]. So-called Dean vortices promote the mass transport [13, 15{18, 21]. In a curved ow ar- rangement, a moving element is a ected both by inertia and centrifugal forces. The latter scales with rotational speed and radius. Thus, an element in the bulk ow is dragged in radial direction of curvature. Elements near the channel wall are slower due to no slip con- dition at the wall. The interplay of both nally result in a double vortex, also known as dean vortices, enabling additional mixing [15, 16, 39, 40]. The formation of dean vortices is commonly described in a mathematical manner using the Dean number. This dimensionless parameter scales the Reynolds number Rewith the radius of curvature. De=Rer di Dc(1) with Dc=D" 1 +b D2# (2) HereDcrepresents the diameter of curvature, corrected by the helical pitch b. The helical pitch determines the length of a 2 twist. The tube internal diameter is denoted by di. [41] Transfer phenomena are often described by dimensionless power-law equations. The correlations for heat and mass transfer are similar due to equal mathematical descriptions of their fundamental principles. There are multiple experimental and theoretical studies on the ow and heat transfer in curved pipes. While the reproduction of all studies is beyond the scope of this paper, Vashisth et al. gave a detailed summary on their ndings and the conditions under which they were derived [39]. The ow eld was applied to reverse osmosis in a curved membrane duct in an early study by Srinivasan and Tien [42] in 1970. Here, potential reduction of concentration polarization in binary salt solutions was proven by a mathematical approach. Nunge and Adams [43] critically evaluated the studies of Srinivasan and Tien and found concentration polarization reduction that is less intense, but still signi cant. 5 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 6 Winzeler and Belfort [44] have prooven the existance of secondary vortices in combination with a at sheet membrane in a spiral channel. They visualized the vortices both optical and by NMR imaging and measured ve times increased ux for dairy whey ltration by presence of spiral channels. Liu et al. [41] investigated the mass transfer in curved hollow ber membrane with di erent wind angles (pitches) and compared the results to straight bers. They found signi cantly increased mass transfer coecients up to 3.5 fold. Finally beeing expressed by mass transfer coecients and Sherwood numbers. Equation 3 represents the Leveque solution for straight tubes. Shs= 1:62di L0:33 Re0:33Sc0:33(3) Liu et. al found correlations for the Sherwood number as a function of Dean number as expressed in Equation 4. Their ndings show dependencies of higher power employing values of up to 0:55 andaof 2:64 regarding bers with a wind angle of 45 degree. Shc=aDe Sc0:33(4) As can be seen by the Dean number exponent, the curved structure o ers higher transfer rates as compared to the straight channel alignment. Also Moulin et al. [17] found increased mass, expressed by improvement factors of 2 to 4 in water oxygenation. In our previous work we developed a manufacturing technique to produce hollow bers based on rope coil spinning without the need of additional manufacturing steps [26]. While the production itself does not demand changes to existing spinning setups, the ber geom- etry intrinsically limits the membrane surface area per module volume in closed module operations. A methodology for the fabrication of integrally twisted multibores does not exist. Here we present a procedure for rapid iterative spinneret improvement, exemplary shown for tri- bore spinnerets based on rapid prototyping. The method of 3D-printing spinnerets o ers freedom of design in the cross section design of hollow bers. In the second part of the 6 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 7 manuscript we investigate the combination of printed spinnerets and a rotational spinneret movement. Thereby, we create bers with potential passive mixing properties and no limita- tion in packing density as compared to conventional hollow bers. The presented method is evaluated by spinning of polymeric bers. Based on our preliminary experience on synthesis of porous metallic bers [45], we also apply the method to produce porous electrodes with previously mentioned properties. 3. Experimental 3.1. Spinning setup For hollow ber production by NIPS (non-solvent induced phase separation), a state-of- the-art hollow ber spinning line was used. The designed spinnerets were connected to the polymer solution and bore uid lines via a rotating construction for the production of bers with twisted bore channels. Figure 1 visualizes the position of the rotating construction in the spinning line. The rotating construction, described in Section 3.3 is located above the coagulation bath, lled with tap water. The ber is spun through the spinneret, passes the air gap and undergoes phase separation in the coagulation bath. The ber is held in place around a small guiding wheel and nally taken out of the bath by a pulling wheel. It is placed in additional water baths for solvent exchange for at least 48 hours. Due to the rotation and the newly designed spinneret, two new production parameters are associated as follows: •Spinneret geometry •Rotational speed of spinneret 7 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 8 Figure 1. Spinning setup: A) Bore uid inlet, B) Polymer solution inlet, C) Spinneret outlet. 8 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 9 Figure 2. Cut view of rotating spinneret assembly. The bore uid line is coloured in blue while the polymer solution is guided through the yellow parts. The stirrer motor is located between the cut lines and is not displayed in this view. A) Marks the inlet of bore uid. B) Marks the inlet of the polymer solution. C) Marks the position of the 3D printed spinneret connected to the bore uid and polymer solution lines. 9 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 10 Figure 3. Comparison of spinneret CAD drawings with printed versions. The left images of each pair are CAD renderings, while the right photographs are taken after printing of the spinnerets. Each spinneret body has a diameter of 25 mm. A) Oval polymer solution opening with clearly visible step size of 100 m in z-direction shown in the magni cation, B) round tri-bore spinneret, C) three edged star shaped bore opening, D) ve edged star shaped bore opening. 3.2. Rapid prototyping of spinnerets All spinnerets, except the ones for the production of at tribore bers were 3D-printed by means of stereolithography with Envisiontec Perfactory®3 mini multi lens. EnvisionTEC e-shell 600 was selected as photo resin. The printing resolution in z-direction on was set to 100mto allow for sucient accuracy. The x- and y-resolution was set to 60 m. The printed slices are cured out of the shallow resin vat with 180 mW=dm2UV emission. The printing process itself operates at a speed of approximately 10mm per hour in z-direction. For post processing of parts printed with e-shell 600 clear EnvisionTEC GmbH states a 10 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 11 suitable method [46]. The parts are cleaned in 2-propanol for a few minutes to remove not polymerized material and afterwards dried at 37 Cfor 30min. Then the part requires to be post cured by the light curing unit Otofash G171 by NK-Optik GmbH with 2000 to 4000 ashes. Figure 3 and 5 show the opening region of a printed spinneret. The step size in z-direction is clearly visible on the top of the spinnerets, while the inner needle is still round and without edges in the horizontal plane. For the at tribore ber production spinnerets were printed with an OBJET Eden260V 3-Dimensional Printing System by Stratasys Ltd., producing parts via the polyjet printing principle. As materials both VeroClear and RGD525 were processed. Closed volume parts that are printed with RGD525 show an in ll that is partly lled with support material. Outer shells of the parts are always printed homogeneous from RGD525. For the at tribore bers Stratasys VeroClear was used. Employing VeroClear and RGD525, a print can either be performed with support on the entire outer surface of the printed part, leaving a matte surface, or without outer support to result in a glossy surface. As NMP was used as a solvent for the polymer, all printed parts have been tested for solvent resistance. A detailed study of the swelling behavior and the solvent resistance can be found in the supplementary material. 3.3. Rotating construction In order to enable a rotational movement of the hollow ber spinneret, a rotation of two uids is necessary while the supply of these has to be realized at static positions. A rotating system as proposed by Femmer et al. [47] has been used. Figure 2 shows the rotating assembly used in the here presented work. Since the bore uid has a lower viscosity, it is guided through the inner capillary of the assembly. It is inserted into the setup from the top (Figure 2 A). Rotation starts directly behind the rst rotary coupling. The polymer solution is fed from the side opening (Figure 2 B) through the metal tubing. The wider tube diameter accounts for potentially high viscosities of polymer solutions. The polymer solution inlet stands still while the tubing above and below the adjacent rotary couplings are turning. In order to be able to connect standard spinnerets with separated inlets for 11 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 12 polymer solution and bore uids, the bore uid is guided out of the shell tube directly before entering the spinneret from the side. 3.4. Spinning of round rotating PES bers A rotating spinneret was assembled into to a spinning line as illustrated in Figure 1. Spinning parameters were set as listed in Table 3. Polymer and bore ow rates have been kept constant as well as the ber draw speed by the pulling wheel and the air gap. The rotational speed of the spinneret has been varied from 0 RPM, over 30 RPM, to 65 RPM for the tri-bore hollow bers. The helical tri-bore hollow bers have been spun at 0 RPM and 30 RPM. The polymer solution consists of polyethersulfone (PES) (BASF Ultrason 6020 P), NMP (1-Methyl-2-pyrrolidinone, 99%, extra pure, ACROS OrganicsTM), polyvinylpyrrolidone (PVP K90, extra pure, CarlRoth), glycerol (purity 98%, Ph.Eur., anhydrous., Carl Roth). The bore uid is a mixture of water and glycerol. Polymer solution and bore uid were used as listed in Table 1. PES PVP NMP Glycerol Water [wt. %] [wt. %] [wt. %] [wt. %] [wt. %] Polymer 15 5.25 75 4.75 - Bore - - - 93 7 Table 1. Solution compositions for rotating spinning of polymeric round tri-bore hollow bers 3.5. Spinning of helical bers In the next step, both static mixing properties on the outside of the membrane as pro- posed by Fritzmann et al. [1] and an integrally twisted multibore monolithic membrane are combined. A rotating spinneret assembly described in Section 3.3 is combined with a at tri-bore spinneret as depicted in Figure 9. In the 3D printed spinneret, the polymer solution or suspension is fed through a tapered hollow space up until exiting the spinneret in the desired dimension of the annular gap. The annular gap of the polymer channel is rounded concentric to the bore channels with spikes between the needles to enable a good 12 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 13 distribution of polymer solution around and between the needles. In case of the bore uid, the total volume ow is divided and fed to three openings of the spinneret, each providing equal volume ow for each needle bore outlet. Table 2 shows the composition of the spinning solution applied for the production of helical polynmer bers. The applies spinning parameters are listed in Table 3. PES PVP NMP Glycerol Water [wt. %] [wt. %] [wt. %] [wt. %] [wt. %] Polymer 16 8 76 - - Bore - - - 5 95 Table 2. Solution compositions for rotating spinning of polymeric helical tri-bore hollow bers VPolymer VBorevPullhAirGap [mL=min ] [mL=min ] [mm=s ] [mm] tri-bore 5.2 2.0 29.7 15 helical tri-bore 7.7 24 3.46 5 Table 3. Spinning parameters for spinning of polymeric tri-bore and helical tri bore bers hollow bers 3.6. Spinning of rotating titanium bers Analog to the spinning of rotating PES bers the dope solution for the titanium bers is polymer based and adapted from the method of our previous work concerning the production of tubular macro-porous titanium membranes by David et al. [45]. 3.6.1. Production of green- bers The dope solution consist of 7 :5wt:% Polyethersulfone (BASF Ultrason 6020 P, dried prior to use) as polymer binder, 22 :5wt:% NMP (1-Methyl-2-pyrrolidinone, 99%, extra pure, ACROS OrganicsTM), and 70wt:% titanium powder with an average particle size of 15 m (ASTM, Grade 2, 99.7% purity, purchased from TLS Technik GmbH & Co. (Germany)). The aqueous bore uid was modi ed by adding 5 wt:% Glycerol (purity 98%, Ph.Eur., anhydrous., Carl Roth) as viscosity enhancer to the de-ionized water. Spinning parameters 13 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 14 were set as listed in Table 5. Polymer solution and bore uid ow rates have been kept constant as well as the ber draw speed by the pulling wheel and the air gap. PES Ti Glycerol NMP Water [wt. %] [wt. %] [wt. %] [wt. %] [wt. %] Polymer 7.5 70 - 22.5 - Bore - - 5 - 95 Table 4. Slurry composition for spinning of PES/Titanium based green- bers VPolymer VBorevPullhAirGap [ml=min ] [ml=min ] [mm=s ] [mm] 5.2 16 2.83 5 Table 5. Spinning parameters for PES/titanium based green- bers 3.6.2. Thermal post-processing of green- bers PES/titanium green- bers were processed in an additional thermal treatment in order to obtain a solid porous metallic monolithic ber. The spun PES/titanium green- bers were sintered in a tubular furnace (Carbolite STF 16/610) under argon atmosphere. Figure 4 shows the applied temperature pro le, proposed by David et al. [45]. The argon atmosphere was applied by a sweep ow of 7 :5mL=min , maintaining a steady supply of argon in order to not cause temperature gradients in the tubular furnace. During the rst plateau the polymer binder (here PES) of the green- ber is combusted. Reaching the second plateau the titanium powder forms sinter connections and builds a solid porous matrix. The heating rate during the heating process towards both plateaus is held constant at 5C=min . Higher heating rates cause production disruptions and lead to an inhomogeneous outcome in terms of shape conservation. Depletion of the polymer binder, and during the progression of the sintering process the ber volume is subject to shrinkage. Depending on the temperature and sinter time, the porosity of the obtained bers varies according to the research by David et al [45]. Here we have shown that higher sinter temperatures and longer sinter times lead 14 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 15 to depletion of free and hidden porosity [45]. Figure 4. Temperature pro le of the thermal post-processing step for PES/Titanium based green- bers. A two plateau method is applied with de ned heating rates of 5C=min . The temperature is held constant for 60min at 600Cfor the polymer binder combustion step and at 1100Cfor the sintering step. 3.7. Optical analysis Photographs have been taken with a Nikon D7100 Digital Single Lens Re ex (DSLR) camera equipped with a 17-105 mm lens. Makro images of bers (Figure 6) have been taken 15 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 16 with a custom made microscope setup with a CMOS uEyeTMLE USB 2.0 camera from iDS and a Micro Nikkor 55 mm f/2.8 Nikon lens which is mounted in retro mode. FeSEM images have been taken with Field Immission Elektron Mikroscope Hitachi S- 4800. SEM images have been taken with Hitachi Table Top TM3030 plus. The acceleration voltage has been set to 15kV. CTimages have been taken with a Bruker, SkyScan 1272 device by Bruker. 4. Results 4.1. Prototyping spinnerets with 3D-printing As 3D-printing presents a higher degree of freedom as compared to traditional manufac- turing techniques during the design process, the shape of the spinnerets was varied from the standard, concentric round, shape. In conventional spinnerets the polymer solution enters the spinneret from the top and is distributed concentrically around the needle through mul- tiple drillings. Employing our prototype spinnerets, the bore uid needle is connected to the outer body at one quarter, leaving three quarters of spinneret for distribution of the polymer solution. To ensure an equal distribution of polymer solution at the outlet, the polymer ow channel was narrowed directly at the opening (see Figure 5). Inner surfaces of 3D printed spinnerets comprise a higher roughness as compared to polished metal spinnerets. This is more pronounced for the spinnerets based on polyjet printing than for the stereolithogra- phy based spinnerets. On the outer surface on the other hand, this is not found for the stereolithography based parts and the glossy polyjet parts. A comparison of designed and printed spinnerets with di erent outlet geometries is dis- played in Figure 3. Rendering of the designed geometry is displayed for each pair on the left side in an isometric view, photographs of the printed versions are displayed on the right. No signi cant deviations from the intended geometry, especially at the outlet are visible. The choice of 3D-printing over conventional manufacturing techniques provides an increased freedom of design, especially concerning the spinneret outlet, where the ber geometry is 16 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 17 de ned during the spinning process. Variations in bore uid channel geometry and channel numbers as well as polymer solution channel geometry are possible to be obtained without increase in fabrication e ort. Figure 5. Close-up of the outlet region of a 3D-printed spinneret cut in half. Bore uid and polymer solution channel maintained its structure during printing. The inner channels show a higher roughness than machined spinnerets. 4.2. Designing tri-bore spinnerets - In uence of needle size The potential for iterative spinneret improvement is exemplary shown for the in uence of needle size using a tri-bore spinneret. The e ect was evaluated by designing two spinnerets, one with a at bore outlet surface, but slitted polymer paths towards the center of the spinneret and the other with extruded bore needles and (Figure 6 A). 17 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 18 Figure 6. In uence of the needle size on ber channel integrity. A) CAD rendering of the spinneret without extruded needles; B) CAD rendering of the spinneret with extruded needles; C) Photograph of a ber cross- section spun with a spinneret without extruded needles (A); D) Photograph of a ber cross-section spun with a spinneret with extruded needles. We observed a more pronounced three-channel cross-section for the extruded needle geometry, as shown in Figure 6 D. Employing the at bore opening the three bore channels are connected to form one channel inside the spun hollow ber as depicted in Figure 6 C. The extruded bore needles (Figure 6 B provide a stable ow channel for the polymer dope solution in-between the bore channels of the spinneret. Therefore, this approach avoids blockage of the dope uid's pathways to the center as occurring utilizing the at openings 18 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 19 (Figure 6A, C). Thus extruded bore needles tend to be more robust against interconnection of bore streams as polymer solution is guided also between the needles. In contrast, utilizing the at bore openings there is no pathway for the polymer solution to enter the space between the bore openings again, as reported also by Wang [35] for the distance of the channels. It was found that hollow geometries at the spinneret outer surface with thin wall thicknesses of below 0:8mmare not feasible to be printed with support material that demands subsequent cleaning steps, as mechanical stress easily leads to material failure in this regions. 4.3. In uence of rotation on tri-bore ber morphology Figure 7 depicts CTimaging, of the ber structure. Figure 7 A shows the ber ma- terial of a short sample detected by CTimaging. One can clearly see the cross-section and the three channels, that are rotating around one another. In Figure 7 B and C the lumen free volume is coloured in blue to visualize the ow channels. Figure7 C is taken from a long ber sample, revealing more information of the whole ber with potential produc- tion instabilities. This CTmeasurements also reveal, that certain chaotic distortions lead to radial channel interconnection (C). We account the characterization of hollow ber membranes via CTsuperior over FeSEM imaging in terms of integral ber properties. Utilizing this techniques in material studies could lead to a better understanding of hollow ber properties in the future. Figure 8 shows the cross-section of three tri-bore bers spun with variation in rotational speed. Figure 8 Ashows a ber, that was spun with no rotation, whereas Figure 8 B and 8 C represent bers spun with 30 RPM and 65 RPM respectively. 19 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 20 Figure 7.-CT imaging of rotating multibore hollow bers spun with 30 RPM rotation of the spinneret. A) Cross-section of a short sample, B) Cross-section of a short sample with colored free volume, C) Colored free volume of a long sample. For all samples a clear tri-bore structure is visible without channel interconnections. Macrovoids are located between the lumen channels towards the outside of the ber. Further, nger like macrovoids are located near the ber outer region. The inner and outer separation layers are not penetrated by the voids. Fibers spun with 30 RPM (Figure8 B) show a 20 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 21 morphology, comparable with the bers spun without rotation. Slight deformation of the lumen structure is visible for the sample spun with 65 RPM (Figure8 B), as one would expect for a rotating co-extrusion due to shear stress at the spinneret opening. The outside diameter of the rotating and non-rotating tri-bore bers show no signi cant di erence and in the nal bers rotational pitches were evaluated to be 23 :3mmfor the bers spun with 65 RPM, being in good agreement with the theoretical value of 27 :5mm. If one applies laminar ows to these bers with Reynolds numbers of Re= 200, Dean numbers of De200;30RPM = 71 andDe200;65RPM = 89 are calculated. Figure 8. Cross-sectional FeSEM images of tri-bore bers spun under rotating conditions. Fibers are spun with 0 RPM, 30 RPM and 65 RPM, respectively. The background was adapted (blackened) for better visibility of the inner ber channels. Original FeSEM Images of the bers can be found in the supplementary material. 4.4. Helical tri-bore hollow bers (PES- and titanum-based) The resulting helical tri-bore hollow bers are displayed in Figure 10 and Figure 11 for classical polymer solution based PES bers and metal-polymer slurries based bers resulting in solid metallic monolithic membrane after thermal post-processing, respectively. 21 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 22 Figure 9. Multibore spinneret for helical and at tri-bore bers. Comparison of A) CAD rendering of the spinneret and B) Photograph taken after printing of the spinneret. 4.4.1. PES based helical multibore bers The classical polymer solution based PES multibore bers were spun with 0 RPM to form at tri-bore bers and with 30 RPM to form helical tri-bore hollow bers. Figure 10 A shows a comparison of both geometries. Figure 10 B shows the cross-section (spun with 0 RPM), presenting a similar macro-void distribution as polymeric tri-bore hollow bers (compare Figure 8). A dense skin formation can be observed on the lumen side and the outer surface. Gradual pore size increase from the skin towards the polymer matrix is due to di erent solvent exchange rates. When exposed to the non-solvent, the phase inversion process takes place rapidly but becomes more and more hindered due to di usion boundaries. The inner bore channels evolve with equal size with slight deformation. The arrangement of bore needles in the spinneret promote the formation of stabilizing grooves on the outer side of the ber. This e ect is enhanced when rotation is applied. The formed grooves show resemblance to buckling e ects of polymer layers on soft substrate [48]. The rotational 22 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 23 pitches were evaluated to be 6 :50mm with the spinning parameters applied (Table 3). Figure 10 C shows a uniform pitch along the whole ber. The theoretical pitch was calculated topitchth= 6:90mmand shows good agreement with the actual value. Figure 10. Helical tri-bore bers PES based: A) Comparison of sample spun with 30 RPM (left) and sample spun with 0 RPM (right) B) Cross-section of sample spun with 0 RPM. The picture is composed of two seperate images with the same magni cation to capture the whole cross-section. C) Side view of the sample spun with 30 RPM showing a uniform pitch along the whole ber length. 4.4.2. Metal based helical multibore bers The metal-polymer slurries based bers resulting in solid metallic monolithic membranes were spun with 0 RPM to form at tri-bore bers and with 30 RPM to form helical tri-bore 23 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 24 hollow bers. Figure 11 A shows a comparison of both geometries in the green form and after thermal post processing. The stable helical and at geometry maintained its shape after extrusion and after consecutive thermal post-processing. The green- ber is subjected to shrinkage during the thermal post-processing step. Shrinkage occurs due to combustion of the binding polymer of the green- ber as well as the progressing sinter stage. The di erence in structure can be observed in the SEM image of Figure 11 B showing the cross-section of the green- ber (bottom) and ber after thermal post-processing (top). Corresponding larger magni cation depicted in Figure 11 C shows the titanium particles embedded between the PES polymer binder and in Figure 11 D the ber after thermal post-processing with clearly visible sintered particles without polymer binder which form a solid porous titanium matrix. The rotational pitches of the bers were evaluated to be 6 :50mm for the green- ber with the spinning parameters applied (Table 5). A uniform pitch along the whole ber can be observed after extrusion as shown in Figure 11 A. The form of the pitch is maintained after the thermal post-processing step, and reduced due to shrinkage to 6 :30mm. The theoretical pitch was calculated to pitchth= 5:65mm. Comparing the spinning parameters and the pitch developed, we experienced that the spinning system is very sensitive towards small changes in parameters and material systems. The curved geometry allow for the calculation of Dean numbers. If one applies ow inside the channels with Reynolds numbers of Re= 200, the Dean numbers were calculated to De200;30RPM = 105 and De200;30RPM = 199:9 for the PES based and sintered bers respectively. 24 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 25 Figure 11. Helical tri-bore bers titanium-based: A) Comparison of green- bers and bers after thermal post-processing spun at 0 RPM and 30 RPM, respectively. B) SEM image of the cross-section of the green- ber (bottom) and ber after thermal post-processing (top) C) Magni ed image of the green- ber showing the Titanium particles with polymer binder D) Magni ed image of the ber after thermal post-processing with clearly visible sintered particles without polymer binder. 25 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 26 4.5. Channel integrity Figure 12 shows the in uences of spinning process parameters on the key geometric prop- erties of the helical multibore ber. Typical machine parameters, such as volume ows, air gap distance and pulling speed act on the channel dimensions as in hollow ber production with simple bore. The key di erences are to be found in the channel integrity and being in uenced by multiple parameters. In this regard a core aspect gured during the conduc- tion of the here presented experiments is the interplay of phase separation, viscosities, and spinneret design. All aspects are in uencing the channel integrity, which is the stability of ber design during fabrication. If, for instance, the ow of bore uid is not constant in all channels of the spinneret, bore channels with di erent diameters evolve. Additionally, multiple bore channels tend to combine to one channel if the geometry is not stabilized. The stability can be generated by two means. First, fast phase separation already takes place in the air-gap region, which is also coupled to the risk of blocking of the spinneret causing ber rupture. Secondly, an enhancement of the bore uid's viscosity hinders mobility of the polymer solution - bore liquid interfaces. Furthermore, the spinneret design is a key aspect to stabilize multiple channel geometries, as explained in Section 4.2. Besides the amount of round channels, the spinneret design also enables the formation of structured channels, as investigated by C  ulfaz [24]. Nevertheless, our approach o ers a more exible route to geometry development compared to the form-de ning spinneret parts being produced employing laser ablation. A combination with the rotational ber production approach would nally o er a way to not only increase speci c surface area of lumen channels but also go beyond an alignment that is parallel to the main direction of ow, thus o ering ow disturbance. 26 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 27 Figure 12. Spinning related in uences on the ber structure during rotating spinning. 5. Conclusion In this manuscript we present a hands-on toolbox for spinneret and hollow ber mem- brane development. We show 3D printing applications in spinneret design and construction for di erent available printing materials as well as a validation of stability against NMP as a typical solvent in phase inversion membrane formation. Examples of iterative spinneret 27 T. Luelf et al. / Journal of Membrane Science 00 (2018) 1{32 28 optimization with a variation of ber geometry are presented for polymeric bers. The produced multibore bers have a stable 3D geometry with separated lumen channels. In addition we take the concept of spinneret design to another level by combining the typically implemented co-extrusion with the design of a rotation assembly. The resulting bers com- bine a multibore approach with high packing density with a ber lumen rotation enabling dean vortice formation during operation. Subsequently, we applied the rotating spinning method to titanium loaded polymer solutions. The phase separated green- bers preserve the spiralling multibore structure during thermal post processing. This enables the develop- ment and production of complex 3D structured metal bers for electrochemical membrane processes. With this approach we are able to fabricate bers with rotating inner or outer geometries. Thus, on the basis of our research one is able to tailor both the membrane inner and outer mixing properties to a separation task. Acknowledgement M.W. acknowledges the support through an Alexander-von-Humboldt Professorship. This work was performed in part at the Center for Chemical Polymer Technology CPT, which is supported by the EU and the federal state of North Rhine-Westphalia (grant no. EFRE 30 00 883 02). This project has received funding from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (grant agreement no. 694946) and German Federal Ministry of Education and Research (BMBF) under the project Tubulair (03SF0436B). The authors would like to thank Karin Faensen for her help with sample preparation and electron microscopy.