How to design, develop and build a fully-integrated melt electrowriting 3D printer

Melt electrowriting (MEW) is a high resolution material extrusion based additive manufacturing process with the unique capability to produce micron to sub-micron diameter fibres and control their deposition in three dimensions. Here, we detail the design, development and build of a custom MEW printer which has full integration between all core components. We detail the design and build of the motion system, print head (including heaters, extrusion system and high voltage power supply) and safety systems


Introduction
Melt electrowriting (MEW) is a high resolution additive manufacturing process which utilises a high voltage between a print nozzle and collector to stabilise the flow of print material, allowing precise controlled deposition of micron to sub-micron diameter fibres. This technique was first demonstrated in 2011 [1,2] and has since gained substantial popularity and widespread use, in particular for the development of tissue engineered scaffolds [3,4]. Its use for tissue engineering applications is popular due to the high porosities and low fibre diameters achievable by MEW fabricated structures, enabling the fabrication of complex microarchitectures tailored to engineering specific biological tissues. In addition, the technique has demonstrated versatility for other industrial applications such as manufacturing embedded microchannel systems [5] and glass chips for applications such as optics, photonics and "lab-on-a-chip" devices [6].
While this technology is still in its relative infancy, MEW has undergone rapid growth and development since its inception. Indeed, the pre-cursor in the development of MEW technology entailed the manual movement of an x-y collecting stage to control the deposition of fibres [7], while present day technology has progressed to the development of highly complex printers such as those with expansive automated systems [8] and machine vision tools [9] for real time altering and monitoring of process variables. There have been several other key milestones within this developmental history. It has been demonstrated that tubular scaffolds can be fabricated using MEW by printing on a rotating mandrel [10], with later work developing an open source software to further enhance ease of use [11]. The printing of fibres with sub-micron diameters has also been demonstrated, further expanding the design space within which scaffolds may be fabricated [12]. Key developments in MEW technology entailed the incorporation of in-process control of print parameters, namely voltage to allow the fabrication of large volume scaffolds exceeding 7 mm in height [13], and pressure to allow variable fibre diameters within a single scaffold and print job [14]. These latter developments require the use of complex printers with integrated motion, pressure (for material extrusion) and high voltage systems which can have variable pre-programmed outputs throughout the duration of the print job. Other key developments include the development of high throughout, multi-print head systems to scale up manufacturing [15], and the development of cell electrowriting technology to print cells within hydrogel microfibers [16]. There are several key components within a MEW printer, including the motion system to define fibre deposition, heating system to melt the print material for deposition, extrusion system to dispense the material though a nozzle/needle, high voltage system to stabilise the print jet, and safety systems due to the various hazards of MEW, in particular the high voltages involved. While several companies have recently developed and began to provide commercially available MEW printers, many researchers continue to use custom built systems. Firstly, due to the relative youth of the technology, researchers previously had no choice but to build custom MEW printers in absence of any commercially available units. In addition, building a custom MEW printer is more cost effective, provided that sufficient expertise and time are present to undertake this process. The greatest argument in favour of implementing a custom built MEW printer, however, lies in the flexibility to design a system specific to the requirements and complexities of the end user in question. Finally, it is worth noting that several commercially available MEW printers are in essence modified 3D printers with MEW print heads attached, which were initially designed for other additive manufacturing technologies. Such systems therefore lack some of the capabilities and features present in custom MEW printers which have been designed from the ground up with MEW printing in mind. End users should also be aware of the potential performance and reproducibility issues some of these systems have and perform due diligence if selecting a commercially available option.
When developing a custom MEW printing system, there are a range of possibilities available, from more simple and inexpensive systems which are relatively simple to implement [17], to complex fully automated systems with real-time process monitoring [18]. More simple designs consistent of discrete systems, for example, a separate high voltage power supply, pressure source or syringe pump, motion system and heating system, all of which are controlled manually and independently. Such a system can be implemented relatively quickly, for example, by utilising a commercial 3-axis computer numerical control (CNC) gantry, high voltage power supply, and components for a print head (including heater(s) and a pressure source). In any case, pneumatic pressure is recommended over a syringe pump or screw extruder as it provides a much smoother flow profile which is crucial for forming a consistent polymer jet and minimising instabilities. The print head is perhaps the most difficult system to develop, as the polymer must be melted while simultaneously applying a high voltage between the nozzle and print substrate. Electrical interference and arcing between the high voltage supply and the heating system is a potential issue here. This can easily be overcome by using heated air [17,19] or water [1] for example, however, these methods can cause issues in terms of slight temperature fluctuations and jet instabilities, or limited melt temperatures achievable. Here, we recommend the use of resistive heating elements, which can be electrically isolated from the high voltage supply as outlined in our methods below. Alternatively, the high voltage can be applied to the print plate while the nozzle can be grounded, a strategy which has successfully been used by other researchers [20], in addition to commonly being used in commercial MEW printers. Finally, the level of component integration is important to consider, and at the very least, we recommend integration between the motion system and the high voltage system if scaffolds above a height of approximately 1-2 mm are required. This allows voltage to be incrementally changed throughout the print job, which is necessary for producing high quality large volume scaffolds [13]. Further integration of other components such as the pressure source will allow for the formation of more complex structures with variable fibre diameters. Integration of the heaters primarily aids in terms of ease of use, as they can be shut down automatically at the end of the print job. In the following work we focus primarily on the hardware and design challenges when developing a custom MEW printer, however, there are other challenges associated with using a MEW printer. Thus, we recommended the following excellent articles, including a video and written tutorial on using MEW printers [21], a review on some of the materials challenges faced when MEW printing and the many materials which have successfully been used for fabricating melt electrowritten structures [22], and some publications which discuss the history and future potential of MEW [3,4,23].
In this work, we detail the complete design and build of a custom MEW printing system, with two almost identical printers (one with a fixed collector and one with an easily removable collector which can be interchanged with a rotating mandrel) being built as outlined. These MEW printers feature advanced capabilities which are not present in the majority of commercially available systems, including the complete integration of all key components which gives the user outright control over processing parameters for the fabrication of a wide range of biomaterial scaffold designs. We detail the development of the framing, motion system, print head assembly (including control over print temperature, extrusion pressure and high voltage), a rotating mandrel for tubular scaffolds, and comprehensive information on the wiring and integration of all these components to a central control unit. We then demonstrate some of the key capabilities of these printers, including the ability to print large volume scaffolds with layer by layer voltage control, the printing of nanoscale fibres, hybrid printing capabilities combining both fused deposition modelling (FDM) and MEW additive manufacturing principles, variable fibre diameter scaffold fabrication via layer by layer extrusion pressure control and automated fibre stabilisation, printing of complex geometries via DXF (Drawing Exchange Format) files, and finally the printing of tubular MEW scaffolds. We also discuss numerous alternative configuration possibilities and varying levels of component integration depending on the requirements of the end user. This work aims to provide a comprehensive guide for anyone considering the implementation of a custom MEW printer, and additionally, to provide a detailed resource for those looking to learn more about this fascinating and powerful additive manufacturing technology.

Overview of custom MEW design
The MEW printer described here is comprised of a 4-axis motion system (3 main axes in XYZ directions and a 4th motion axis which may be used for a rotating mandrel), a print head assembly (comprised of heaters to control material temperature and a system for high voltage application), digital pressure controller, high voltage power supply and various safety systems. Two such MEW printers were built, with one being show in Fig. 1A. All of the systems within the MEW printers are fully integrated and controlled through a central "main control unit", as shown in the general control schematic in Fig. 1B. Custom parts were designed with PTC Creo Parametric 3.0 computer aided design (CAD) software. Wiring diagrams were created with QElectroTech 0.80 software.
In the below Materials and methods sub-sections, we will outline each of the components within this custom designed MEW in greater detail. While all components are fully integrated in this system, it is important to note that this is not essential, and more simple configurations may be built according to the needs and cost requirements of the user. For example, in a previous study, the authors built a low-cost custom MEW with separate systems, with the exception of the high voltage power supply which was connected to a safety interlock [17]. Other examples and possible configurations will be discussed in greater detail in the Results and discussion section further below.

Framing and motion system
The chassis and frame was built using Bosch Rexroth 45 × 45 mm square aluminium profile. This provides a stable and rigid structure upon which other components can easily be mounted using the integrated attachment grooves. Fig. 2A illustrates the construction of the frame and various fittings. A magnetic latch was used to hold the door closed ( Fig. 2A-i). Ethylene propylene diene monomer (EPDM) rubber strips were affixed to the exterior of the frame and polycarbonate panels of thickness 5 mm bolted on to ensure effective sealing ( Fig. 2A-ii). External angle brackets were used to securely support the printer gantry ( Fig. 2A-iii), while inner brackets were used in all other joints ( Fig. 2Aiv). Adjustable levelling feet with damping rings were mounted underneath the frame ( Fig. 2A-v).
The design concept for the print plate assembly is illustrated in Fig. 2B. This assembly consists of two motors and two linear slides (defining the XY motion of the printer), with a print plate mounted on top using either 4 pins or screws. Of note are the spacer block, which provides clearance for the print plate over the motor housing, and the insulating plate, which further isolates the aluminium plate from the linear slides and surrounding electrically conductive components in the chassis. This is achieved by the thickness of the insulating plate, and its larger outer dimensions of 230 × 230 mm (compared to the outer dimensions of 210 ×210 mm for the aluminium plate), giving a 10 mm insulating buffer region all around the aluminium plate. Nylon screws are further used to attach the insulating plate to the spacer block, as seen in the exploded assembly view in Fig. 2 C. This design helps to completely isolate the grounded aluminium plate from all surrounding electrically conductive components, ensuring that a stable ground is defined upon which to perform MEW printing. This design allows an inverse configuration whereby the user may safely apply the high voltage to the print plate (and ground the nozzle) while avoiding electrical arcing to surrounding components. An image of the entire print plate assembly is shown in Fig. 2D, also illustrating the mounting of a USB microscope at the side of the frame for process visualisation and imaging of MEW fibres. In one of the MEW printers built, the aluminium plate is mounted using pins to allow quick removal of the plate for attachment of a rotating mandrel (Fig. 2D). In the second MEW printer, the print plate is mounted using countersunk bolts (Fig. 2D, inset).
It is important to consider the leadscrew pitch and lead for each axis in a MEW printer. The pitch is the distance between two adjacent threads, while the lead is the distance the carriage moves for one full rotation of the leadscrew (Fig. 2E). A high lead allows higher maximum linear speeds to be reached at the expense of resolution, while a lower lead results in a lower maximum speed but greater resolution ( Fig. 2E-i). For the 2 slides defining XY motion, an intermediate to high lead is desirable, with a Velmex E02 leadscrew being chosen here, which has a lead of 0.2 ′′ (5.08 mm) and pitch of 1.27 mm. This is due to the fact that relatively high translation speeds are required to print straight fibres, particularly for lower fibre diameters. The XY slides have a four-start leadscrew design, meaning that there are four intertwined threads running parallel to one another, where the lead is 4 times the pitch (Fig. 2E-ii). This allows greater leads to be used without compromising contact area between the leadscrew and the carriage nut. For the Z slide defining vertical movement of the print head, resolution is a much greater consideration and high translation speeds are not required. Thus, a low lead is desirable here allowing for the print head to be moved in tiny increments of several microns or less. Here, a Velmex E25 single start leadscrew was chosen, with a lead and pitch of 0.025 ′′ (0.635 mm). These leadscrews were mounted within Velmex XSlide assemblies with integrated limit switches. Arcus NEMA 23 stepper motors with a step angle of 1.8 • (200 steps per rotation) were mounted to each of the XYZ linear slides. A microstep setting of 50 was further applied to each motor to reduce the effective step size and further allow smoother motion. Framing and motion system assembly components are listed in Table 1. Technical drawings for custom motion assembly parts are included in the supplementary information.

Print head assembly
The print head assembly is comprised of components responsible for 3 primary functions: (1) application of heat to melt the print material, (2) application of pressure to extrude the print material through the nozzle and (3) application of a high voltage for the stabilisation of micro/nano-fibrous jets for MEW. A similar configuration was used to that from a custom MEW in the Hutmacher lab [24], whereby a tube heater is used to heat the syringe barrel and a ring heater is used to heat the nozzle. An exploded view demonstrating the design of the main print head assembly housing the heaters is illustrated in Fig. 3 A, with the inset illustrating the assembled print head. Fig. 3B shows the actual exploded view of the main print head components with the inset again showing the assembled parts. Ketron® PEEK was used for the upper and lower housing components, which is rated for a 250 • C continuously, while the print head mount was fabricated using a Formlabs 3D printer. Nylon bolts were used to attach the upper and lower housing components to eliminate any possible interference with the high voltage Table 1 Components for framing and motion system. applied to the nozzle. The main obstacle when designing a MEW print head is eliminating any potential interference and electrical arcing between the high voltage applied to the nozzle, and the heaters (or any other electrically conducting components) in the print head. To help mitigate this, an aluminium oxide ceramic tube heater (Rauschert GmbH, Germany) and silicon nitride type HI ceramic ring heater (BACH Resistor Ceramics GmbH, Germany) were used to heat the syringe barrel and nozzle respectively. Nonetheless, there are still metallic components within these heaters and within the wiring and thermocouples, and thus, further measures were taken to physically isolate the nozzle from the heaters. This was done in the form of a PEEK isolator ring designed to fit tightly around the circular plastic manifold at the interface between the nozzle and Luer-Lock connector ( Fig. 3 C, Fig. 3C-i). Voltage is applied to a brass ring which fits tightly around a 22 G nozzle/needle and contains a bore in the side to plug in the high voltage lead with a banana plug soldered to the end (Inset, Fig. 3D). The ring heater, isolator ring and high voltage ring can be seen in a plan view of the assembled print head ( Fig. 3C-ii). A limit tab on the print head mount is designed to contact the limit switch on the end of the linear Z-slide when the nozzle is 0.5 mm from the build plate (Fig. 3E). This ensures no collisions can occur between the print plate and the print head, and also allows a datum program to be run to calibrate the axes and set the nozzle distance to a known value before printing. A temperature control unit with 2 proportional-integral-derivative (PID) controllers are used to define temperatures of the tube and ring heaters (Fig. 3 F). The internal design and wiring of this unit will be outlined in further detail in the "Wiring and integration of components and safety systems" section below. A PID digital pressure controller mounted to the printer gantry was used to control air pressure to extrude the print material ( Fig. 3 G). A mains air pressure supply of 5 Bar is fed to the inlet, while a T-splitter fitting at the output goes to a 3 ml syringe containing the print material in addition to an analogue pressure gauge. This pressure gauge gives the user a rough indication of the output pressure to visually verify that the pressure is set as desired. The PID unit is controlled digitally via the main control unit to give a precise output pressure, with details of this being outlined further in the "Wiring and integration of components and safety systems" section below. Print head assembly components are listed in Table 2. Technical drawings for custom print head assembly parts are included in the supplementary information.

Rotating mandrel
A rotating mandrel was built to allow for the fabrication of tubular scaffolds. An exploded view of the design is illustrated in Fig. 4 A, with main considerations of the design being a long mandrel for the fabrication of multiple or large scaffolds, and a system by which to easily change mandrel sizes. An offset stepper motor which drives the mandrel via a belt allows the mandrel length to be maximised within the space limitations of the MEW enclosure. The belt is tensioned by sliding the motor and fastening it on the 4 mounting slots. In addition, the system of jaw couplings allows the mandrel to be quickly changed by loosening and tightening a single grub screw on a shaft collar between the bearing mounts at the left hand side of the mandrel in Fig. 4 A. A print plate is also mounted on the mandrel chassis at a similar height to the mandrel, allowing space for initial fibre stabilisation before printing. A photograph of the assembled mandrel is shown in Fig. 4B with another view clearly showing the drive belt in Fig. 4 C. The mandrel is grounded using a wrap-around copper braid with good electrically conductive contact being maintained with the small grounded aluminium print plate via spring tensioning (Fig. 4D). Fig. 4E shows the mandrel mounted on the same spacer block used for the print plate with the USB microscope being present on the left for process visualisation. A view from the USB microscope of a tubular scaffold being printed is shown in Fig. 4 F. Mandrel assembly components are listed in Table 3. Technical drawings for custom mandrel assembly parts are included in the supplementary information.   (Fig. 5I). This was done to keep connectors small on the print head side to allow ease of complete assembly and disassembly of the print head. A proximity switch is mounted to the inner side of the enclosure as a safety interlock for the high voltage power supply ( Fig. 5 J), as detailed previously. Components used in the main control unit and temperature control unit are listed in Table 4 and Table 5 respectively. Wiring diagrams are included in the supplementary information.

Programming and scaffold fabrication
Programming to integrate components is primarily carried out using a form of BASIC programming language called TrioBASIC, which controls the motion/output controller in the main control unit from the "Motion Perfect v4.3 ′′ software. Initial set up of all axes is first carried out with the "1_initialise_system.txt" program. This program is set to run automatically on software start-up, after which the "2_zero_stage.txt" program may be run to home all axes and set the print head to a defined height above the collector. Programming for the initial parameters for the auxiliary outputs is done with the "3_set_auxial_variables.txt" program. These programs are included in the supplementary information. Of note here is the "DAC AXIS(X)" command, which is a "digital-toanalog converter" command to convert a numerical value to an output voltage which specifies how an external item of equipment should behave. For example, considering the below series of commands: pressure = 1.0 DAC AXIS(4) = (pressure/10) * 2048 current of 40 mA using a Cressington 208HR sputter coater. Imaging was conducted in a Zeiss ULTRA plus SEM with an accelerating voltage of 5 kV, with the SE2 detector being used for lower magnification imaging to give good topological information and the InLens detector being used for high magnification imaging.

MEW design
The configuration of the custom MEW printer reported here, with full integration between all of the core components, allows for a high degree of dynamic control over print parameters throughout a print job, allowing for the fabrication of a range of complex architectures. With this configuration, voltage may be regulated throughout the print to fabricate MEW scaffolds with a high thickness [13], and pressure may be controlled throughout the print to fabricate complex architectures with variable fibre diameters [14]. Controlling all components from a central unit also has several practical advantages. For example, all components can be programmed to automatically shut down at the end of a print job, which is advantageous for MEW due to the sometimes long printing times ensuring the user does not need to be present at the end of the print. Indeed, once all physical switches are powered on, the MEW can also completely be controlled remotely via another computer. This integrated configuration is also advantageous from a safety standpoint, allowing any of the components to easily be linked to the E-stop and safety interlock. A general operating procedure is included in the supplementary information (Operating procedure for trained users.docx).

Motion system
The MEW system reported here uses mechanical bearing linear slides as the foundation of the motion system, a similar configuration to that used by several different research groups to produce custom MEW printers [12,21,25,26], with stepper motors often being used to drive these slides as is also the case in the present study. The popularity of this configuration lies in the relatively low costs and servicing requirements associated with it, while also being capable of achieving a high degree of positional accuracy and repeatability. Stepper motors are primarily run in an open-loop configuration, relying on pre-defined step counts to define the amount by which to rotate the motor without positional feedback. Nonetheless, these motors are highly capable for MEW applications and in our experience provide exceptional accuracy with no issues even after long, multi-day print jobs. Servo motors have also been used in custom MEW printers [27], and provide additional positional feedback as part of a closed-loop system via the use of encoders to verify and ensure positional accuracy. Various approaches have also been taken for controlling rotating mandrels for the fabrication of tubular scaffolds, with a stepper motor being used in the present work and DC motors being used by others [11,28]. Another good approach which may be taken is to purchase a ready-made motion system, such a multi axis CNC gantry system, and use this as the basis for a custom MEW printer.
Perhaps more important is the design of the system to take into account the differing requirements for the X-Y print plate in comparison to the vertical Z movement of the print head. As detailed previously, the X-Y slides in this work have a larger leadscrew lead value to facilitate the high translation speeds which are required in MEW to print low to submicron fibres above their critical translation speed. In contrast, the Zslide has a much lower leadscrew lead value, prioritising high accuracy and the possibility of extremely low translation distances, which allows for the print head height to be incrementally increased by the thickness of a MEW fibre after each layer of printing if desired. Indeed, depending on the requirements and applications of the user, a motorised Z axis may not even be required [29], however it is advised if there is a requirement to print scaffolds greater than approximately 1 mm in height.
Finally, the configuration of the 3-axis print system also has implications in terms of space restrictions and enclosure size. The MEW printer reported here, as with many other custom systems, has the print plate mounted on an X-Y stage, with the print head moving in the Z direction. This is a relatively simple and intuitive design, however, if a smaller space footprint is required, the print head could be designed to move in 2 -3 axes, with the print plate moving in either 1 axis or remaining static. In the printer reported here, the print plate has dimensions 21 × 21 cm, with the footprint of the enclosure being 60 × 60 cm to allow for movement of the plate while allowing buffer regions from the framing to electrically isolate the plate and reduce potential pinch hazards. The smaller footprint of the print head vs the print plate means that designing for the print head to move in additional axes would reduce the overall enclosure size required for the same size print plate, at the possible expense of a more complex design.

Print head configurations
One of the main challenges with building a custom MEW printer is designing the print head such that the print material can be heated while implementing a high voltage (usually several kilovolts, in the range of approximately 1-10 kV depending on nozzle distance) between the nozzle and collector. The high voltage, if applied to the nozzle, has the propensity to arc towards nearby electrically conductive materials, for example, the electrical circuitry in the heaters. This is due to the dielectric strength of air, which between flat electrodes has a value of 3 kV/mm [30,31], meaning that above this threshold the insulating properties of air can break down and allow a flow of charge. In all commercially available MEW printers to date, this issue is overcome by applying the high voltage to the collector and grounding the nozzle. In certain cases this may not be desirable however, and may cause issues with arcing, for example, if conducing MEW printing onto a non-planar or irregularly shaped conductive objects or printing on electrically sensitive substrates. In addition, while safety controls are of the utmost importance and should mitigate the possibility in either case, it is easier to accidently come in contact with residual charge on a large print plate at high voltage in comparison to a nozzle at high voltage in the print head. In this work, we have designed the system such that high voltage is applied to the nozzle with the print plate being grounded, with several measures being taken to mitigate electrical arcing within the print head. Firstly, ceramic heaters have been used, and while these contain embedded electrically conductive materials and several locations with exposed electrical connections, they provide a robust initial barrier to electrical arcing. Secondly, exposed electrical contacts and wires have been insulated as best possible using sheaths and heat shrink tubing. And finally, we implemented a physical barrier in the form of a 2.5 mm thick polyether ether ketone (PEEK) isolator ring which fits tightly around the plastic manifold of the syringe nozzle and sits between the nozzle and the heaters. The much higher dielectric strength of 24 kV/mm of Ketron PEEK compared to air thus greatly increases the threshold above which arcing may potentially occur. This allows the maximum voltage of 20 kV to be reached in the system without any issues, as has been experimentally verified. Other controls to apply heat to the syringe while overcoming arcing are also possible, such as using a water jacket heating, [1], air heating [17,19] or oil heating [20] approach. Using water however limits the maximum temperature achievable, and in our experience, using air does not provide as stable a temperature and consistent a fibre as using resistive heating elements. We have also designed the present system such that an inverse configuration of high voltage at the print plate and ground at the nozzle is possible if desired.
Here, the 210 × 210 mm aluminium print plate sits on a 230×230 mm Ertacetal isolator plate with a 10 mm buffer region all around and thickness of 20 mm, thus allowing ample space all around to overcome the potential of arcing to surrounding objects such as the framing. Aluminium was used for the print plate due to its lightweight nature, however, other conductive materials may be used, for example, steel [14,32] or copper [33]. In addition, the use of Ertacetal as a material for the isolator plate is not critical, and other electrically insulating plastics may be used.
In the print head, we have used two heaters, with one heating the syringe barrel and one heating the nozzle, as has also been reported previously in custom built MEW printers [21,34]. This allows the bulk of K.F. Eichholz et al. material in the barrel to be kept at a lower temperature and reduce thermal degradation, while maintaining a higher temperature in the more constricted manifold and nozzle zone where blockages are more likely to occur with insufficient heating. Even in the case with highly stable materials such as PCL, we have found that keeping the temperature of the barrel approximately 5 • C lower than the nozzle results in a more consistent fibre than equal temperatures or a greater temperature in the barrel heater. If the high voltage will be applied to the print plate only with the nozzle always being grounded, a simpler and more inexpensive alternative may be to implement a metallic barrel with a standard resistive heating element, as is done in several commercially available MEW printers.
The material of the print head housing is also of importance to ensure stability at the required temperatures. The Ketron® PEEK used here is stable at up to 250 • C continuously, however, if only lower processing temperature materials such as PCL will be used, a more inexpensive material or 3D printed print head housing can be implemented. In this work for example, initial testing of the print head housing design was conducted via a 3D printed version using a commercial 3D printer (Formlabs). For the print head mount, we have used High Temp resin (Formlabs), which was used for its temperature resistance and ease of manufacture via 3D printing. We have experience with this material being stable in this particular use case for extended periods of several years, and ongoing. In certain use cases, for example, if the part will be exposed to UV light for extended periods or if additional loads are placed on the print head, ageing and brittleness of the part may be an issue and other materials may be more appropriate. Alternatively, this mount could also be machined using PEEK, or 3D printed in another material, taking care that its temperature resistance is high enough for desired processing temperatures of the polymer of choice for MEW. The top of the ceramic tube heater does not get as hot as its core temperature, thus, the print head mount does not need excessive temperature resistance.
Finally, the extrusion method used is important both in terms of achieving a consistent flow rate and MEW fibre, and also in terms of limiting degradation of the material. Much of the early research with MEW incorporated syringe pumps to provide flow [10,35,36], however, due to the extremely low flow rates of 5 -50 μL/hr typically used, getting a consistent flow at this rate is difficult and introduces instabilities into the MEW fibre. The majority of recent work uses pneumatic pressure to extrude material. Air pressure was used in the present work, while nitrogen has also been used [22,37], which as an inert gas may be more desirable to reduce material degradation depending on the material being processed. In the present work, a PID digital pressure regulator was used to give fine control over output pressure while also allowing dynamic pressure changes within the program. Alternatively, while not as precise, favourable results are also possible with manually operated pressure regulators if dynamic pressure changes are not required [17,38].

Considerations for component integration and MEW safety
In the MEW design reported here, all of the core components are fully integrated and linked to a central "main control unit". Namely, the motion system, pressure regulator, high voltage power supply and heaters are all linked, allowing a great deal of dynamic control over  MEW printing. This also provides additional ease of use benefits, such as automating start up sequences and beginning a print automatically, automatically shutting down all components after a print, and allowing remote control over the MEW via the main software interface from an external computer. It is also possible to build a highly capable MEW printer with discrete components which are not interconnected, or to design a MEW printer with any level of component integration in between. For example, if large height scaffolds are desired, it is a necessity to have some form of dynamic voltage control to increase voltage as the print head rises throughout the scaffold height. If variable fibre diameters or hybrid FDM/MEW printing is desired, dynamic pressure or flow rate control is advantageous to vary material flow rates. However, K.F. Eichholz et al. it is worth noting that a certain degree of dynamic fibre diameter control is also possible using only velocity changes to mechanically stretch the fibre, as has previously been reported [14]. With regard to the heaters, there are limited cases where temperature changes may be required throughout the print, and thus in the present work, initial temperatures are defined and only the power to the heaters is controlled from the main control unit via a relay. This allows the heaters to be shut down automatically when a print is complete. Regardless of the level of complexity of component integration chosen, it is paramount that no effort is spared on implementing effective safety systems. The main concern here is in relation to the high voltages required by MEW. In this work, a magnetic safety switch is mounted to the inner surface of the door and frame, with this being done to eliminate the possibility of overriding the switch should it be mounted on the outer door surface. An emergency stop button is also mounted to the main control unit, with both this and the safety switch being connected to a safety contactor which controls mains electrical power to the high voltage power supply. Also of importance is that the high voltage output does not automatically turn on again if the door is closed or emergency stop button reset, and must be manually switched on again on the high voltage power supply panel. In this work, the emergency stop button (but not the door safety switch) is connected to the motors also, with a similar restart procedure whereby if the emergency stop button is pushed and reset, motion does not automatically re-start, and a reset button must first be pressed on the main control unit. Depending on the requirements of the end user, the magnetic safety switch could also be connected to the motors such that motion is stopped should the door be opened.

Printing capabilities
Here, scaffolds are primarily fabricated using TrioBASIC programs as outlined in the materials and methods section above. Fig. 6A-B illustrates views from the USB microscope showing scaffolds being fabricated, while Fig. 6 C shows a stereomicroscope image of a typical MEW scaffold. The capability to fabricate large volume scaffolds is shown in Fig. 6D, where a 5 mm high scaffold is shown, with the inset showing a 4 mm diameter cut from one of these scaffolds. The top and sides of this scaffold is shown in Fig. 6E-F, demonstrating the precise stacking of fibres. The pore structure of the top of the scaffold is shown in Fig. 6 G, demonstrating that good control over fibre placement was maintained right to the end of the print job at 5 mm high. This is important to verify for large scaffolds, and is not trivial to achieve compared to the bottom of a MEW scaffold where good control over fibre placement is more straightforward to control in the initial print phase (Fig. 6H). The capability to produce smaller diameter fibres was also verified, with a dense 4 µm fibre diameter fibrous scaffold shown in Fig. 6I, and nanofiber printing down to 350 nm being shown in Fig. 6J-K. The latter scaffolds were fabricated using a combination of low pressures and high translation speeds to draw out small diameter fibres. The ability to conduct FDM printing and fabricate hybrid scaffolds was also demonstrated (Fig. 6L-M), where 20 µm diameter MEW fibres were stacked on 200 µm diameter FDM fibres. This was done in a single print job with pre-programmed and automated powering on of high voltage and fibre stabilisation sequences before depositing MEW fibres. The ability to define variable fibre diameters via pressure changes was also shown (Fig. 6 N), with 30 µm fibres being deposited in one orientation and 10 µm in another at a 90 • offset. The capability to print more complex shapes is shown in Fig. 6 O for the printing of a meniscus shape scaffold. Finally, the ability to control the stacking of fibres to create a welldefined tubular scaffold architecture is demonstrated in Fig. 6 P.

Conclusions
MEW is a powerful additive manufacturing technology which has abundant potential for the fabrication of complex, micrometric resolution scaffolds for a range of applications in tissue engineering and beyond. Here, we have outlined the design and build of a custom MEW printer with advanced fabrication and automation capabilities. We have also discussed the relevance of the various systems used in a MEW printer, and described the various other options and approaches which may be taken depending on the specific requirements of the end user. This work aims to make MEW technology more accessible by firstly informing others interested in developing similar custom devices, and additionally, by providing an insight into the inner workings of MEW technology for potential and existing practitioners in this emerging field of additive manufacturing research.