Feature first published in the May/June 2022 issue of 3D ADEPT Mag and posted online in September 2022. Interviewee roles, product offerings and reported performance reflect that period.
Miniaturisation in electronics, biomedical devices, automotive and aerospace is driving interest in micro-additive manufacturing. Alongside the large parts that often attract attention, this feature explores processes for making small components or micrometre-scale features.
Micro-additive manufacturing, or micro-3D printing, is part of a wider set of microfabrication methods. “Additive manufacturing” and “3D printing” do not automatically distinguish industrial production from prototyping. Likewise, “high precision” is not a sufficient specification without tolerances, a measurement method and reference dimensions.
“3D microfabrication is a general term that describes all methods. The main part of this world would come from clean room fabrication such as lithography methods that are very common and widely used in MEMS manufacturing (this is a huge well-established market and the methods are very mature). There are a bunch of other 3D microfabrication methods, for example methods used for microfluidics, digital methods based on, say, e-beam litho, and many more.
High precision AM is probably not well defined and someone who has resolution of 100 microns would market it as high precision, so it very much depends on the context”, Jon Donner, the General Manager of Fabrica, a Nano Dimension company, states from the outset.
A voxel is a volume element in a digital representation; depending on the process, it may also describe a region of solidified material. Its size alone does not define part accuracy. Micro-AM targets fine features and geometries that may be difficult to produce otherwise, without making micro-injection moulding, lithography or machining obsolete.
The present exclusive feature ambitions to discuss the various ways in which micro-AM challenges the traditional manufacturing status quo. It will shed light on:
- The fundamentals and different types of micro-AM technology.
- Main advantages and areas for improvement that may help this market move forward.
- The place of these processes in vertical industries, and in the medical and healthcare industries in particular.
Fundamentals and different types of micro-AM technology
One micrometre, also called a micron, is 0.001 millimetres or 1,000 nanometres. Overall part size, minimum feature size, layer thickness, dimensional accuracy and repeatability are different quantities. They depend on process, material and measurement.
Layers of 5 µm or features of 2 µm are examples of capabilities, not a universal threshold defining micro-AM. Some processes produce submicrometre features on larger objects: a nanoscale feature does not mean that the entire part is nanoscale.
“It is hard to say but in general we consider 10 microns and below as micro–AM. Specifically one can talk about pixel size or layer height or tolerance or repeatability. For sure, if all of these are in the 1-3 micron range then it is micro-AM”, Donner explains.
Publications discuss approaches including fused filament deposition (FFD), direct ink writing (DIW), directed energy deposition (DED), digital light processing (DLP), laminated object manufacturing (LOM), electrohydrodynamic redox printing (EHDP), powder bed fusion (PBF), photopolymerization-based printing (P3DP) and laser chemical vapour deposition (LCVD). This list combines process families and demonstrators at different maturity levels; not every industrial implementation has micrometre resolution.
Photopolymerisation has an important place among the solutions discussed here. Resolution, throughput and repeatability vary by configuration. The table combines different studies: it does not establish that industrial DED is generally more precise than resin printing. Cost, throughput and material need comparison for the same application.
“The smallest feature sizes achieved by Two-Photon Polymerization extend far into the submicrometer range. This exceeds the capabilities of conventional 3D printing technologies. For example, Fused Deposition Modeling (FDM) or Selective Laser Sintering (SLS) are typically limited in their vertical and lateral resolution. The achieved feature sizes are in the range of several hundred micrometers. Resin cross-linking technologies, such as stereolithography (SLA) and digital light processing (DLP), achieve a slightly higher precision than the above-mentioned AM technologies (in the range of 5 to 10 micrometers).
However, they still have limitations in realizing small high-precision parts or structures due to their limited resolution. In contrast to these methods, 2PP enables the manufacturing of minimal feature sizes down to 100 nanometers. The printing rate of 2PP-based 3D printing processes is at least competitive with digital light projection (DLP) stereolithography. One general metric for print speed refers to the rate in voxels printed per second and is an indicator of how digital information is converted into solidified material” Jörg Smolenski, Business Development Manager at Nanoscribe adds.
The values cited by Nanoscribe describe its comparisons and systems at that time. They are not a common measurement of every machine. Minimum feature size, tolerance and throughput in voxels per second are not interchangeable metrics.
The 2020 review by Lin, Chen and Chen examines optical methods for micro-AM. Wavelength, numerical aperture, material reaction threshold and exposure strategy contribute to the result. Two-photon polymerisation exploits a nonlinear interaction; it is not simply a matter of using shorter-wavelength ultraviolet light.
Exposure control and development or post-curing steps influence final geometry and properties. An optical process alone does not guarantee better cohesion or repeatability than every other method.
That being said, the most widely known micro-AM processes include DLP, Microstereolithography (µSLA), Projection Microstereolithography (PµSL), Two-Photon Polymerization (2PP or TPP), Lithography-based Metal Manufacturing (LMM), Electrochemical Deposition and Microscale Selective Laser Sintering (μSLS).
Digital light processing (DLP)
DLP projects a light pattern to cure an area of resin. In laser-scanning SLA, a beam traces the layer. Projected pixel size, optics, resin and exposure determine the resulting features; adaptive optics is not a universal requirement for DLP machines.
Microstereolithography (µSLA)
Microstereolithography adapts photopolymerisation to small structures. Resin exposure, optical calibration, positioning and development require control. Experience with a resin printer does not replace training for the system and materials being used.
Projection Microstereolithography (PµSL)
Projection microstereolithography uses projected patterns to build layers of microstructures. Reported performance depends on optics, formulation and geometry, and should not be confused with overall build dimensions. Ceramic-filled formulations also require suitable processing, including debinding and sintering where applicable.
Lithography-based Metal Manufacturing
Lithography-based Metal Manufacturing (LMM), discussed in our interview with MetShape, uses metal powder dispersed in a photosensitive binder. Light cures the polymer phase; the metal itself does not polymerise. Green parts then undergo debinding and sintering, with shrinkage accounted for in the design.
Two-Photon Polymerization (2PP or TPP)
Two-photon polymerisation uses a femtosecond pulsed laser focused in a suitable resin. It can produce submicrometre features depending on exposure conditions and material. In this 2022 interview, Nanoscribe’s Jörg Smolenski explains the process:
“2PP unfolds its full potential at the interface between maskless lithography and high-precision additive manufacturing. The process belongs to a family of AM technologies, in which light is used to cure a liquid photoresin to create a digitally defined structure. The combination of 2PP with a robust 3D printing workflow enables multiple manufacturing scenarios. 2PP is a direct laser writing approach and works without the need of the costly generation of masks and the use of multiple lithographic steps for 3D and 2.5D microstructures. 2PP advances the microfabrication of parts on flat substrates at wafer level, but also enables to directly print complex structures on pre-existing patterns and topographies, e.g., on optical fibers, photonic chips and inside sealed microfluidic channels. 2PP requires polymer printing materials and the dedicated photoresins for an easy handling and best possible resolution and shape accuracy as well as tailor-made for different applications.
This allows to benefit from submicron features, overhanging elements, optical-quality surfaces, high-speed mesoscale fabrication, biocompatibility or high refractive index. High-precision 3D printing based on Two-Photon Polymerization is ideal for rapid prototyping of application designs to realize biomedical devices, microoptics, microelectromechanical systems (MEMS), microfluidic devices, photonic packaging (e.g. PIC), surface engineering projects and many more. Wafer handling capabilities make batch processing and small series production of 3D microparts easier than ever.”
Electrochemical Deposition
In the electrochemical deposition approach described here, an iontip dispenses a solution containing metal ions within an electrolyte bath. An electrochemical reaction reduces them to solid metal at the desired location. This route does not require debinding or sintering to consolidate the metal; rinsing, handling and inspection remain application-dependent.
Microscale Selective Laser Sintering (μSLS)
The microscale selective laser sintering (µSLS) approach described here deposits and dries a metal nanoparticle ink. A laser pattern defined with micromirrors sinters selected regions, and the cycle repeats. This research approach should not be treated as identical to powder spreading in an industrial SLS machine.
So, what are the decisive factors to consider when choosing one technology over another?
The market reviewed for this 2022 feature includes approaches at different maturity levels. Supplier count alone does not determine a process’s suitability: industrial availability, material qualification, maintenance and the ability to make the required geometry also matter.
Nevertheless, some of the key items that could tilt the balance in favour of one technology, may include materials (obviously), robustness, speed, quality and size.
“First and foremost, the materials each technology enables are key factors in any choice. Second is how robust and precise the system is and specifically the speed and tolerances you can reach. Another critical factor is the size you can print in the bounding box. Finally, throughput and repeatability are key decision points”, Jon Donner comments.
Needless to say that each process cannot deliver the same advantages or have the same limitations. Sometimes, the choice of the user will be based on the sought advantages for a given application or the limitations to avoid for another one. For example, “Two-Photon Polymerization is a new technology known in the industrial context primarily for its quality and high precision. Similar to DLP, it uses light to crosslink a photosensitive resin to fabricate nano-, micro- and mesoscale structures, but it does so only in a very small volume of significantly less than 1 micrometer in diameter. 2PP is said to be slow when it comes to micro- and mesoscale structures.
This has changed recently with the advance of new processing technologies such as Two Photon Grayscale Lithography (2GL ®) and the combination of higher power laser that have come on the market and improved hardware such as stages and scanners. Other conventional AM technologies such as DLP, SLA, and Projection Micro Stereolithography (PµSL) can only fabricate larger structures with less filigree feature details in the range of several micrometers. These technologies are clearly suitable for some microfabrication tasks.
However, they encounter geometric limitations when it comes to high- resolution (<1 micrometer) 3D Microfabrication. Resolution and design geometry are limited due to the inherent direct illumination with UV light. 2PP provides the freedom of full 3D design to fabricate freeform, porous and even organic 3D geometries with excellent shape accuracy in the submicrometer to millimeter range. The inherent geometric limitations associated with subtractive methods are overcome with 2PP due to the additive approach.”, Smolenski states, reflecting on the advantages and limitations of some of the aforementioned technologies.
In general, here are the key benefits and areas for improvement we will keep in mind when exploring micro-3D printing technologies:
| Benefit to assess | Limitation or consideration |
|---|---|
| Cost: compare tooling, preparation, manufacture and finishing at the intended volume. Micro-AM is not always cheaper than micro-injection moulding. | Materials: like in traditional AM technologies, materials remain the hurdle that continuously requires advancements. “As new materials are developed, as yields increase, we will see many new areas where micro-AM will be applied”, Donner comments. |
| Design: access to some complex geometries within material, process and post-processing limits. | “A challenge in the fields of photonic integration, optical computing and data communications is to advance the alignment and packaging of photonic components. Specialized hardware- and software- based printing solutions can in turn enable efficient microoptical coupling. That’s why we have recently introduced and will continue to develop precisely aligned 3D printing on optical fibers and photonic chips”, Smolenski adds. |
| Lead time: avoiding tooling may accelerate an iteration; total time also includes preparation, cleaning and inspection. | Time: Micro-AM remains a relatively new entrant on the market and as seen with any new technology, it takes time to convince companies to use it. |
The place of these processes in vertical industries, and in the medical and healthcare industries in particular
Electronics, optics, microfluidics and medical devices are among the fields cited by the interviewees. Selection depends on useful dimensions, material, production volume and qualification requirements.
To date, “all industries that deal with small and precision parts are applicable. Making small parts traditionally has always been expensive, and Micro-AM is now delivering much cheaper and accessible solutions. In miniaturization, control over form factor is critical and enables a “next level” of miniaturization. Specifically: electronics, optics, semicon, medical devices, medical tooling, micro-injection molding, microfluidics, sensors, and then these applications of course are found in auto, aerospace, consumer goods, etc.” Fabrica’s expert points out.
Fabrica sees medical device research as a promising field. Donner’s following observation concerns the medical applications discussed in the 2022 interview; it does not mean that all micro-AM is limited to prototyping.
“As micro-AM is a “new” field it is currently only used for prototyping. But we see many opportunities. Some examples are micro-needles, parts for small blood vessels like blood clot-related tooling, tooling for ophthalmology, and many more. We are also seeing many miniaturized medical devices”, Donner notes.
Nanoscribe goes one step further and highlights the fact that it has recently made 2PP accessible for high-precision 3D bioprinting:
“High-precision 3D bioprinting enables to create advanced microenvironments for tissue engineering, custom scaffolds for cell studies, and many other innovative biomedical applications where precision, speed, material diversity, and sterility matter. In cell cultivation or cell seeding applications, microstructures are first printed using a material such as hydrogels or one of Nanoscribe’s biocompatible IP Photoresins. After printing, the structures are rinsed and cells are seeded onto the structures. Typically, the substrates are kept in incubators to monitor cell behavior over time. Cell seeding is the most common technique to combine 2PP microfabrication with cell research. Moreover, the dedicated high-precision bioprinter Quantum X bio covers biomedical applications like customized microfluidic elements and microneedle arrays or microrobots for drug delivery. Connecting the micro to the macro world, 3D structures can be placed intuitively and with highest precision into microfluidic channels or wells by simply tapping on the machine’s touchscreen.”
These applications include research and development. Using a material described as biocompatible does not amount to clinical authorisation of a device: its intended use and complete manufacturing process require evaluation.
At Boston University, a study published in 2021 combines direct laser writing and soft lithography to fabricate a heart-on-a-chip platform. Printed sites anchor cardiac microtissues; a microfluidic system applies mechanical loading and measures their response. It is a tissue research tool, not an implantable heart or a demonstrated clinical treatment.
Concluding thoughts
Micro-AM expands the design options for small structures. 2PP, PµSL and DLP feature prominently in this 2022 dossier. Process selection should nevertheless rely on test parts, comparable measurements and the cost of the complete production chain. Collaboration between suppliers, laboratories and users can help qualify applications.
Resources
This dossier has first been published in the May/June edition of 3D ADEPT Mag.
This exclusive feature has been written thanks to several external resources and main contributions from industry insiders. Exclusive interviews have been conducted with Fabrica, a Nano Dimension company as well as Nanoscribe.
In its 2022 presentation, Nano Dimension describes Fabrica 2.0 as a DLP solution for small parts, with stated layer thicknesses of 1–5 µm. Materials cited are ABS-like Precision N-800 and reinforced composite Performance N-900. Layer thickness is not the same as final tolerance; lead times and performance depend on the part.
In its 2022 contribution, Nanoscribe presents Quantum X align for printing aligned microoptics on fibres and photonic chips. It also describes two-photon grayscale lithography (2GL) for 2.5D structures, including freeform microoptics, microlens arrays and diffractive elements. The company reports surface roughness Ra ≤ 5 nm under the relevant conditions. This is a roughness value, not universal dimensional accuracy.
To increase volumes, Nanoscribe discusses trials combining 2GL master fabrication with nanoimprint lithography and injection moulding, with EV Group and kdg opticomp. Replication is then a separate step from additive manufacturing of the master.
Projection micro stereolithography based 3D printing and its applications, IOP Publishing Ltd on behalf of the IMMT.
Bert Huis in ‘t Veld, Micro additive manufacturing using ultra short laser pulses
Wei Lin, Dihan Chen and Shih-Chi Chen, Emerging micro-additive manufacturing technologies enabled by novel optical methods, Photonics Research, 8, 1827–1842, 2020.
V. Hahn et al., Rapid Assembly of Small Materials Building Blocks (Voxels) into Large Functional 3D Metamaterials, Advanced Functional Materials, 30, 1907795, 2020.
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