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polyHWG: 3D Printed Substrate-Integrated Hollow Waveguides for Mid-Infrared Gas Sensing Robert Stach, Julian Haas, Erhan Tütüncü, Sven Daboss, Christine Kranz, and Boris Mizaikoff ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00649 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 1, 2017
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ACS Sensors
polyHWG: 3D Printed Substrate-Integrated Hollow Waveguides for Mid-Infrared Gas Sensing Robert Stach, Julian Haas, Erhan Tütüncü, Sven Daboss, Christine Kranz, Boris Mizaikoff* Institute of Analytical and Bioanalytical Chemistry, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany KEYWORDS: substrate-integrated hollow waveguide, iHWG, 3D printing, polymer-based hollow waveguide, polyHWG, mid-infrared, quantum cascade laser, breath diagnostics
Abstract: Gas analysis via mid-infrared (MIR) spectroscopic techniques has gained significance due to its inherent molecular selectivity and sensitivity probing pronounced vibrational, rotational, and roto-vibrational modes. In addition, MIR gas sensors are suitable for real-time monitoring in a wide variety of sensing scenarios. Our research team has recently introduced so-called substrate-integrated hollow waveguides (iHWGs) fabricated by precision milling, which have been demonstrated useful in online process monitoring, environmental sensing, and exhaled breath analysis especially if low sample volumes (i.e., few hundreds of microliters) shall be probed with rapid signal transients. A logical next step is to establish ultra-lightweight, potentially disposable, and low-cost substrate-integrated hollow waveguides, which may be readily customized and tailored to specific applications using 3D printing techniques. 3D printing provides access to an unprecedented variety of thermoplastic materials including bio-compatible polylactides, readily etchable styrene copolymers, and magnetic or conductive materials. Thus, the properties of the waveguide may be adapted suiting its designated application, e.g., drone-mounted ultra-lightweight waveguides for environmental monitoring or biocompatible disposable sensor interfaces in medical/clinical applications. Recent advances in 2D and 3D printing render these light pipe) usually integrated into a metal substrate. Gas techniques increasingly interesting for scientific applicain- and outlets enable the sample to populate the same tions. Ink-jet-based 2D printing has readily matured for hollow structure. Hence, while radiation – nowadays from the deposition of, e.g., receptor layers 1,2, for defining the UV to the THz spectral regime - propagates along the electronic circuits 3–5, and for the development of sohollow waveguide structure, it may interact with the gaseous sample constituents flowing through the very same called ‘lab-on- paper’ devices, which combine separation hollow core 16–20. iHWG structures are usually fabricated and detection principles onto cheap paper substrates 6. Especially, mass-production of such devices may drastivia CNC milling procedures, which are time and cost cally reduce cost and benefit time-effective production intensive 21. A detailed description of iHWG fabrication, 7,8 due to usage of proven printing concepts . Whereas advantages, and a comparison to other techniques on the conventional 2D printing techniques are intrinsically market is given by Wilk et al. 22–24. limited in the dimensions achievable for elevated strucAs a step towards more design flexibility and ontures at a substrate surface, 3D printing adds another demand – potentially even on-site - fabrication of iHWGs degree of freedom by adding the z-dimension enabling is the usage of 3D printing techniques. Next to establishmore elaborate structures. ing the structure, the usually IR absorbing polymer surNowadays, 3D printing at a microscopic and macroscopic level has evolved utilizing a continuously increasing number of materials facilitating prototyping and production of, e.g., home-made lab ware 9, components for (bio)medical devices such as prosthetics and engineered tissues 10,11, and sub-micrometre structures for more specialised applications, e.g., in multiphoton lithography for fabricating smallest objectives 12,13. Besides such highly specialized applications, 3D printing particularly enables rapid and flexible prototyping, as demonstrated for photocatalysis reactors 14 or for robotical structures 15. Herein, we present a novel approach towards 3D printing optical sensor components. A fully 3D printed substrate-integrated hollow waveguide (polyHWG) was designed, printed, and tested for mid-infrared gas sensing applications in combination with a quantum cascade laser (QCL) light source. An iHWG mainly comprises a hollow optical path with a highly reflective surface, which simultaneously serves as a miniaturized gas cell and as a photon conduit (a.k.a.,
face has to be converted into an IR-reflective surface for propagating radiation. In order to obtain a highly IRreflective surface on the inside of the light-guiding channel, the polymer surface was chemically etched and subsequently coated with a gold layer deposited by sputtering from ultra-high vacuum. This innovative approach allows manufacturing customizable, lightweight, lowcost, and disposable polyHWGs, which is especially relevant for, e.g., monitoring scenarios via drone-mounted devices, and for medical/clinical in-field applications such as physiological, pharmaceutical, and disease screening 25. As an application example in the present study, exhaled breath analysis will be used. An inevitable by-product of the human metabolism is exhaled breath, which is of substantial interest serving as a readily available, yet potentially highly complex carrier of potential biomarkers. Hence, molecules that are linked to certain diseases, disease conditions or therapy progress are readily available via this diagnostic window without invasive sampling. Carbon dioxide (CO2) is a major metabolite providing direct information on vital functions,
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e.g., in intensive care scenarios 26,27. Pulmonary diseases such as asthma, cystic fibrosis, and lung cancer are readily linked to selected molecular biomarkers present in exhaled breath including nitric oxide, hydrogen peroxide, etc. 28,29. However, not only pulmonary diseases directly affecting the airways, but in fact any disease eventually produces metabolic byproducts that reflect in an altered composition of the exhaled breath matrik, e.g., cardiovascular diseases 30. Frequently, such diseases give rise to pronouncedly elevated levels of various volatile organic compounds (VOC) – e.g., isoprene and isopropyl alcohol 31 – which may be detectable via MIR sensor technologies, in particular if bright light sources such as tuneable quantum cascade lasers (tQCLs) or interband cascade lasers (ICL) are used 16,32–37. The polyHWGs presented herein are ideally suited for exhaled breath diagnostics in combination with broadband IR light sources (e.g., coupling to an FTIR spectrometer) or tQCLs, and may potentially be used as a consumable sampling interface when screening series of patients. Experimental Materials and Methods. Acetone and isopropyl alcohol (IPA) were purchased from VWR (VWR International GmbH, Darmstadt, Germany). Carbon dioxide (CO2) gas was purchased from MTI (MTI IndustrieGase AG, NeuUlm, Germany). ABS and Proto Pasta 3D printing filaments were purchased from Filamentworld (Ulm, Germany). Fabrication. The polyHWG consists of two parts, i.e. a bottom and a top plate, which contain all relevant structural features and auxiliary mounts. Figure 1 shows a detailed technical drawing of the assembled waveguide structure and the features of the top plate. After 3D printing and post-processing (i.e., etching and gold sputtering), both parts are mounted together establishing the final polyHWG structure. To seal the waveguide, IR transparent windows (i.e., silicon disks) are fitted and glued into the window intrusions. Luer-lock® systems can be mounted onto the upper side of the top-plate, and readily connect with the gas in- and outlet channels feeding a gaseous sample into the hollow channel of the polyHWG. After assembly, the polyHWG bottom side is attached to a 3D printed magnetic mount, which matches magnetic positioning systems (here, kinematic mounts by Thorlabs, Dachau, Germany). To ensure precise positioning, the mount was printed from a ferromagnetic material.
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Figure 1: CAD design/rendering of the assembled 3D printed polyHWG (left), and the top plate only (right) illustrating the interior structural features. a) Waveguiding hollow channel/gas flow cell with diameter of 2 mm. b) 6 mm diameter intrusions for silicon windows. c) Gas in- and outlet channels ® d) Gas in- and outlet extensions to fit the Luer-lock system.
The hollow waveguide structure was printed from acrylonitrile-butadiene-styrene (ABS) using an Ultimaker 2+ 3D printer with the process settings given in Table 1. Table 1. Process variables for polyHWG production via 3D printing. PARAMETER PROCESS
FUSED-FILAMENT DEPOSITION
EXTRUDER TYPE
BOWDEN SETUP
FILAMENT
2.85 MM ABS
BUILDPLATE
GLASS WITH BUILDTAK ADHESION FOIL
NOZZLE DIAMETER
250 µM
NOZZLE TEMPERATURE
234 °C
BUILDPLATE TEMPERATURE
130 °C
INFILL
40 %
RETRACTION DISTANCE
3.5 MM
LAYER HEIGHT
150 µM
To ensure a high-quality waveguide structure and to prevent the warping effects characteristic for ABS, the temperature gradient between nozzle and build plate was kept as small as possible, and a high infill percentage was applied. Additionally, a BuildTak layer was applied to the build platform, which ensures ultra-strong adhesion during the printing process, and thus, a counterforce to potential warping effects. For positioning, a magnetic mount was printed using Proto Pasta, a ferromagnetic material at the process settings given in Table 2. This mount fits conventional kinematic optical mounting platforms, and was then attached to the bottom plate of the polyHWG.
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Table 2. Process variables the magnetic mount production via 3D printing. PARAMETER PROCESS
FUSED DEPOSITION
EXTRUDER TYPE
BOWDEN SETUP
FILAMENT
2.85 MM PROTO PASTA FERROMAGNETIC
BUILDPLATE
GLASS WITH BUILDTAK ADHESION FOIL
NOZZLE DIAMETER
400 µM
NOZZLE TEMPERATURE
220 °C
BUILDPLATE TEMPERATURE
50 °C
INFILL
100 %
RETRACTION DISTANCE
4 MM
LAYER HEIGHT
200 µM
FILAMENT
Figure 3: polyHWG before and after gold coatingprocess.
The overall material costs for an individually manufactured polyHWG device were estimated at approx. 0.35 €, which is significantly more cost-effective vs. CNC-milled components, which can be estimated at 30€ for single manufactured pieces and approx. 5€ in mass production. In addition, the polyHWG may be printed and assembled in
