polyHWG: 3D Printed Substrate-Integrated Hollow Waveguides for Mid-Infrared Gas Sensing

Programmable and Modularized Gas Sensor Integrated by 3D Printing

The rapid advancement of intelligent manufacturing technology has enabled electronic equipment to achieve synergistic design and programmable optimization through computer-aided engineering. Three-dimensional (3D) printing, with the unique characteristics of near-net-shape forming and mold-free fabrication, serves as an effective medium for the materialization of digital designs into usable devices. This methodology is particularly applicable to gas sensors, where performance can be collaboratively optimized by the tailored design of each internal module including composition, microstructure, and architecture. Meanwhile, diverse 3D printing technologies can realize modularized fabrication according to the application requirements. The integration of artificial intelligence software systems further facilitates the output of precise and dependable signals. Simultaneously, the self-learning capabilities of the system also promote programmable optimization for the hardware, fostering continuous improvement of gas sensors for dynamic environments. This review investigates the latest studies on 3D-printed gas sensor devices and relevant components, elucidating the technical features and advantages of different 3D printing processes. A general testing framework for the performance evaluation of customized gas sensors is proposed. Additionally, it highlights the superiority and challenges of programmable and modularized gas sensors, providing a comprehensive reference for material adjustments, structure design, and process modifications for advanced gas sensor devices.

Fabrication of Low-Cost Miniaturized Gas Cells via SLA 3D-Printing for UV-Based Gas Sensors

The use of 3D-printing technology for producing optical devices (i.e., mirrors and waveguides) remains challenging, especially in the UV spectral regime. Gas sensors based on absorbance measurements in the UV region are suitable for determining numerous volatile species in a variety of samples and analytical scenarios. The performance of absorbance-based gas sensors is dependent on the ability of the gas cell to propagate radiation across the absorption path length and facilitate interaction between photons and analytes. In this technical note, we present a 3D-printed substrate-integrated hollow waveguide (iHWG) to be used as a miniaturized and ultralightweight gas cell used in UV gas-sensing schemes. The substrates were fabricated via UV stereolithography and polished, and the light-guiding channel was coated with aluminum for UV reflectivity. This procedure resulted in a surface roughness of 11.2 nm for the reflective coating, yielding a radiation attenuation of 2.25 W/cm2. The 3D-printed iHWG was coupled to a UV light source and a portable USB-connected spectrometer. The sensing device was applied for the quantification of isoprene and acetone, serving as a proof-of-concept study. Detection limits of 0.22 and 0.03% in air were obtained for acetone and isoprene, respectively, with a nearly instantaneous sensor response. The development of portable, low-cost, and ultralightweight UV optical sensors enables their use in a wide range of scenarios ranging from environmental monitoring to clinical/medical applications.

Infrared Spectroscopy–Quo Vadis?

Given the exquisite capability of direct, non-destructive label-free sensing of molecular transitions, IR spectroscopy has become a ubiquitous and versatile analytical tool. IR application scenarios range from industrial manufacturing processes, surveillance tasks and environmental monitoring to elaborate evaluation of (bio)medical samples. Given recent developments in associated fields, IR spectroscopic devices increasingly evolve into reliable and robust tools for quality control purposes, for rapid analysis within at-line, in-line or on-line processes, and even for bed-side monitoring of patient health indicators. With the opportunity to guide light at or within dedicated optical structures, remote sensing as well as high-throughput sensing scenarios are being addressed by appropriate IR methodologies. In the present focused article, selected perspectives on future directions for IR spectroscopic tools and their applications are discussed. These visions are accompanied by a short introduction to the historic development, current trends, and emerging technological opportunities guiding the future path IR spectroscopy may take. Highlighted state-of-the art implementations along with novel concepts enhancing the performance of IR sensors are presented together with cutting-edge developments in related fields that drive IR spectroscopy forward in its role as a versatile analytical technology with a bright past and an even brighter future.

Innovative Substrate-Integrated Hollow Waveguide Coupled Attenuated Total Reflection Sensors for Quantum Cascade Laser Based Infrared Spectroscopy in Harsh Environments

An innovative mid-infrared spectroscopic sensor system based on quantum cascade lasers has been developed. The system combines the versatility of substrate-integrated hollow waveguides (IHWGs) with the robustness of attenuated total reflection (ATR) crystals employed as internal reflection waveguides for evanescent field sensing. IHWGs are highly reflective metal structures that propagate infrared (IR) radiation and were used as light pipes for coupling radiation into the ATR waveguide. The combined IHWG-ATR device has been designed such that the utmost stability and robustness of the optical alignment were ensured. This novel assembly enables evanescent field absorption measurements at yet unprecedently harsh conditions, that is, high pressure and temperature. Combining these advantages, this innovative sensor assembly is perfectly suited for taking ATR spectroscopy into the field where the robustness of the assembly and optical alignment is essential.

From Light Pipes to Substrate-Integrated Hollow Waveguides for Gas Sensing: A Review

Absorption-based spectroscopy in the mid-infrared (MIR) spectral range (i.e., 2.5-25 μm) is an excellent choice for directly sensing trace gas analytes providing discriminatory molecular information due to inherently specific fundamental vibrational, rovibrational, and rotational transitions. Complimentarily, the miniaturization of optical components has aided the utility of optical sensing techniques in a wide variety of application scenarios that demand compact, portable, easy-to-use, and robust analytical platforms yet providing suitable accuracy, sensitivity, and selectivity. While MIR sensing technologies have clearly benefitted from the development of advanced on-chip light sources such as quantum cascade and interband cascade lasers and equally small MIR detectors, less attention has been paid to the development of modular/tailored waveguide technologies reproducibly and reliably interfacing photons with sample molecules in a compact format. In this context, the first generation of a new type of hollow waveguides gas cells-the so-called substrate-integrated hollow waveguides (iHWG)-with unprecedented compact dimensions published by the research team of Mizaikoff and collaborators has led to a paradigm change in optical transducer technology for gas sensors. Features of iHWGs included an adaptable (i.e., designable) well-defined optical path length via the integration of meandered hollow waveguide structures at virtually any desired dimension and geometry into an otherwise planar substrate, a high degree of robustness, compactness, and cost-effectiveness in fabrication. Moreover, only a few hundred microliters of gas samples are required for analysis, resulting in short sample transient times facilitating a real-time monitoring of gaseous species in virtually any concentration range. In this review, we give an overview of recent advancements and achievements since their introduction eight years ago, focusing on the development of iHWG-based mid-infrared sensor technologies. Highlighted applications ranging from clinical diagnostics to environmental and industrial monitoring scenarios will be contrasted by future trends, challenges, and opportunities for the development of next-generation portable optical gas-sensing platforms that take advantage of a modular and tailorable device design.

Optical transmittance of 3D printing materials

The increasing prevalence of three-dimensional (3D) printing of optical housings and mounts necessitates a better understanding of the optical properties of printing materials. This paper describes a method for using multithickness samples of 3D printing materials to measure transmittance spectra at wavelengths from 400 to 2400 nm [visible to short-wave infrared (IR)]. In this method, 3D samples with material thicknesses of 1, 2, 3, and 4 mm were positioned in front of a uniform light source with a spectrometer probe on the opposing side to measure the light transmittance. Transmission depended primarily on the thickness and color of the sample, and multiple scattering prevented the use of a simple exponential model to relate transmittance, extinction, and thickness. A Solidworks file and a 3D printer file are included with the paper to enable measurements of additional materials with the same method.

Advances in Mid-Infrared Spectroscopy-Based Sensing Techniques for Exhaled Breath Diagnostics

Human exhaled breath consists of more than 3000 volatile organic compounds, many of which are relevant biomarkers for various diseases. Although gas chromatography has been the gold standard for volatile organic compound (VOC) detection in exhaled breath, recent developments in mid-infrared (MIR) laser spectroscopy have led to the promise of compact point-of-care (POC) optical instruments enabling even single breath diagnostics. In this review, we discuss the evolution of MIR sensing technologies with a special focus on photoacoustic spectroscopy, and its application in exhaled breath biomarker detection. While mid-infrared point-of-care instrumentation promises high sensitivity and inherent molecular selectivity, the lack of standardization of the various techniques has to be overcome for translating these techniques into more widespread real-time clinical use.

Optimizing the Analytical Performance of Substrate-Integrated Hollow Waveguides: Experiment and Simulation

The goal of this technical note was to compare experimentally and via simulation of eight substrate-integrated hollow waveguide (iHWG) designs, and to predict promising future iHWG structures in lieu of experiments. The iHWGs differed in their geometry (i.e., inlet funnel cross-section and inner channel cross-section), as well as in their material properties (i.e., type of metal, polish of inner channel). Experimentally, calibration functions of isobutane as a model analyte were determined, and the analytical figures of merit, i.e., signal-to-noise ratio, limit of detection, were evaluated for each iHWG. Evaluation of the amount of radiation incident at the real-world and simulated detector revealed that experiment and simulation were in excellent agreement. While material and quality of the inner channel wall did not have a significant influence on the performance, the iHWG geometry profoundly affected the performance in terms of light throughput: Increasing the inlet funnel dimensions and the inner channel cross-section benefits light throughout, and thus, the analytical signal. Based on these results, simulations of not yet fabricated iHWGs were performed and promising new iHWG structures were suggested.

Characterization of Gas Absorption Modules Based on Flexible Mid-Infrared Hollow Waveguides

A new gas absorption module, the substrate-embedded hollow waveguide (eHWG) model, is proposed. It consists of a substrate with a curved channel and a hollow waveguide. The hollow waveguide is curved into the channel and works as a gas absorption cell as well as a transmission medium for mid-infrared light. Owing to the low loss property of the hollow waveguide, the signal-to-noise ratio (SNR) was improved for the sensing system. A polycarbonate (PC) base tube was used to obtain flexibility in the fabrication of the hollow waveguide. A silver (Ag) layer and a silver iodide (AgI) layer were inner-coated to ensure a low loss property at the fingerprint wavelength of methane gas. A sensing system was established using a Fourier transform infrared spectrometer (FTIR), an external detector, and an eHWG. Experimental investigations were carried on the sensing performance of eHWGs with various channel shapes. Comparison studies were made on eHWGs embedded with Ag-coated or Ag- and AgI-coated hollow waveguides. The Ag- and AgI-coated hollow waveguides with inner diameters of 0.7, 1.4, and 2.0 mm were used in the eHWGs. The large bore waveguide had low loss but high bending additional loss. The large bore waveguide had a low detection limit due to high coupling efficiency with the light source. A limit of detection (LOD) as low as 2.7 ppm was attained for the system using the eHWG with the long and large bore waveguide.

Developing Microfluidic Sensing Devices Using 3D Printing

This short perspective assesses the present landscape for desktop 3D printing to design and fabricate sensors, in particular, those associated with microfluidics and multiplexing. Lots of advanced devices have already been reported, and this article briefly surveys interesting achievements. Microfluidics can be designed and optimized faster and more easily on low cost desktop 3D printers that with competing methods. Rapid prototyping leads directly to a final (marketable) product fabricated on the same 3D printer. While resolution is not as good as lithographic approaches, very often channel and feature resolution on the order of 100 μm obtainable with SLA 3D printers is perfectly suitable for the desired sensing device. Two examples from our team's research are used to illustrate how using a 3D printer along with simple automation can reduce a complex microfluidic sensing procedure to a much simpler automated one. Future possibilities for sensor technology are discussed.