Optical spectroscopy is a well-suited technique for the nondestructive and real-time maturation monitoring of fruits and vegetables. Although many commercial spectroscopy systems exist, including some for portable use in the field, a significant gap in agricultural monitoring is the ability to continuously measure the condition of fruits in the field over longer periods of time. To this end, we present here a fully integrated, flexible microspectrometer consisting of multiple light sources and multiple broadband photodiodes for the spectral evaluation of grape maturation in the field. To enable the microspectrometer design, a customized grape berry model generated from the optical properties, primarily light scattering, of grape berries was developed. The microspectrometer was fabricated using a scalable fabrication process based on a spin-coated, flexible polyimide substrate. Experiments were conducted both in a controlled laboratory environment as well as during the grape maturation period in the vineyard and these demonstrated that the spectral properties of grapes at different maturation stages can be accurately measured. Using suitable chemometric models, the amount of total soluble solids in °Br, which is the most important factor for the maturation estimation of grapes, was determined from the microspectrometer data.
SignificanceFor the development and routine characterization of optical devices used in medicine, tissue-equivalent phantoms mimicking a broad spectrum of human skin properties are indispensable.AimOur work aims to develop a tissue-equivalent phantom suitable for photoplethysmography applications. The phantom includes the optical and mechanical properties of the three uppermost human skin layers (dermis, epidermis, and hypodermis, each containing different types of blood vessels) plus the ability to mimic pulsation.ApproachWhile the mechanical properties of the polydimethylsiloxane base material are adjusted by different mixing ratios of a base and curing agent, the optical properties are tuned by adding titanium dioxide particles, India ink, and synthetic melanin in different concentrations. The layered structure of the phantom is realized using a doctor blade technique, and blood vessels are fabricated using molding wires of different diameters. The tissue-mimicking phantom is then integrated into an artificial circulatory system employing piezo-actuated double diaphragm pumps for testing.ResultsThe optical and mechanical properties of human skin were successfully replicated. The diameter of the artificial blood vessels is linearly dependent on pump actuation, and the time-dependent expansion profile of real pulse forms were mimicked.ConclusionsA tissue equivalent phantom suitable for the ex-vivo testing of opto-medical devices was demonstrated.
The process of computed axial lithography (CAL) has been established as one of the fastest available photopolymer 3D printing methods, offering smooth surfaces (r.m.s. surface roughness as low as 6 nm) and the ability to process high-viscosity precursor materials (100,000 cP demonstrated). Recently we showed successful printing of microscale geometries into dispersions of silica nanoparticles in a refractive-index-matched photopolymer. After exposing the 3D geometry via patterned tomographic illumination the material is debinded and sintered. In this way, external features of 50 µm and internal channels of 150 µm diameter have been achieved. This processing technique offers a promising route to production of 3D glass microfluidic devices and complex monolithic micro-optical devices. We will describe the status of optics fabrication via CAL. We will also consider the influence of light scattering on spatial resolution and possible ways of addressing this effect.
UV/VIS spectroscopy is a useful and efficient tool for non-destructive remote and real-time monitoring of the maturation of grapes and other agricultural products. For this purpose, we have developed a fully-integrated micro-spectrometer consisting of four LEDs emitting at different wavelengths, which will allow a spectral analysis of the sample under investigation in reflection mode. The positioning of the emitters and photodiodes with respect to each other was optimized using simulations employing a grape model based on the Henyey-Greenstein scattering model, with parameters calculated from reflectance and transmission data of real grapes. The sensor was fabricated on a polyimide stripe on which the optical components were placed and interconnected on one side and an array of electrical contacts on the other side allowed interfacing of the sensor with the driving electronics. With an overall thickness of 11 μm, this sensor stripe can be placed inside a grape bunch and acquire reflectance data continuously in the vineyard during the maturation period.
Tissue-mimicking optical phantoms are used for a wide variety of purposes, especially the development and routine quality control of bio-optical measurement techniques. We present here the fabrication and evaluation of a multilayer human skin phantom with a pulsating vascular network. The skin-mimicking phantom comprises the three upper skin layers (dermis, epidermis and hypodermis) and the corresponding blood vessels. It is based on polydimethylsiloxane (PDMS) in which absorption and scattering are induced by adding ink as an absorber and titanium dioxide particles as scattering agent. The vessels are fabricated by inserting wires of different diameter at different heights into a mold frame before adding the uncured PDMS and removing them after curing. A circulatory system is constructed using micro-displacement pumps that are connected to the artificial blood vessels. The expansion of the microvasculature in pure PDMS is monitored using a microscope.
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