2.2. CONTROL
2.2.2. Tipos de controladores
(a) Solid silicone phantoms
The protocol for fabricating silicone phantoms was adapted from33. Two parts, A and B of RHODORSIL RTV 141 silicone were used in a 1:10 ratio, 4% of alumina powder was thoroughly mixed to the polymer and left at room temperature to vulcanise. The phantoms normally vulcanise and develop resin-like texture after 72 hours. They can be moulded in any shape and
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98 easily come off the mould after the silicone vulcanisation. A simple rectangular-shape phantom was made for diffuse reflectance measurements.
(b) Absorption properties of silicone pigments and dyes
Various inks from Silc Pig® Silicone Pigments (Smooth-On, Macungie USA) and Cirius Urethane Resin Dye (ArtMolds, Summit, NJ USA) were mixed with the silicone to explore the absorption properties with the view to potentially mix different dyes and pigments in the phantom to match the complex absorption spectrum of the skin. In order to place the phantoms in an absorption spectrometer, 1 mm thick phantom layers were prepared in a sandwich-like container formed from 1 mm thick microscope glass slides. When silicone vulcanises it does not stick to any of the surfaces but forms quite a tight contact as it pushes out all the air trapped at the interface with the container. It is easy to use a spatula to lift up and peel off the phantom from the container, but in the case of small tight containers such as a regular glass cuvette taking the phantom out would become a challenge without damaging the cuvette. So cheap disposable glass slides were used as phantom holders in the film holder of an absorption spectrometer. Two 1/4 pieces of the glass slide were placed as side walls and two full glass slides were used as top and bottom base of the container. A few drops of the silicone mixture were dropped in the middle of the semi-open cavity, and then the phantom was covered from the top with another slide. The Silicone is viscous enough to stay in the cavity and vulcanise on the slides almost without leaking out from the two open sides. Different colour phantoms in the glass semi-open containers are shown in
Figure 4.5
, a. A LAMBDA 950 UV/Vis/NIR spectrometer was used to measure the absorption spectrum. A broad range of wavelengths from 400 nm up to 2 µm was investigated for potential use in many applications, including PDT, OCT and other devices. A glass container with air and also one with pure silicone were measured for the baseline. The absorption spectra are shown inFigure 4.5
, b.4.2 Methods
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Figure 4.5:
Absorption in phantoms using of Silicone dyes.a
–.phantoms with the dyes.b –.absorption spectrums.
Considering that there are many chromophores in skin with complex spectra (
Figure
4.3
) the task of imitating the ultimate absorption in skin is quite challenging. Necessary ink or dye or a combination of them can be selected to meet specific measurement requirements. However, for the purposes of the work in this chapter the ultimate phantoms did not have any absorbers.(c) Incorporating PpIX fluorescence in silicone phantoms
Fluorescent phantoms mentioned in the previous chapter for testing the fluorescence imaging camera were made by incorporating fluorescent powder of PpIX in a similar way to that mentioned in [33]. Protoporphyrin IX P8293 - ≥95% from SIGMA-ALDRICH was dissolved in methanol. The methanol solution of PpIX was mixed with alumina powder to provide a more uniform suspension of alumina particles and PpIX fluorophores. This suspension was then thoroughly mixed to the silicone polymer. Methanol evaporated under a fume hood and the silicone vulcanised after three days. If kept in the dark the phantoms retain their fluorescence properties for months.
(d) Gel synthetic silicate Laponite® RD phantoms
Gel scattering phantoms for depth-dependent transmission measurements were made of a Laponite® RD38 aqueous suspension. The protocol for this phantom was developed by Dr Mario Giardini, former research fellow at the School of Medicine, the University of St Andrews. In addition to the optical properties this phantom mimics the mechanical properties of the brain
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100 which enables various opto-mechanical manipulations in a brain-like medium. Laponite® RD is a synthetic layered silicate which is insoluble but hydrates and swells in water to give clear and colourless colloidal dispersions39. It stays in gel phase for as long as the water does not evaporate. 3% of Laponite® RD was mixed with water and 4% of alumina powder. Gel-like phantoms were thoroughly mixed in a plastic container and left overnight to swell and jellify. Before the measurement the phantoms were mixed up again to obtain a uniform gel structure. Optical properties of this phantom were dictated by all the components. Laponite® does not absorb in visible and near IR, so the only absorber in IR was water40. The refractive index of Laponite® RD powder is 1.54, in water suspensions it changes depending on the concentration Laponite® RD and ambient temperature41,42,43. Due to the fact that only 3% of the powder was
added in the phantom, the refractive index of the ultimate phantom was only slightly increased to 1.35 compared to 1.33 refractive index of water (this value was not confirmed by measurements). Scattering properties of the phantom were the same as in silicone phantoms with alumina powder33,29 as the same quantities were added.
Two types of phantoms - gel and solid - enabled measurements to be conducted inside phantoms and on the surface. Gel Laponite® RD phantom had similar mechanical and optical properties of brain; solid silicone phantom had a flexible resin-like texture similar to the mechanical properties of skin and was close in optical properties. It was demonstrated that these phantoms could be mixed with any combination of absorbers or fluorophores depending on the requirements of the measurement.