Sensitive layer development

Imaging plates are exposed similar to radiographic films. They are read out by a LASER-scanner to a digital image without any developing process. After optical erasing of the virtual picture the same IP can be used cyclic up to more than 1000 times. The life time is limited by the mechanical stability of the IP s. An IP consists of a flexible polymer carrier which is coated with the sensitive layer. This layer is covered with a thin transparent protective foil. 

The initiation of development in the activator solution is more rapid than in conventional processes because the developer molecules need not diffuse into the light-sensitive layers from the processing solution. In spite of the low activity of the coated developer, some unintentional reduction sensitization may occur, which produces unwanted fog. Therefore, coating the developer in a separate layer usually is preferred. Because of simplicity, rapid access, and solution stabihty, incorporated developer papers have been used for office copying appHcations. 

Negative image in red-sensitized layer Metallized cyan dye developer layer... 

Moreover, in many cases, a shift of Tg to lower values of temperature has been detected, but in these cases the quality of adhesion between phases may be the main reason for the reversing of this attitude 11,14). If calorimetric measurements are executed in the neighbourhood of the glass transition zone, it is easy to show that jumps of energies appear in this neighbourhood. These jumps are very sensitive to the amount of filler added to the matrix polymer and they were used for the evaluation of the boundary layers developed around fillers. 

After discussing various sensitive layers later, both principles will be applied to monitor effects in biomolecular or chemical sensitive layers. The applications will demonstrate the feasibility of the methods as well as their advantages and their disadvantages. Therefore various applications are given, sometimes even for the same analyte, in order to demonstrate the normal approach in sensor development to select the best transduction principle for a specific application. 

Some earlier developments and applications of various implantable pH sensors or measurement systems have been reported [128, 129, 130, 131]. However, reliable pH sensors for long-term implantations are still not available, and widespread clinical usage of implantable pH sensors has not been reached. Similar to other implantable sensors, the development of implantable pH microelectrodes, either fully implanted in the body or needle type sensors applied through the skin (percutaneous), has faced serious obstacles including sensor stability deterioration, corrosion, and adverse body reactions [48, 132, 133], Among them, encapsulation to prevent corrosion represents a major challenge for the implantable sensor devices [51]. Failure of encapsulation can cause corrosion damage on internal components, substrate materials, and electrical contacts [48], The dissolution of very thin pH sensitive layers will also limit the stability and lifetime of implantable micro pH sensors. 

During the last years, so-called microhotplates (pHP) have been developed in order to shrink the overall dimensions and to reduce the thermal mass of metal-oxide gas sensors [7,9,15]. Microhotplates consist of a thermally isolated stage with a heater structure, a temperature sensor and a set of contact electrodes for the sensitive layer. By using such microstructures, high operation temperatures can be reached at comparably low power consumption (< 100 mW). Moreover, small time constants on the order of 10 ms enable applying temperature modulation techniques with the aim to improve sensor selectivity and sensitivity. 

The third block in Fig. 2.1 shows the various possible sensing modes. The basic operation mode of a micromachined metal-oxide sensor is the measurement of the resistance or impedance [69] of the sensitive layer at constant temperature. A well-known problem of metal-oxide-based sensors is their lack of selectivity. Additional information on the interaction of analyte and sensitive layer may lead to better gas discrimination. Micromachined sensors exhibit a low thermal time constant, which can be used to advantage by applying temperature-modulation techniques. The gas/oxide interaction characteristics and dynamics are observable in the measured sensor resistance. Various temperature modulation methods have been explored. The first method relies on a train of rectangular temperature pulses at variable temperature step heights [70-72]. This method was further developed to find optimized modulation curves [73]. Sinusoidal temperature modulation also has been applied, and the data were evaluated by Fourier transformation [75]. Another idea included the simultaneous measurement of the resistive and calorimetric microhotplate response by additionally monitoring the change in the heater resistance upon gas exposure [74-76]. 

There are two basic methods by which a developed silver image can be obtained. The one commonly employed in practice is termed chemical or, better, direct development. The exposed sensitive layer is placed directly in a suitable reducing solution, and the silver image is derived from the reduction of the silver halide grains. This is the type already considered to some extent in the preceding section. 

In a modified and more complicated form (pre-fixation physical development) the exposed sensitive layer is placed directly in the developing solution without first dissolving out the silver halide. The developing solution contains a silver halide solvent in addition to the soluble silver salt, and part of the developed silver in this process comes from the original silver halide. Some direct development also occurs. 

Replacement of the latent image silver by gold in the exposed photographic sensitive layer increases the developability (James et at., 31). Since the mass of developed silver depends only upon the number of developed nuclei, the gold treatment has obviously increased the number of active nuclei. Hence, smaller gold nuclei (in terms of numbers of atoms) than silver can initiate development, unless the treatment used to effect a replacement of silver by gold has resulted in something more than a simple replacement. 

It is important to keep in mind that, in the development of the sensitive layer as a whole, we are dealing with an ensemble of reaction units where reaction may or may not proceed in a parallel fashion among the many units. Under certain conditions the kinetics of development of a typical single grain can be inferred directly from a measurement of the overall rate of formation of silver, but this is not true as a general proposition. Studies of development of the individual grains are of fundamental importance, since the grain is the real unit of development. 

Within (say) the red-sensitive layer of a color-negative film, silver halide grains become sensitized by red light to form a latent image and are reduced to silver metal by a developer just as in black-and-white photography. In... 

Several sensors have been developed to measure substrates in biological fluids. The structure of the membrane in these biosensors is quite complex. It generally contains a substrate-sensitive layer that includes a specific enzyme to catalyse a reaction and transform the analyte (or substrate). 

In a modification of the process the sensitized layers are arranged as before but nonwandering couplers are incorporated into the respective layers during manufacture of the film. This means that, after the hydroquinone development has been carried out, the differently coloured images can be formed in one common developer solution. Silver and silver halide are removed as previously. 

Figure 28-11 Schematic representation of the layer structure of color film and the color changes that occur on development. The actual film also contains a filter below the blue-sensitive layer to remove the blue light passing through this layer (because all emulsions are sensitive to blue), an antihalation layer below the red to prevent scattering of the light back through the emulsion, and a film base, such as cellulose acetate or poly-1,2-ethanediyl 1,4-benzenedioate, to support the emulsion.

Figure 28-12 Schematic changes in the development of a color film exposed to green light. The color couplers are present in the original sensitized layers.

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