Monolithic Matrices Research Articles

Active silicon CMOS addressing matrices for light-valve projection displays

In the second part of this three-paper series, the design and the fabrication of monolithic active matrices for addressing metallized viscoelastic light-valve control layers are described together with the required external electronics. The active matrices were fabricated by means of standard silicon CMOS technology. Special matrix features are signal-handling capabilities of 30 V, optimized surface planarity and extremely low light sensitivity. The on-chip addressing circuitry exceeds HDTV speed requirements. Three identical matrix samples coated with metallized viscoelastic control layers were successfully demonstrated as spatial light modulators for the read-out light from a xenon arc lamp in a full-colour schlieren-optical dark-field projection system. Results obtained with medium-sized active matrices are presented; larger matrix versions were already fabricated in small numbers, but not yet tested.

Microstructure and mechanical behaviour of self-reinforced Si3N4 and Si3N4-SiC whisker composites

Abstract Monolithic Si3N4 and Si3N4-SiC whisker composites were fabricated by hot pressing or hot isostatic pressing. They were sintered in the 1600–1800°C temperature range with 6 wt% Y2O3 and 3 wt% Al2O3 as additives. Morphological aspects of whiskers were statistically determined by image analysis and different matrix microstructures were observed after chemical etching. Then, a correlation was established with mechanical properties. When the same sintering conditions are used, the composite rupture stress increases or decreases with respect to that of the corresponding monolithic Si3N4 matrices. The increase is attributed to an effective load transfer mechanism which involves stress concentration at fibre-matrix interfaces. These interfaces can become the new critical defects in the microstructure when the whiskers are too large. The resistance to short cracks was determined by indentation. The single edge precracked beam (SEPB) method allowed characterization of the resistance to long crack propagation. The toughness increases both with the aspect ratio of whiskers and/or elongated β-Si3N4 grains and with the precrack length (R-curve effect). The improvement is mainly due to the bridging of crack borders by acicular shapes or ligaments of unbroken matter. The R-curve corresponds to the enlargement of the active clamping zone as the crack extends. At a given precrack length the fracture toughness is strongly dependent upon the potential diameter of bridges, whereas the R-curve steepness rises with the density of clamping sites.

On the strength of ductile particle reinforced brittle matrix composites

This study examines theoretically the strength characteristics of ductile particle reinforced brittle materials in which the strength enhancement is derived from the crack bridging process. The crack bridging is characterized by rectilinear and linear softening bridging laws that relate the crack surface traction to the crack opening displacement. The composite strength is expressed in terms of two non-dimensional parameters that combine the effects of the flaw size, elastic modulus, matrix toughness, and the bridging-law parameters. It is shown that such composites can be substantially more flaw tolerant than the monolithic matrices owing to a narrowing of the strength distribution. The role of interface debond length is also examined. It is shown that there exists optimal debond lengths of which the composite strength is maximized. In contrast, the steady state toughness increases monotonically with debond length. The implications of these results on the design of composite microstructures are briefly described.

Some formulation factors influencing the rate of drug release from bioadhesiw matrices

The development of monolithic matrices with controlled release and mucosa-adhesive properties was investigated. After an initial screening procedure for formulations that showed stability and minimal swelling, the rate of release of a model water soluble drug from various polyacrylic acid containing matrices was evaluated. All the formulations gave a prolonged drug release relative to a lactose containing control formulation. A formulation containing Carbopol 934P and CaCl2, was found to give the slowest rate of drug release (t50% of 7.77h), with release kinetics nearest to the ideal zero order. When tested in a modified tensiometer it was found that the inclusion of a relatively high loading of a model drug did not adversely affect the adhesive properties of these formulations.

Factors in zero-order release of clonidine hydrochloride from monolithic polydimethylsiloxane matrices

Clonidine hydrochloride loading doses in the 20–40% range are released from monolithic polydimethylsiloxane (PDS) matrices through a self-triggered osmotic pumping mechanism via cracks having formed in the stressed polymer. This mechanism results in zero-order release kinetics when drug particles in the 50–150 μm size range and filler-reinforced PDS are used. Changes in matrix shape (disk vs cylinder) are inconsequential to the zero-order pattern, whereas they do affect release rate. A strong influence of drug loading dose on this rate, as quantified by a highly significant log-log correlation, allows the rate to be modulated over a wide range of pharmacologically interesting values. The zero-order regimen, which lasts several days or months depending on matrix shape and drug load, is preceded by a burst effect and followed by a decline of rate when osmotic pumping ends and the matrix starts shrinking. By this time, around 50% of initial drug load has been released. Release rate is also affected by the pH and buffering competence of the receptor medium.

38] Membrane systems: Practical applications

Key representative applications of membrane systems are presented in this chapter, discussing their structure, function, and release kinetics with reference to the earlier review of theoretical applications of drug release from membrane systems. In each type of membrane system considered -- membrane-coated reservoirs and monolithic matrices -- the drug is released by diffusion through a rate-controlling membrane or matrix. Applications range from ocular inserts to transdermal patches to oral dosage forms. Examples illustrate the principles involved in most membrane systems. The 1st major application of membrane coated reservoirs for controlled drug delivery was the use of silicone-rubber-tubing implants for delivery of cardiovascular drugs. To overcome the problem of not very reproducible release rates and the significant decrease in release rates with time, Alza Corporation developed Progestasert, a device that incorporates a reservoir consisting of drug (progesterone) crystals suspended in silicone oil. A more recently developed steroid-releasing IUD is similar in several ways to the Progestasert but contains estriol rather than progesterone. The primary advantage of using estriol is that it is effective at much lower doses than is progesterone, and therefore, theoretically, an estriol IUD can be used for several years without replacement. At about the same time that the Progestasert system was being developed, a membrane-based ocular system also was developed for the delivery of pilocarpine to treat glaucoma. Key Pharmaceuticals began to market a controlled release tablet in 1977 for the treatment of asthma and bronchitis. This was the 1st theophylline product that allowed 12-hour dosage and has become widely accepted by physicians and patients. Transdermal drug delivery has received considerable publicity over the past 3-4 years. Some drugs, primarily nitroglycerin, are particularly attractive for transdermal delivery. By monolithic system is meant a mixture of a dispersed or dissolved drug in a polymeric matrix. The drug release from such a system is controlled by diffusion of the drug through the polymeric matrix. The drug release rate is not constant but decreases with time in a nonlinear manner. Several representative examples are reviewed -- nitrodisc and nitro-dur transdermal nitroglycerin systems, synchron hydrogel matrix, and aprotinin-releasing hydrogel.

37] Membrane systems: Theoretical aspects

This chapter discusses two types of membrane systems: membrane-coated reservoirs and monolithic matrices. These two systems involve diffusion of drug through a polymer membrane. Principles of diffusion in polymers are presented. The most important type of membrane system for controlled release consists of a nonporous, homogeneous polymer. Transport through membranes occurs by a solution-diffusion process in which the solute (drug) dissolves in the membrane and diffuses through the membrane in the direction of lower solute concentration. Membrane-coated reservoirs consist of a reservoir of active ingredient (drug) enclosed by a polymeric membrane. Drug release occurs by diffusion from the reservoir through the membrane to the external medium. A monolithic system consists of a polymeric matrix containing a uniformly dispersed or dissolved drug. Drug release occurs by diffusion through the polymeric matrix to the surface of the device.