Polycrystalline Silicon Films for Microelectromechanical Devices

Polycrystalline silicon is the most widely used structural material for surface micromachined microelectromechanical systems (MEMS). There are many advantages to using thick polysilicon films; however, due to process equipment limitations, these devices are typically fabricated from polysilicon films less than 3 μm thick. In this work, microelectromechanical test structures were designed and processed from thick (up to 10 μm) in situ boron-doped polysilicon films. The elastic modulus of these films was about 150 GPa, independent of film thickness. The thermal oxidation of the polysilicon induced a compressive stress into the top surface of the films, which was detected as a residual stress in the polysilicon after the device fabrication was complete.

Adhesion Failure in Bonded Rubber Cylinders Part 1: Internal Penny-Shaped Cracks

Rubber disks bonded between flat parallel metal plates are often used as adhesion test specimens such as ASTMD 429 1999, Method A. However, the mechanics of adhesion failure (debonding) for this geometry have not been fully analyzed previously. Therefore, a study has been conducted of the strain energy release rate (tearing energy) for bonded rubber disks having cracks at the rubber-to-metal bond. In this paper, we consider internal penny-shaped cracks. A future paper will discuss external ring cracks.

Finite element analysis was used to determine the tearing energy as a function of crack length for disks of various dimensions (shape factors). The crack configurations considered were an internal penny shaped crack located at the center of either one or both rubber-to-metal bonds. The rubber was assumed to be linearly elastic and nearly incompressible.

For any bonded disk held in constant tension, the tearing energy was found to be a non-linear function of crack length. For small cracks, the tearing energy was linearly related to the crack length. As the crack grew, the tearing energy increased until it passed through a maximum value. The peak tearing energy was found to depend on the height of the disk. Finally, for large cracks, the tearing energy decreased as the crack grew. Analytical and empirical models were developed and shown to be in good agreement for both small and large cracks in disks of different dimensions.

Theoretical study of the effects of tooth and implant mobility differences on occlusal force transmission in tooth/implant-supported partial prostheses

STATEMENT OF PROBLEM

Despite their mobility differences under occlusal loads, a natural tooth and an implant are often used together to support fixed prostheses. In some situations, tooth/implant-supported partial prostheses include cantilever extensions, especially in the posterior region where the bone is inadequate for placement of an additional implant.

PURPOSE

In this study, engineering beam theory was used to study the effects of the mobility differences between the implant and the tooth on the force and moment distribution, due to occlusal loads in tooth/implant-supported prostheses.

METHODS

The prosthesis was treated as a linear elastic beam and the supports were modeled as springs with (vertical) translational and rotational stiffness. The bending moments and forces on the supports were calculated as functions of the parameters that describe the geometry, position of the occlusal load, and stiffness ratios (namely, implant or tooth) of the springs.

RESULTS

Bending moments on the supports were more sensitive to the relative rotational mobility between the supports and their individual values than to the relative translational mobility. The moment at the implant was minimized when the supports had similar mobilities. A preliminary design concept was introduced and eliminated the moment at the implant without significantly increasing the magnitude of the moment at the tooth. Cantilevering the prosthesis resulted in moderately increased bending moments and considerable tensile forces on the supports for a broad range of the parameters that describe the geometry and loading.

CONCLUSIONS

From this simulation, it is suggested that cantilever extensions should be avoided or supported by a short implant, which will only restrain the vertical movement of the cantilever end.

Effects of attachment clips on occlusal force transmission in removable implant-supported overdentures and cantilevered superstructures

Attachment clips are commonly used to provide retention for removable implant-supported overdentures. In this study, the effects of attachment clips on occlusal force transmission in four implant-supported overdentures with cantilever extensions were investigated using beam theory. Distributions of moments and of forces in overdenture, clips, cantilevered superstructure, and implants were calculated as functions of position, number, and stiffness of attachment clips. Three-, four-, and five-attachment clip configurations were analyzed. Results showed that maximum bending moments and forces in all components are strong functions of position, number, and retention of attachment clips, while the effects of attachment clip stiffness are negligible. Increasing the number of attachment clips from three to five results in a significant decrease of maximum bending moments in the superstructure and implant. For a wide range of attachment clip positioning, the maximum tensile force on the implants is as high as half the applied load. At least one attachment clip for all configurations is also subjected to tensile force, which can cause it to slip out from the superstructure and increase maximum bending moment in the superstructure and implants.

Biomechanics of the facial skeleton

Several concepts have been discussed in this article. (1) The geometry and muscle attachments of the mandible are such that it is inconsistent with physics as we know today to have this structural arrangement always in tension at the upper surface and compression always at the lower. (2) The idea behind rigid internal fixation (RIF) is to stabilize the fracture to allow the mandible to work (function) as a nonfractured structure. We should therefore attempt to simulate conditions that most accurately approximate the unfractured structure. (3) The larger, older plates worked in most situations if the fracture was reduced anatomically. As physicians use smaller devices and combinations with unicortical screws, there will be less room for error, and the stability provided by the devices will be closer to the critical load characteristics of the fractures. (4) The "neutral" axis does not remain constant in a fixed location during mastication, either in unfractured or fractured mandible scenarios. Even if a standard neutral zone existed, it would be the worst place for plate placement because devices there would not function with mechanical advantage either in tension or compression loading. (5) Tangential and comminuted fractures generally should be addressed with caution and require special attention. The probability of the smaller plates providing adequate stability in these conditions is low, and the likelihood of failure is high. Physician discretion and judgment should be used to select a more stable system if early mobilization is the goal. (6) The plating systems available rely on adequate bone apposition at the fracture site. This, coupled with a degree of compression in many scenarios, provides a stable condition under functional loading. If movement occurs at the fracture site, the load characteristics change dramatically, with much higher demands placed on the plate systems, and failures will increase based on mechanics alone. When biomechanics are not considered, the incidence of infection, nonunion, and tissue injury may increase. (7) Treatment of structural defects of the midface should be directed to the reconstruction of "normal" pretraumatic load paths. (8) When dealing with plating systems in midfacial fractures, the placement of multiple screws on each side of the fracture provides for a more even distribution of loading (load sharing between the plate and the bone). (9) Stabilization of a fracture requires prevention of translation in all three directions and rotation about all three axes. Restraining a point solves translation but not rotation. Plates provide some rotational stability. The best mechanical advantage is obtained during fixation when plates are not placed along the same axis.(ABSTRACT TRUNCATED AT 400 WORDS)