Monthly Archives: February 2014

Metallic Biomaterials

Metals in the body can corrode and possibly cause damage to an implant and harmful interactions with its corrosion prod­ucts. Some metals, such at iron (Fe) and cobalt (Co), are re­quired by the body for normal function but are still harmful if available in more than minute quantities. Implants con­sisting of stainless steels, cobalt-chromium (CoCr) alloys, and titanium (Ti) and titanium alloys are corrosion resistant and biocompatible. Stainless steels with molybdenum (Mo), types 316 and 316L, have increased salt-water corrosion resistance and are commonly found in temporary implants like fracture plates or screws. Type 316L, which differs from type 316 only in carbon content, is more widely recommended. Cobalt-chro – mium alloys are used in dentistry and artificial joints (cast – able CoCrMo) and in knee and hip prostheses (wrought CoN – iCrMo). Titanium is a strong, lightweight metal that is ideal for implants. However, its poor shear strength precludes it from being used in bone screws and plates. Titanium-nickel alloys (TiNi) have the uncommon property of shape memory effect (SME). This is involved when, after deformation, a ma­terial returns to its previous shape when heat is applied. At­tempts to take advantage of this property include research into intracranial aneurysm clips, contractile artificial muscles for an artificial heart, and orthopedic implants. There are other specialized uses for metals, such as platinum alloys for electrodes and tantalum for wire sutures. In dentistry, gold provides durable, corrosion-resistant fillings, and gold alloys are implemented in cast restorations, inlays, crowns, and cusps. Dental amalgam for cavities is a mixture of liquid mer­cury with silver, tin, copper, and zinc (5).

METHODS OF PRODUCTION Surface Diffusion Process

In this process, a tape of V or Nb is dipped into a Ga or Sn bath. The heat treatment then forms V3Ga or Nb3Sn. It is also possible to hold the bath at the reaction temperature, forming the A15 phase during immersion. To use the better flexibility of tapes and getting the relatively thin layers of the brittle A15 only, other methods, such as sputtering the V or Nb to the core material by cathodic deposition or chemical vapor de­position, or by condensing under high vacuum conditions have been applied. For special MRI magnets, tapes of (Nb, Zr)3Sn are introduced, operating at 9 K (16).

ANTENNA, APERTURE

See Aperture antennas.

ANTENNA ARRAYS

Consisting of two to thousands of antenna elements, an ar­ray antenna presents the ultimate in flexible antenna pat­tern control. This capability includes electronic scanning, planar or conformable apertures, the ability to transmit or receive multiple shaped patterns, and adaptive control for jammers, clutter, and multipath suppression and for the reduction of cosite interference. Arrays benefit from tech­nological advances, and they are the key technology drivers for solid state T/R (transmit/receive) modules, advanced signal processing, and photonic technology.

Antenna arrays are often simply called “phased” arrays, a term that refers to the progressive phase shift intro­duced to scan the beam. We will use this term through­out this section, although the beam can be scanned by ei­ther time delay devices or phase shifters, depending on the required system’s bandwidth. The distinction will be ad­dressed later.

The basic principle behind the operation of the phased array, as illustrated in Fig. 1, is that the RF power be di­vided among a number of elements by a power divider, with each element signal shifted appropriately in phase or in time. Relative time delay is portrayed in the sketch by cir­cular phase fronts emanating from each element of the ar­ray, with signals either radiated at the same time [Fig. 1(a)] or delayed in time by an increasing amount from left to right so that the rightmost signal radiates last. The figure shows how radiation from each element adds in space so as to create an outgoing wave with an appropriate scan angle. Although not shown in the figure, the array power divider usually provides equal line lengths from the source to each element because this leads to optimum system bandwidth.

The power divider can also be used to control the signal amplitude at each element. This unequal power division is called amplitude “tapering,” and it provides for sidelobe control. At broadside, with element spacing chosen prop­erly, the array directivity is that of the broadside aperture,

ANTENNA, APERTURE

where A is the aperture area and ea is the aperture ef­ficiency, which depends on the array’s taper design. The scanned beam of Fig. 1(b) will have reduced directivity and (usually) additional losses that further reduce the array gain.

DISPLAY SYSTEM

The two-dimensional projection of Fig. 2 must be presented to the simulator pilot within the original viewing angles Ф and

0. This may be accomplished by a small image nearby or a larger image further away (Fig. 3). It may be a ‘‘real image’’ projected on a screen or traced on a CRT or a ‘‘virtual image’’ created by optics. A real image is limited by practical consid­erations to be within a few meters from the eyepoint. A vir­tual image can be as far away as desired and even infinitely far. With the pilot’s eye at the eyepoint, all the images in Fig. 3 create the same impression on the retina, with the same resolution. But there are significant differences:

• Accommodation. The pilot’s eye must accommodate opti­cally to the distance at which the image is located rather than the real-world distance of the objects it contains. Should the pilot need corrective lenses to aid in accom­modation, these would not necessarily be the same in the simulator as in flight.

• Parallax. Even when seated, the pilot’s upper body and head is free to move to some extent. As the eye moves, nearby objects (e. g., the cab environment) change their apparent position relative to objects that are further away. With the simulator display this would be governed by the distance of the image rather than the distance to the objects it represents. Objects in the image will not

DISPLAY SYSTEM

Figure 3. Image planes at varying distances from the viewer create the same impression on the retina, with the same resolution. But accommodation, parallax, and stereopsis effects differ and betray a close by image for what it is—a small, flat picture.

move relative to each other. Should the pilot’s eye devi­ate from the nominal eyepoint, the perspective would be­come distorted. During forward flight this would create the impression of a spurious sideways component of mo­tion.

• Stereopsis. When the pilot’s two eyes observe the same image from slightly different vantage points, the two ret­inal impressions differ. This difference is the raw mate­rial for stereopsis, which determines apparent distance. The distance so determined is that of the image rather than of the objects it represents. The stereopsis cue might conflict with other cues—for example, perspective cues and cues based on the size of familiar objects.

These effects are most pronounced with a small, nearby dis­play, such as a monitor screen. They flag the image as a small, flat picture. A human being can transcend this detail when appreciating art. To some extent, one can transcend it during training of specific tasks. Screen displays as close as one meter have been used successfully and accepted well by experienced pilots. However, to attempt physical equiva­lence, one must do better. This is where the display system comes in.

A screen projection is a significant improvement over a monitor screen. The image may be projected either from the front of the screen or, with a suitable screen, from the rear. Back projection has the advantage that the projector is out of the way of the pilot and cab structure. It is possible to place the projector so as to avoid distortion and the need for distor­tion correction.

A larger image placed, typically, three meters away is eas­ier to perceive as real. The accommodation is only 0.3 diopter from infinity. Parallax with nearby objects, such as the cock­pit structure and instruments, is approximately correct.

Infinity optics is a more effective solution. The image is optically placed infinitely far away. Accommodation is exactly
correct for distant objects as is parallax with the cab envi­ronment.

To avoid color fringes, infinity optics must employ mirrors rather than lenses. A collimator, illustrated in Fig. 4, is a common example. The monitor is set at 90° to the pilot’s line of sight. A “beam splitter’’ semireflective glass plate, set at 45°, reflects the monitor screen into the concave spherical mirror. The pilot views the mirror through the beam splitter. The monitor face is at the mirror’s focal point (half radius as measured along the broken optical path). Light originating from a point on the monitor comes out of the mirror as a par­allel pencil of rays, putting the image out at infinity.

A collimator typically covers the field of view of one chan­nel. Three channels may be combined by a battery of three collimators set at an angle to each other. Such batteries are designed with ‘‘overfill.’’ This means that the pictures in adja­cent monitors overlap. When the pilot’s head moves, parts of the scenery that were near the edge of one collimator are now seen in the other. This way, the three collimators offer a seamless combined view.

The collimated image at infinity can be seen only when the viewer’s eye is within the fairly narrow collimated beam. Collimators act as funnels with an opening to the distant scene. Eyepoint movement does not distort the scene, but ex­cessive movement blocks it. Two pilots cannot share a colli­mator. They must be given two separate collimators even when the same IG channel drives both with an identical im­age. Supplying more than one crewmember with a wide field of view is impractical because of mechanical interference of the systems of collimators.

DISPLAY SYSTEM

Figure 4. A collimator serving as infinity optics. The monitor faces down. The screen is reflected into a concave spherical mirror by a diagonal semi-reflective glass plate. The pilot views the mirror through the plate. The mirror creates an image located at infinity.

DISPLAY SYSTEM

Collimators cannot match the field of view offered by a spherical dome that encloses the pilot and makes a borderless projection screen. But sharing of a dome or screen projection by two crew members is problematic. The basic image at in­finity is the same, but the distortion correction is different for the two eyepoints.

DISPLAY SYSTEM

Figure 5. A six-post motion platform is capable of six DOF motion. The platform carries a simulator cab and a display system with wide – angle infinity optics. The display system employs back projection on a spherical screen which the crew views reflected in a large spheri­cal mirror.

Figure 5 shows an elegant solution: an infinity optics sys­tem that can serve several crewmembers and provide them with a correct, wide-angle outside view regardless of their po­sition in the cockpit. The picture is back-projected by a num­ber of projectors (only one is shown) onto a spherical screen. The simulator crew views this display through a large con­cave spherical mirror. The screen and mirror are concentric with their radii matched to put the screen at the focal surface of the mirror as viewed from the cab. The mirror creates a virtual image located out at infinity that can be seen from anywhere in the cab.

Neither the projected image nor the one viewed through infinity optics offers correct stereopsis, parallax, or accommo­dation for objects that are not far away. This is significant for operations where nearby objects play a role, including aerial refueling, spacecraft docking, and maneuvering helicopters near terrain and objects.

Stereopsis can be achieved by offering separate images for the two eyes. When this is done, the stereo cue is expected to overpower the accommodation cue and the parallax cue with which it is not consistent.

Three-dimensional images that are inherently correct in stereopsis, accommodation, and parallax for any viewer and for multiple viewers at the same time can be produced by ho­lography. But holography requires creation of an interference pattern with resolution of the order of the wavelength of visi­
ble light (in the order of 10~8 m). This capability is not yet available in real time.

Separate images for the two eyes (or for that matter, for two crew members) can be offered with projection systems and infinity optics systems by use of polarized light or of elec­tronically timed shutters. In the former case, two separate images are projected on the screen using mutually orthogonal polarization. The pilot views the display through polarizing lenses, so that each eye sees only one image. In the latter case, the two images alternate. The pilot views the display through electronically timed liquid crystal shutters. These block each eye when the image intended for the other is pro­jected.

Head (or helmet)-mounted displays (HMD) offer separate collimator-like display systems for the two eyes. The HMD requires head tracking to determine the instantaneous orien­tation of the eyepoint. Head movement can sweep a narrow field of view over a much wider field of regard. These systems typically induce the pilot to substitute head movement for eye movement, and the natural ability to notice moving objects in one’s peripheral vision cannot be exercised.)

The quality of HMD depends on the precision of head tracking and its latency. The display requires a fast update rate to keep up with fast image changes due to abrupt head movement. HMDs typically require individual fitting. The size and weight of an HMD is a burden on the civilian pilots. Even military pilots, used to flying with a helmet, often ob­ject. Besides, the HMD precludes the use of operational hel­mets and viewing devices in the simulator.

The eyepoints used for the HMD are generic. They repre­sent the eye positions of a typical pilot. Static adjustment to the pilot’s seat position, torso height, and eye separation is feasible. Dynamic adjustment to body and head movement is not in the current systems.

For use with an HMD, the database models the inside of the cab as a black silhouette. The HMD reflects its images on beam-splitters that allow the pilot to see through into the cab. Even so, there is a potential problem when two crew members sit side by side. The silhouette of the other crew member’s head cannot be predicted perfectly and will not register accu­rately. Bright outside scenery may ‘‘show through’’ the edges of the other crew member’s helmet.

Brightness is an issue for all simulator displays. One must assess the brightness available at the source and how much of it reaches the observer’s eye through the display system optics. These estimates are too involved to be presented here. The bottom line is that there is no difficulty in creating what an observer will accept as a daylight scene. The brightness of this scene is far below actual daylight. Pilots do not use their sunglasses in simulators. Simulator cabs are darkened during operation unlike aircraft cockpits in daytime. By the same token, problems of observing certain dimly lit displays in sun­light do not arise in the simulator.

It was not possible to describe in this section all the types of display systems in current use. Some of the ones not cov­ered are calligraphic displays, multi-resolution displays, and area of interest displays.

BIOLOGICAL EFFECTS OF ELECTROMAGNETIC FIELDS

A biomedical engineer involved in the study of the biologic effects of electromagnetic fields studies a variety of diagnostic and therapeutic applications of electromagnetic fields and the adverse effects of such fields. The diagnostic uses of extremely low-frequency (ELF) magnetic fields include magnetic reso­nance imaging (MRI), which also uses a radiofrequency (RF) field, and neural stimulation by strong magnetic pulses. Bone and cartilage repair, soft-tissue healing, and nerve repair or regeneration are among the therapeutic applications under investigation for applications of low-frequency, pulsed electro­magnetic fields (PEMF). There is considerable concern in to­day’s society regarding the bioeffects of electromagnetic fields, but no deleterious effects have been shown to be associated with long-term exposure to low-level electromagnetic fields.

In cancer treatment, heat generated by radio-frequency en­ergy (3 kHz to 300 GHz) or microwave energy (300 MHz to 300 GHz) kills radiation-resistant tumor cells. This therapeu­tic use of electromagnetic energy is termed hyperthermia. An­other use of electromagnetic energy, called electroporation, involves the use of an electrical pulse to disrupt the mem­branes of cells. This is a common DNA transformation tech­nique used in biotechnology. There is additional interest in electroporation that stems from its possible use in drug deliv­ery systems.

BIOMATERIALS

Biomedical engineers working in this area are concerned with researching and designing safe and reliable synthetic materi­als that can intimately contact living systems and tissues. This contact makes it essential that these materials are phys­iologically acceptable and pharmacologically inert, that is, nontoxic and noncarcinogenic. Additional requirements in­clude (1) adequate mechanical strength, (2) adequate fatigue life, (3) proper weight and density, and (4) usable in reproduc­ible and cost effective large-scale fabrication (4). Examples of biomaterials range from replacement parts to sutures, diag­nostic aids, and tooth fillings. The three main classes of bio­materials are metals, ceramics, and polymers.