Tuesday, August 26, 2008

Adeno-associated viruses

Adeno-associated viruses, from the parvovirus family, are small viruses with a genome of single stranded DNA. The wild type AAV can insert genetic material at a specific site on chromosome 19 with near 100% certainty. But the recombinant AAV, which does not contain any viral genes and only the therapeutic gene, does not integrate into the genome. Instead the recombinant viral genome fuses at its ends via the ITR (inverted terminal repeats) recombination to form circular, episomal forms which are predicted to be the primary cause of the long term gene expression. There are a few disadvantages to using AAV, including the small amount of DNA it can carry (low capacity) and the difficulty in producing it. This type of virus is being used, however, because it is non-pathogenic (most people carry this harmless virus). In contrast to adenoviruses, most people treated with AAV will not build an immune response to remove the virus and the cells that have been successfully treated with it. Several trials with AAV are on-going or in preparation, mainly trying to treat muscle and eye diseases; the two tissues where the virus seems particularly useful. However, clinical trials have also been initiated where AAV vectors are used to deliver genes to the brain. This is possible because AAV viruses can infect non-dividing (quiescent) cells, such as neurons in which their genomes are expressed for a long time.

Friday, August 22, 2008

Active Object

This article is about a multi-threading technique. For the lockstep protocol variant, see Active objects.

The Active Object design pattern decouples method execution from method invocation that reside in their own thread of control.s The goal is to introduce concurrency, by using asynchronous method invocation and a scheduler for handling requests.

The pattern consists of six elements:

* a proxy, which provides an interface towards clients with publicly accessible methods
* an interface which defines the method request on an active object
* a list of pending requests from clients
* a scheduler, which decides which request to execute next
* the implementation of the active object method.
* a callback or variable for the client to receive the result.

Tuesday, August 12, 2008

Wide Area Augmentation System

The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to augment the Global Positioning System (GPS), with the goal of improving its accuracy, integrity, and availability. Essentially, WAAS is intended to enable aircraft to rely on GPS for all phases of flight, including precision approaches to any airport within its coverage area.

WAAS uses a network of ground-based reference stations (Benchmark DGPSRs transmitting differential corrections (DCs, located within spaces protected from the public inside airportsin North America and Hawaii, to measure small variations in the GPS satellites' signals in the western hemisphere. Measurements from the reference stations are routed to master stations, which queue the received DCs and send the correction messages to geostationary WAAS satellites in a timely manner (at least every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receiver uses the corrections while computing its position to improve accuracy. The longer any given DC has been delayed, the less benefit it will produce.

The International Civil Aviation Organization (ICAO) calls this type of system a Satellite Based Augmentation System (SBAS). Europe and Asia are developing their own SBASs, the Indian Gagan, the European Geostationary Navigation Overlay Service (EGNOS) and the Japanese Multi-functional Satellite Augmentation System (MSAS), respectively. Commercial systems include StarFire and OmniSTAR.

Wednesday, August 06, 2008

Quartz rate sensors

This system is usually integrated on a silicon chip. It has two mass-balanced quartz tuning forks, arranged "handle-to-handle" so forces cancel. Aluminum electrodes evaporated onto the forks and the underlying chip both drive and sense the motion. The system is both manufacturable and inexpensive. Since quartz is dimensionally stable, the system can be accurate.

As the forks are twisted about the axis of the handle, the vibration of the tines tends to continue in the same plane of motion. This motion has to be resisted by electrostatic forces from the electrodes under the tines. By measuring the difference in capacitance between the two tines of a fork, the system can determine the rate of angular motion.

Current state of the art non-military technology (2005) can build small solid state sensors that can measure human body movements. These devices have no moving parts, and weigh about 50 grams.

Solid state devices using the same physical principles are used to stabilize images taken with small cameras or camcorders. These can be extremely small (≈5 mm) and are built with MEMS (Microelectromechanical Systems) technologies.