Acoustic Micro Imaging Speeds Development of Blood Tester
An emerging testing method helped a diagnostic product manufacturer ensure the integrity of its cartridge housing.
You'll have to wait a little longer—we're waiting for the results of your tests."
Soon, healthcare practitioners in hospital emergency rooms will no longer have to wait hours after taking a patient's blood sample to obtain test results. Instead, they will be able to receive results by wireless communication in less than 15 minutes through the use of a new automated system that tests whole-blood samples right in the emergency room.
Called FasTraq, the automated system is nearing final development by Quantech Ltd. (Eagan, MN). The countertop system relies on the principle of surface plasmon resonance, which identifies and quantifies both native and foreign molecules by the interaction of light and electrons. FasTraq will look for molecules associated with specific diseases or conditions, including heart attacks, pregnancy, red and white blood cell counts, blood coagulation, kidney function, pancreas function, and electrolytes. The system will also perform tests for liver function, drugs of abuse, therapeutic drugs, and other conditions. Quantech has already received FDA 510(k) approval for its cardiac markers for myoglobin, for the heart attack marker CK-MB, and for a pregnancy test. A third heart attack marker, for Troponin I, meets clinical requirements. Quantech is developing numerous other tests as well.
A specialized cartridge called Prepaq, designed to hold blood samples for testing, is one of the most critical components of the system. A single cartridge can be used for up to six tests, and the system can accommodate up to 20 cartridges simultaneously. To ensure the reliability of the cartridges, Quantech turned to Sonoscan Inc. (Elk Grove Village, IL) to verify, through the use of acoustic micro imaging (AMI), the bond between the cartridge's base and a top film. An announcement in the March 2000 issue of PMP News introduced Sonoscan to Quantech.
Blood samples to be tested are collected by traditional means and placed in the cartridge, which consists of a solid plastic base, on top of which a film is bonded. In the top surface of the plastic base are numerous channels through which the blood or other fluids will flow during testing. The film forms the top boundary of these channels. Bonding the film to the base, however, must not block any channels, and there can be no delamination between the film and the base, which would permit leakage between channels.
Quantech could have used testing methods designed for less-demanding applications, such as pull tests or liquid flow tests. But since the functionality of the channels is key to FasTraq's operation, Quantech determined that it needed a reliable method that could permit development engineers to actually see the bond between the film and the Prepaq cartridge. AMI appeared to offer such capability.
In its basic form, AMI images and analyzes the integrity of internal bonds in materials. To do this, an AMI system pulses very high frequency ultrasound into a sample and makes images based on the intensity and polarity of the returned echoes. The frequencies used in AMI range from 10 MHz to more than 200 MHz; as the frequency increases, so does resolution. Penetration is less at high frequencies, but many applications do not require deep penetration. What matters is the behavior of ultrasound pulsed into a sample: air-filled gaps such as delaminations and voids in the sample reflect all of the ultrasound, and these gaps have the highest contrast in the acoustic image. Well-bonded interfaces between materials reflect a portion of the ultrasound and thus have moderate contrast. The degree of reflection from an interface depends on the difference in acoustic impedance (density times acoustic velocity) between the two materials involved. If one of the materials is air (or any other gas), the difference is maximized, since the density of air is so slight.
In making an acoustic image, an ultrasonic transducer of a selected frequency scans over the sample, pulsing ultrasound and receiving the return echoes several thousand times per second. The speed of ultrasound through most production materials (polymers, metals, ceramics) is very high, and the round trip typically takes less than 1 microsecond, a fact that enables the transducer to scan at high speed and still collect the very large number of data points needed for high-resolution imaging.
An important point in AMI is that echoes are reflected from various depths within the sample, and therefore arrive back at the transducer at slightly different times. This makes it possible to gate the return echoes electronically, using only those return echoes within a specified time window. The acoustic image then displays only the desired depth within the sample.
The Prepaq cartridge presented three layers for acoustic imaging: the film, the bond layer, and the bulk plastic below. The return echoes could therefore be gated rather widely to view the entire bond by including in the gate the lower part of the film, all of the bonding layer, and the top part of the plastic. But narrower gates could be used to image only the interface between the film and the top of the bonding layer, or only the interface between the bottom of the bonding layer and the top of the plastic. Gating at various depths gives precise information about a defect such as a delamination, and helps to evaluate the overall quality of the bond or to diagnose the cause of a defect.
Figure 1 is the AMI image of the seal of the film to the plastic cartridge during an early stage of cartridge development. White and gray areas are well bonded, while red areas are gaps. Since the film encloses the tops of the channels in the cartridge, the channels should appear as red, gap-type features. Some of the channels in Figure 1 are white, indicating that the process of sealing the film has also sealed the channel shut. In addition, there are several red areas away from the channels. These are delaminations of the film from the plastic base, and are troublesome because they could develop into leakers between adjacent channels.
Figure 1. High-resolution acoustic image of a portion of one film-cartridge bond at an early stage of development. Open channels are red, but some channels are blocked (white). There are also voids (red) in nonchannel areas.
Figure 2 is a similar AMI image of the same Prepaq cartridge type, but at a later point in the development process. All of the channels are now open (red), and, while there is slight variation in the bond quality in the nonchannel areas, there are no delaminations or other anomalies.
Figure 2. Similar acoustic image at a later stage of cartridge development. All channels are open, and there are no voids.
Direct, nondestructive information about internal features such as bond quality can greatly speed development and ensure reliability of a product in the field. In this application, the ability to evaluate the bond of the film to the test cartridge accelerated the development of an automated system that will cut costs, improve patient care, and save lives.