Study On Measurement System For The Thickness Of Drug-eluting Stent Coating Based On Laser CCD Sensor

With the improvement of living standard, cardiovascular disease has become one of the most serious diseases which do great harm to people’s health. Drug-eluting stent(DES) is a kind of implantable medical device used in the treatment of cardiovascular disease, which has been widely applied in clinical. DES can prevent vascular restenosis after surgery, which is the outstanding advantage when compared with traditional stents. DES uses polymers to carry medicine. So there is usually a layer of a few decades to more than one hundred microns thick on its surface. When implanted into people’s blood vessels, the polymers release medicine to prevent hyperblastosis. The thickness and distribution of polymers must be strictly restricted, as studies have shown that they have important implications for DES’s performance and effect. However, the common methods for coating thickness measurement are complex, part of the methods are destructive.This paper describes a kind of non-contact displacement measurement system based on laser CCD sensor, with the advantage of fast measurement speed, high precision, simple operation, high degree of automation and non-destructive. It can basically meet the needs of drug-eluting stents coating thickness measurement.This system is based on laser trigonometry measurement principle. When laser falls on an object, light from the surface gets scattered. Use a focusing mirror to gather the light and focus it to the CCD. We can get the displacement of the object after converting analog CCD signal to digital signal and processing it. The hardware of this system has three parts: probe, mainboard and interaction panel. Probe consists of laser driver, brightness control module, CPLD, CCD driver and signal processing circuit based on programmable amplifier. Mainboard consists of DSP, memory module, high speed ADC, FIFO, serial port communication module, power supply module. Interaction panel consists of nixie tubes, LEDs and keys, which are all driven by CH452. This paper expounds the design principle and methods of the hardware modules. Moreover, the PCB board has been made and tested.In the aspect of software design, this paper firstly discusses the method of CPLD design based on Verilog HDL. Solving the problem of how to use CPLD to adjust brightness and drive CCD normally. Secondly, this paper discusses the design method of DSP software based on C language and CCS3.3, and describes the program flow chart and function of each module. Finally, this paper discusses the PC software function and design method based on VB language.This measurement system has three characteristics. First, a CCD of 7500 pixels is used to guarantee the accuracy and resolution. Second, it has two speed modes. Low speed mode is fit to measure static objects and high speed mode is fit to measure moving objects. Third, by using programmable amplifiers and automatic brightness adjustment module, this system can satisfy the requirement of different environment.In the experiment, we firstly tested the function of each hardware module, built the complete system when each module worked normally. Then we used a self-made test platform to measure the coating thickness of a simulative drug-eluting stent. The collected data showed that this system could measure the coating thickness correctly, the accuracy was guaranteed. Then the coating thickness of a real drug-eluting stent was tested. The result showed that laser CCD sensor could be used to measure the coating thickness of drug-eluting stent. Finally we discussed the structure of a platform which was used to fabricate drug-eluting stent. This measurement system was included in the platform, so the coating thickness of drug-eluting stent could be measured when fabricating.