Time-Resolved 2D Shrinkage of Commercial Resin-Based Composites Using Laser Interferometry With PCIe-8231 and myDAQ

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"The precision and sampling rates of the PCIe-8231 and myDAQ, as well as the customization possible with LabVIEW, helped our research team realize the innovative shrinkage strain measurement method"

- Daniel Labrie, Dalhousie University Department of Physics and Atmospheric Science

The Challenge:
Measuring shrinkage strain during the curing of resin-based composites in dentistry applications

The Solution:
Capturing the topography of a resin sample as it cures in real time by using a Michelson interferometer with a CCD camera connected through a PCIe-8231 GigE Vision adapter interface card, while using a photodiode connected to a myDAQ device to validate the measurements

Author(s):
Daniel Labrie - Dalhousie University Department of Physics and Atmospheric Science

Dalhousie University, a public research university in Nova Scotia, Canada, has a history of successes and a long list of notable alumni. The Department of Physics and Atmospheric Science and the Department of Dental Clinical Sciences at Dalhousie University partnered to tackle a major challenge in dental restoration: measurement of shrinkage when curing resin-based composites (RBCs).

Shrinkage strain is a difficult issue with RBCs that can result in bond failure between the tooth and resin, leading to a variety of problems either with the filling or the tooth. Contemporary materials research has reduced the shrinkage in cured RBC, but there is still an appreciable effect in clinical applications. The ability to accurately measure the physical change in the RBCs during or after curing allows for better anticipation and prevention of the many adverse consequences of shrinkage strain.

An Optical Solution to Shrinkage Strain Measurement

There are many shrinkage strain measurement techniques; however, they all have shortcomings including high sensitivity to temperature changes and sample impurities during measurement, low data collection rates, and the inability to measure shrinkage in real time. Our research team developed a powerful new optical method using a Michelson interferometer and taking advantage of the precision and sampling rate of the PCIe-8231 controller and the myDAQ device to solve these problems. We chose NI products for the experiment for their specific capabilities, the ease of use and programming, and the exceptional support that NI delivers.

The devised experimental setup uses laser interferometry to analyze a sample prepared with the bonded disc method, wherein a disc-shaped RBC sample of 1.22 mm thickness and 10 mm diameter is placed on a 3 mm thick quartz plate of 25.4 mm diameter, as illustrated in Figure 1. In essence, the Michelson interferometer uses a beam splitter to divide one laser beam into two, with one reflecting off a reference mirror and another reflecting off the top surface of the sample, then it recombines the two beams to result in an interference pattern caused by the differences in path lengths. This pattern reveals the surface geometry and thus axial shrinkage of the sample.

Figure 1. Bonded Disc Geometry

 

The apparatus consists mainly of the Michelson interferometer, a 632.8 nm emission wavelength 1 mW output power helium–neon (HeNe) laser, a CCD camera with a data acquisition rate of 122 frames per second (fps) connected to the PCIe-8231 interface, as well as a photodiode connected to a myDAQ device to verify the measurements from the CCD camera, both using LabVIEW software programs for video acquisition. Figure 2 shows a schematic and a photograph of the setup.

Figure 2a. Apparatus Layout
From “Time-resolved 2D shrinkage field of dental resins using laser interferometry,” Applied Optics 54(7), 1852-1860 (2015). Used with permission.

 

Figure 2b. Apparatus Photo
[1] HeNe laser (632.8 nm), [2] 6 mm focal length plano-convex lens to expand the laser beam, [3] Aperture to block stray laser light, [4] Plane mirror, [5] 750 mm focal length lens to collimate the laser beam, [6] Plane mirror, [7] Aperture to define the collimated laser beam to 10 mm diameter, [8] Michelson interferometer, [9] CCD camera, [10] Si photodiode, [11] Photodiode amplifier, [12] Light curing unit and sample holder.

 

Addressing Challenges Using PCIe-8231 and myDAQ

The experiment has a few immediate challenges, including the precision and capture rate of the image and the calibration of the apparatus. We used the PCIe-8231 GigE Vision adapter interface with a Basler scA40-120 gm CCD camera to capture the interference patterns at 122 fps with each camera pixel imaging a 20.6 μm × 20.6 μm area on the sample surface. The resulting maximum axial shrinkage rate that the system can measure without aliasing is 19.3 μm∕s.

To verify that this sampling rate is sufficient for the experiment, we used a Si photodiode with an active area diameter of 1 mm to observe the central area of the sample with a diameter of 0.3 mm. We amplified the photodiode signal and connected it to a myDAQ device, which has an acquisition rate of 1,000 samples per second. Using a separate measurement method with the photodiode and myDAQ also verifies the synchronicity of the camera data collection system. Figure 3 shows one visualization of these verifications and compares the signal collected using the photodiode with that from a single pixel at the center of the CCD camera throughout the sample polymerization. This comparison shows that the CCD camera is not under sampling as the irradiance measurement is seen with peaks and troughs matching those seen by the photodiode even at the millisecond scale.

Figure 3. Sampling and Synchronization Verification With Si Photodiode and myDAQ
From “Time-resolved 2D shrinkage field of dental resins using laser interferometry,” Applied Optics 54(7), 1852-1860 (2015). Used with permission.

 

Results

The precision and sampling rates of the PCIe-8231 and myDAQ, as well as the customization possible with LabVIEW, helped our research team realize the innovative shrinkage strain measurement method. Our newly developed method makes it possible to precisely measure fast and minute changes in dental filling material geometry in real time, which could help address major challenges in dental restoration. An example of real-time measurements of the sample shrinkage from one experiment is shown below.

Figure 4 shows an instance when the sample RBC used is the Filtek Supreme Ultra CT from 3M ESPE. Photo curing is done by illuminating the sample with a Plasma Arc LCU (Sapphire) on bleach mode (1.38 W∕cm2) with a turbo light guide for an exposure time of 30 s. Polymerization continues even after the LCU is switched off, and the selected frames show the interference pattern recorded up to 81.1 s after the LCU is switched on.

Figure 4. Real-time Capture of Interference Patterns on RBC Sample Surface
The time frames selected were 0 s (1) when the LCU was turned on, 0.98 s (2), 2.95 s (3), 4.92 s (4), 7.38 s (5), 9.84 s (6), 18.0 s (7), and 81.1 s (8).
From “Time-resolved 2D shrinkage field of dental resins using laser interferometry,” Applied Optics 54(7), 1852-1860 (2015). Used with permission.

 

As further context, Figure 5 shows a 3D contour plot reconstructed from the real-time measurements with a plot near the time displayed in frame 7 of Figure 4.

Figure 5. 3D Contour Plot at 18.162 s of Photo Curing
From “Time-resolved 2D shrinkage field of dental resins using laser interferometry,” Applied Optics 54(7), 1852-1860 (2015). Used with permission.

Author Information:
Daniel Labrie
Dalhousie University Department of Physics and Atmospheric Science
Dalhousie University, 6310 Coburg Rd
Halifax, NC B3H 1Z9
Canada
Tel: 9024942322
Daniel.Labrie@Dal.ca

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