AUTOMATED HEMATOCRIT MEASUREMENT AND
PATIENT DATA LABELING BY A COMMERCIAL DVDWRITER WITH A LOW-COST OPTICAL ADD-ON
L. Riegger1, J. Steigert1, S. Lutz1, W. Streule1, R. Zengerle1,2, and J. Ducrée2
1
Laboratory for MEMS Applications, Department of Microsystems
Engineering (IMTEK), University of Freiburg, Germany
2
HSG-IMIT, Institute for Micromachining and Information Technology, Germany
ABSTRACT
This contribution derives an actuation, detection and labeling unit for a recently
presented centrifugal hematocrit (Hct) test [1] based on an „off-the-shelf“ LightscribeTM
DVD-writer and corresponding disk media. While centrifugation is enabled by the DVDdrive, the LightscribeTM technology allows to directly store individual patient data and the
obtained diagnostic results in a human-readable format on the LightscribeTM surface of the
processed disk. To implement an automated, absorption-based readout of the hematocrit
after centrifugation, we added a low-cost green power LED and a PIN-photodiode to the
commercial writer. Using the device, fully automated Hct measurements and labeling have
been conducted on untreated whole blood at a very good reproducibility (CV = 2.9 %) and a
high linearity (R2= 0.995) within 3 minutes, only!
Keywords: DVD-writer, low-cost optics, hematocrit, LightScribeTM, centrifugal
microfluidics
1. INTRODUCTION
For the screening of blood in centralized laboratories, small sample volumes as well as a
high degree of parallelization and automation to limit the number of critical manual
handling steps is of great importance. These demands can be met by so-called “lab-on-achip” technologies [2] which seek to enable full process integration, reduced consumption
of sample and reagents as well as short times-to-result and ease of handling. Among the
various lab-on-a-chip concepts, “lab-on-a-disk” [3,4] systems exploit centrifugal forces for
flow control, metering, accelerated sedimentation of particles in biological suspensions and
mixing in order to run fully process integrated assays.
In our lab-on-a-disk approach, we modify a conventional DVD-writer which acts as a
cost-efficient centrifuge with low-cost optics to enable the automated, absorption-based
determination of one of the most relevant parameters in medical diagnostics and routine
blood screening - the hematocrit. Further, by utilizing the LightscribeTM-technology, the
obtained patient data can be stored next the sample for displaying and archiving of the
diagnostic results.
2. PRINCIPLE AND OPTICAL CONCEPT
The concept is based on the high differential absorption ('A ~ 2) of a cellular pellet of
RBCs with respect to the plasma at the designated probe wavelength (Opeak = 535 nm)
(Fig. 1-A). Assuming full absorption by the cellular pellet, the Hct of the blood sample can
hence be determined from the intensity of the transmitted light in a predefined area as
recorded by the onboard photodiode (Fig. 1-B).
Eleventh International Conference on Miniaturized Systems for Chemistry and Life Sciences
7 – 11 October 2007, Paris, FRANCE
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Figure 1. (A) Light absorption of packed RBCs in contrast to blood plasma. 'A ~ 2 at a
wavelength of 535 nm essentially implies that the probe beam is transmitted by the blood
plasma and almost completely absorbed by the RBCs. (B) Optical concept for the
hematocrit determination. The emitted light of a green power LED propagates through an
aperture to define the spot size. The impinging light is partially absorbed by a centrifuged
blood sample. The light intensity captured by a photodiode is thus inversely proportional to
the Hct value.
3. HYBRID DISK FABRICATION AND MODIFICATION OF THE DVD-WRITER
First, the hematocrit structure constituted by an inlet, an overflow channel with a
hydrophobic barrier for metering and a capillary for sedimentation is micromachined in a
polycarbonate disk (Fig. 2-A). Next, the reflective coating is removed from the outer part of
a conventional LightscribeTM-CD to permit the transmission of the probe beam. The two
disks (Fig. 2-B) are finally bonded by an adhesive layer.
The standard LightscribeTM-writer (Fig. 2-C) comprises a milled aperture, a green
power-LED, a low-cost PIN-photodiode with an amplification circuit and a USBoscilloscope for data logging (not shown). The oscilloscope could be easily substituted by a
digital signal processor (DSP) in a product development.
Figure 2. (A) The channel structure features an inlet, a hydrophobic barrier for metering
and a measurement capillary (l = 13 mm, h = 300 µm, w = 1 mm) for sedimentation. The
spot size of the light beam defines the working range of the Hct determination. (B) The
microfluidic disk is bonded to a conventional LightscribeTM-CDROM with a partially
removed reflective surface. (C) The experimental setup comprises a standard DVD-writer
with a milled aperture, a green power LED, a low-cost PIN-photodiode with an
amplification circuit and a USB-oscilloscope for data logging (not shown).
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7 – 11 October 2007, Paris, FRANCE
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4. EXPERIMENTAL PROCESSING, RESULTS AND PATIENT DATA LABELING
After an unmetered sample of whole blood (V > 5 µL) is introduced via the inlets, the
disk is inserted into the DVD-drive. While reading the inner data track, the disk spins to
precisely meter the blood sample and subsequently sediment the RBCs (t = 2 min) [1]. The
photodiode signal of the transmitted light is measured at Q = 50 Hz to determine the Hct by
comparing the values of the measurement capillaries with the calibration curve (Fig. 3-A).
The method displays a very good reproducibility (CV = 2.9 %) and a high linearity (R2 =
0.995). Finally, after flipping the disk, the LightscribeTM surface is permanently labeled
with the Hct values and the patient data next to the respective blood sample (Fig. 3-B).
Figure 3. (A) Calibration curve for the hematocrit determination. The measurements
feature a very good reproducibility (CV = 2.9 %) and a high linearity (R2 = 0.995) over the
entire physiological range. (B) Hct value and patient code labelling by the LightscribeTMtechnology. The results are permanently written next to the respective blood samples into
the disk surface.
5. CONCLUSION
We introduced a novel, absorption-based hematocrit test on an unmetered amount of
whole blood drawn from the finger tip which is carried out on a device derived from a
commercial DVD-writer. The processing device features low-cost components (< 100 $),
only, while the disposable microfluidic substrate can easily be fabricated by standard
polymer microfabrication. The possibility to write permanent, human-readable information
next to the blood sample onto the LightscribeTM surface of individual disks offers promising
options for point-of-care settings as well as quality control measures in centralized labs.
REFERENCES
[1] L. Riegger et al., Single-step centrifugal hematocrit determination on a 10$ processing
device, Biomedical Microdevices, publ. online, DOI 10.1007/s10544-007-9091-1.
[2] R. Oosterbroek and A. van den Berg, Lab-on-a-Chip: Miniaturized systems for
biochemical analysis and synthesis, Elsevier Science, Amsterdam, Netherland, 2003.
[3] M. Madou and G. Kellogg, LabCD: A centrifuge-based platform for diagnostics,
Proceedings of SPIE, SPIE publisher, San Jose, 3259, pp 80–93, 1998.
[4] Jens Ducrée, Stefan Haeberle, Sascha Lutz, Sarah Pausch, Felix von Stetten, Roland
Zengerle. The centrifugal microfluidic Bio-Disk platform. Journal of Micromechanics
& Microengineering, 2007. Status: in press.
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