Zurich Instruments Newsletter - Edition Q1/2012
- Interview with Loes Segerink, University of Twente: Development of a fertility chip
- Reducing Lab Complexity: Powerful cross-platform programming support with HF2LI/HF2IS
- Detailed Application Know-How: Optical phase measurements with HF2LI
- Premium Customer Support: PASCA project update
- Tips & Tricks: How to set the parameters of a PID Controller
Interview with Loes Segerink, University of Twente
Development of a fertility chip
Hello Loes, your research subject sounds straightforward: "Development of a fertility chip". What does this chip actually do and what problem does it address?
This chip measures the quality of semen by looking at the concentration and motility of spermatozoa, two important parameters that provide information concerning a man’s fertility. If a couple can't have children they will end up consulting a medical doctor and one of the first steps will be to carry out a semen assessment. The best method currently for semen assessment is a manual procedure but this has the disadvantage that it is not really accurate due to it being subjective analysis as well as being time consuming. To overcome these drawbacks we are developing the fertility chip.
This addresses a hot topic in the health sector. How would a fertility test work, and what are the benefits of this technique?
The idea is that the male involved will use our system, carrying out several tests with disposable chips at home. As our system allows objective measurements to be carried out several times, it is possible to make a more reliable statement about the semen quality. This will aid the gynecologist in choosing an appropriate treatment.
This research will likely result in a product. What is your current view about that? When might a market introduction take place?
At the moment we are trying to form a spin-off company, however, a lot of work has to be carried out first before it becomes a product. In our work we have shown that a microfluidic chip can be used to assess the semen, but a hand-held system has not yet been developed. Furthermore, our system needs to be validated before it can be released on the market, so we think it will take us at least 5 years before it is available for you to buy.
Will men eventually be able to buy a fertility test in the drugstore around the corner?
We don't know. Our idea is that the male involved will acquire the system from the gynecologist, this professional can then also provide advice and information to him. We feel this is necessary as couples need help if they are unable to have children and only a medical doctor can provide such assistance. If however people convince us of the idea to sell our system at the drugstore then this would also be possible.
Which other research teams do you know that are employing electrical measurements to analyze semen cells? How is your work different from theirs?
There is currently a research group in Taiwan developing a test for the assessment of semen by looking at the concentration of motile spermatozoa, which is a combined parameter. They use a Coulter counter in combination, which detects the motile cells that have a tendency to swim against the flow .
At the beginning of 2010, there was a press release from your institute which resulted in a great deal of important media coverage of your research. How did you experience those times?
It was a bit of a hectic period. The press release was the result of an article that was published in Lab on a Chip . This was the first article published concerning our research and we were really surprised by the amount of media attention. The feedback was nice to have and it also made us realize that our research is very useful to society.
 Chen, Y.A., et al., Analysis of sperm concentration and motility in a microfluidic device. Microfluidics and Nanofluidics, 2011. 10(1): p. 59-67. doi:10.1007/s10404-010-0646-8
 Segerink, L.I., et al., On-chip determination of spermatozoa concentration using electrical impedance measurements. Lab on a Chip, 2010. 10: p. 1018-1024. doi:10.1039/B923970G
Reducing Lab Complexity
Powerful cross-platform programming support with HF2LI/HF2IS
How often in the past have you been stuck with legacy software in your measurement setup? When was the last time you had to train LabVIEW to a skilled MATLAB programmer (or vice versa)? Why would someone prefer GPIB at 1 Mbit/s instead of USB at 480 Mbit/s rate? All of these issues have arisen in the past and as you prefer to invest your time carrying out research rather than re-engineering software, you would certainly prefer if you could write code in the programming language most suited to you. The Zurich Instruments software for the HF2LI/HF2IS provides a best-in-class example for cross-platform programming support.
Independent of whether you prefer working with C++, Python, MATLAB, or LabVIEW, Zurich Instruments supports all of these programming languages. Furthermore, they can all be used at the same time. From the Zurich Instruments viewpoint all user programs are clients that access the same server. Legacy code can run concurrently with new software written in another language, while the settings for the ZI Instrument will remain consistent as they are centrally managed by the server. Appropriately designed user software can benefit from this management and regularly read back the effective settings, which may have been changed by another client.
A user interface on the instrument front-panel, with one button for one function, used to be an attractive approach. However, the advantages of having the graphical user interface on a computer or notebook largely outweigh the drawbacks. In particular, intrinsic cross-platform programming as part of the Zurich Instruments standard software delivery is a consequence: once the user interface is on a computer adding support for multiple concurrent user interfaces is a natural step. This of course requires powerful APIs being made available to our users. Considering that setups in research labs are in use for several years and the lifetime of measurement instrumentation is counted in tens of years, Zurich Instruments provides an effective solution for managing legacy software and available software skills.
In the ZI Blogs, experts share their programming experiences with the community. Don't forget to check these blogs when starting a new project. Programming languages have their strengths and weaknesses, and ZI allows the user to choose the most appropriate one for his requirements.
Features of ZI supported programming languages.
(C is also supported, but not listed below)
- Popular in the academic and industrial world
- Efficient for simple user interfaces
- Provides many advanced mathematical functions
- Strong plotting and display options
- Increasingly awkward for complex user interfaces
- Poor multi- language support
- Deployment requires installation of large run time environment
- Limited performance for some tasks
- Matrix manipulation
- Publishable graphics
- Highly portable
- Widely used in academic research and industrial R&D
- Expensive and specialized toolboxes are sold separately
- Deployment requires installation of large run time environment
- Limited performance when built‐in functions can't be used
- Free and open source
- Good readability
- Low maintenance cost
- Simple syntax for ease of learning
- Simple import of external modules
- Multi-language support
- Practical as a glue language between modules of different origins
- Applicable for small scripts to large interactive projects
- Object oriented
- Performance limitations
- Available for free
- Object oriented
- Most common programming language in academia and industry
- Capable of high-performance
- Difficult to learn
- Cluttered with legacy constructs
- No enforcements for robustness
MATLAB® and LabVIEW™ are trademarks of their respective owners.
Detailed Application Know-How
Optical phase of a fiber optical Mach-Zehnder interferometer measured with the HF2LI by pseudoheterodyne detection (courtesy of Dr. Rolf Brönnimann, EMPA Dübendorf, Switzerland)
Optical phase measurements with HF2LI
Measurement of the optical phase is useful in displacement sensing and surface characterization, as well as for dispersion measurements in laser spectroscopy and photonics, nevertheless, direct access to the optical phase is an experimental challenge as today’s photo-detectors based on the photoelectric effect only measure the intensity of the absorbed light field. An elegant solution to this measurement problem is the use of a reference light field, which allows for the extraction of phase and amplitude from the resulting interference signal. An example of such a measurement configuration is the Mach-Zehnder Interferometer (MZI).
An additional challenge arises in experiments which yield only weak optical intensities, an example being near-field imaging and in certain spectroscopic experiments. In such situations, lock-in amplification combined with optical modulation in the interferometer can be applied for detection sensitivity well below the limit of the photo-detector used. If for instance a photo-detector with a detection limit in the lower picowatt regime is used, phase sensitive detection of signals in the lower femtowatt regime is feasible. The key for this sensitivity enhancement is the reference field having a much higher intensity than the measurement signal, typically by six orders of magnitude. The fingerprint of the weak optical measurement signal in the resulting interference signal is therefore substantially enhanced. The lock-in amplifier combined with optical modulation enables detection of this fingerprint with very high noise rejection at the shot noise limit.
In the Mach-Zehnder Interferometer, light is split into two paths and recombined for interference. Typically the reference path is held at a constant optical path length. The signal path is used to stimulate the experiment and provides the optical measurement signal of interest. To enable lock-in detection, modulation of the optical phase is incorporated into the interferometer. The measured interference intensity then has oscillating components containing optical amplitude and phase information of the measurement signal, both relative to the reference field amplitude and phase. Knowing the reference field intensity even allows for an absolute determination of the measurement signal intensity. Phase instabilities generated inside the interferometer are the source for limitations in the data interpretation.
The demodulation scheme for the lock-in amplifier depends on the modulation technique, as demonstrated in the two examples below.
Example 1 (optical heterodyne detection): The reference field laser frequency is shifted by an acousto-optic modulator. Accordingly, the interference signal intensity contains one single beat oscillating at the shift frequency. Changes in the phase and amplitude of the demodulated interference signal relate to changes in the optical phase and amplitude, respectively. In analogy to heterodyne detection in the radio frequency domain, this phase sensitive optical detection technique is termed optical heterodyne detection.
Example 2 (optical pseudoheterodyne detection): The reference field phase is modulated sinusoidally by electro-optic or piezo-electric modulators. In the frequency spectrum of the phase modulated reference field, multiple sidebands around the laser frequency arise, these are all harmonics of the modulation frequency and depend on the phase modulation depth via a Bessel function weighting. The interference signal intensity therefore oscillates at several beat frequencies. The even harmonics are proportional to the optical in-phase component, which is the part of the measurement signal that is in phase to the reference field, whereas odd harmonics are proportional to the optical quadrature component, which is the part of the measurement signal that has a phase shift of 90° to the reference field. In order to fully resolve the optical phase up to 2π, both in-phase and quadrature components are required. Accordingly, at least one even and one odd harmonic of the modulation frequency need to be demodulated. The dependence on the phase modulation depth is usually cancelled by modulating with a depth of 2.63 radians, where the first and second harmonics are equally weighted. As this detection technique gathers the same information as heterodyne detection with frequency-shifting, it is usually termed optical pseudoheterodyne detection.
High Performance Lock-in Amplifier required
The Zurich Instruments HF2LI is a particularly well suited lock-in amplifier to analyze interference beats in heterodyne and pseudoheterodyne detection setups. Its frequency range up to 50 MHz covers the operation frequency of many modulators: piezo-electric phase modulators (kHz), electro-optic phase modulators (kHz to MHz) and even certain acousto-optic modulators (40 MHz). Thanks to the high noise rejection, detection limited by shot noise is enabled at MHz frequencies. With the ability to demodulate higher harmonics and up to 6 arbitrary frequencies simultaneously (HF2LI-MF), advanced demodulation schemes such as pseudoheterodyne detection are implemented in a straightforward manner. For measurements at the shot noise limit, researchers benefit from 120 dB dynamic reserve to capture the weakest signals. Finally, direct sideband demodulation (HF2LI-MOD) and an integrated phase locked loop (HF2LI-PLL) enable sophisticated measurements, where light interacts with mechanical resonators (AFM/SNOM and MEMS applications).
Premium Customer Support
PASCA project update
The PASCA project has entered its second year after passing its first annual review meeting with flying colors and Zurich Instruments being ahead of schedule on its deliverables. The European Union funded PASCA project (Platform for advanced single cell manipulation and analysis) forms a pan-European consortium of seven research and industrial partners to develop an innovative platform for the manipulation and analysis of single living cells. The process of printing cells is a hot topic in biotechnology research, and relates to the positioning of single cells onto specific locations, very much like ink-jet printers do on paper. Such technology will be essential in the future for the manipulation, culture and analysis of individual living biological cells.
During the first year, the PASCA team has produced the first prototype of the single cell manipulation instrument which has demonstrated how living single cells are printed at specified locations (SCM,http://www.pasca.eu/news/article/single-cell-manipulation-scm-prototypes-ready-for-evaluation). This involved combining a broad scope of different technologies including a microfluidic dispenser chip, an automated robotic XYZ stage for positioning the dispenser and an optical imaging and detection system for initiating and verifying cell printing. Furthermore, a microfluidic chip with electrodes to measure impedances was developed and produced.
Zurich Instruments, as the technology leader for the measurement of fast dynamic electrical signals, has contributed to the project by providing the project's principal technology partner with an HF2IS 50 MHz Impedance Spectroscope and HF2TA current amplifier in order to develop impedance-based cell classification algorithms. Furthermore, Zurich Instruments has provided impedance expertise to help perform the first dynamic impedance measurements on single biological cells and provided its real-time software for single cell detection and recognition.
Additional information can be found on the PASCA website, www.pasca.eu.
Tips & Tricks
How to set the parameters of a PID Controller
A proportional-integral-derivative (PID) controller is very useful for keeping a control system at a fixed set point. The regulation feedback is composed of a direct feedback (P: proportional), a past oriented feedback (I: integral) and a predictive feedback (D: derivative). Textbooks often take temperature and water level regulation systems as examples. Paired with lock-in amplifiers, PID controllers are employed e.g. for distance control in scanning probe microscopy (SPM), or frequency stabilization of lasers.
The performance of the regulation feedback loop essentially depends on the setup of the three feedback paths and on the open loop properties of the system to be controlled. In many cases the feedback loop is adjusted manually to best fulfill the required regulation behavior. The manual tuning of the feedback loop balances regulation bandwidth, overshoot and steady state error towards the user’s requirements.
The manual tuning procedure is as follows:
- Begin by increasing the direct feedback P, while keeping I and D at zero. At one point, the system starts oscillating. Set P to half this value for fast operation. If your system is susceptible to overshoot, you should decrease P to a quarter of this value, which results in a smaller regulation bandwidth.
- Increase the past oriented feedback I to reduce the steady state error. Be aware that a too high value will again result in an oscillation.
- Carefully increase the predictive feedback D for a faster settling time.
Users who prefer automatic tuning of the PID feedback loop can find several application-specific quick tuning recipes (heuristics) in the paper mentioned below.
 Anthony S. McCormack and Keith R. Godfrey, Rule-Based Auto-tuning Based on Frequency Domain Identification, IEEE Transactions on control systems technology, Vol. 6, No. 1, January 1998. doi:10.1109/87.654876