F.
Schwemmer
*a,
T.
Hutzenlaub
b,
D.
Buselmeier
a,
N.
Paust
ab,
F.
von Stetten
ab,
D.
Mark
b,
R.
Zengerle
abc and
D.
Kosse
ab
aLaboratory for MEMS Applications, IMTEK – Department of Microsystems Engineering, University of Freiburg, Georges-Koehler-Allee 103, 79110, Freiburg, Germany. E-mail: frank.schwemmer@imtek.de
bHahn-Schickard, Georges-Koehler-Allee 103, 79110 Freiburg, Germany
cBIOSS – Centre for Biological Signalling Studies, University of Freiburg, 79110 Freiburg, Germany
First published on 24th June 2015
The generation of mixtures with precisely metered volumes is essential for reproducible automation of laboratory workflows. Splitting a given liquid into well-defined metered sub-volumes, the so-called aliquoting, has been frequently demonstrated on centrifugal microfluidics. However, so far no solution exists for assays that require simultaneous aliquoting of multiple, different liquids and the subsequent pairwise combination of aliquots with full fluidic separation before combination. Here, we introduce the centrifugo-pneumatic multi-liquid aliquoting designed for parallel aliquoting and pairwise combination of multiple liquids. All pumping and aliquoting steps are based on a combination of centrifugal forces and pneumatic forces. The pneumatic forces are thereby provided intrinsically by centrifugal transport of the assay liquids into dead end chambers to compress the enclosed air. As an example, we demonstrate simultaneous aliquoting of 1.) a common assay reagent into twenty 5 μl aliquots and 2.) five different sample liquids, each into four aliquots of 5 μl. Subsequently, the reagent and sample aliquots are simultaneously transported and combined into twenty collection chambers. All coefficients of variation for metered volumes were between 0.4%–1.0% for intra-run variations and 0.5%–1.2% for inter-run variations. The aliquoting structure is compatible to common assay reagents with a wide range of liquid and material properties, demonstrated here for contact angles between 20° and 60°, densities between 789 and 1855 kg m−3 and viscosities between 0.89 and 4.1 mPa s. The centrifugo-pneumatic multi-liquid aliquoting is implemented as a passive fluidic structure into a single fluidic layer. Fabrication is compatible to scalable fabrication technologies such as injection molding or thermoforming and does not require any additional fabrication steps such as hydrophilic or hydrophobic coatings or integration of active valves.
In two-stage aliquoting, the metering chamber is combined with a valve for transport of the liquid after metering. Besides ensuring a full separation of liquids, e.g. for geometric multiplex PCR,6 two-stage aliquoting allows further liquid processing after metering. Typical valves for two-stage aliquoting are hydrophobic valves7–9 or geometric valves.10–12 Both types of valves depend strongly on the contact angle and surface tension of the metered liquid. Highly wetting liquids cannot be handled with these types of valves. Furthermore, geometric valves are additionally based on sharp edges, which require complex tooling for injection molding. For hydrophobic valves, localized surface modifications are introduced by an additional surface coating during manufacturing of the cartridge. Centrifugo-pneumatic aliquoting13,14 is based on pneumatic counter-pressure from an enclosed air volume in the collection chamber. Since the counter pressure from the trapped air is independent of liquid properties, the centrifugo-pneumatic aliquoting can be used for liquids with a wide range of contact angles. Strohmeier et al. aliquoted two liquids into shared collection chambers by sequential processing of the two liquids within a centrifugo-pneumatic aliquoting structure. However, such sequential aliquoting is limited to the same aliquoted volumes and aliquoting pattern. Moreover, the second liquid comes into contact with residue from the first liquid, which can lead to undesired premature reactions outside of the collection chambers.15
For assays where liquids of different volumes need to be combined in one reaction, the centrifugo-pneumatic valves can be combined with dissolvable films.16–18 Dissolvable films require a multi-layer cartridge by design and need additional fabrication steps for integration of the dissolvable films during fabrication. Aliquoting based on laser actuated ferro-wax valves19,20 is also not limited with respect to contact angles or surface tensions. However, additional process steps are required for integration of the wax valves and the respective processing device needs to include a laser for melting of the ferro-wax valves.
So far, no aliquoting principle is available for aliquoting of multiple liquids in a single fluidic layer, which guarantees liquid separation till the collection chamber. A likely reason no such aliquoting principle is available is the traditional radially outwards transport of liquids in combination with fundamental geometric limitations of crossing channels (see ESI† Fig. S1). With new unit operations for centripedal pumping it now becomes possible to overcome such geometric limitations.21–26 In this manuscript we introduce the centrifugo-pneumatic multi-liquid aliquoting. This aliquoting principle allows aliquoting in both the radially outwards and the radially inwards direction. The aliquoting overcomes previous limitations and allows for the first time a pairwise combination of aliquots with full fluidic separation before combination. It is based on pneumatic pumping,27–30 which has recently been extended to centrifugo-dynamic inward pumping.26 The fluidic principle relies only on pneumatic pressure and viscous dissipation and thus allows for aliquoting that is largely independent of surface tensions and contact angles. Since it does not require hydrophobic patches, capillary siphons or sharp edges, it can be easily fabricated with standard fabrication technologies, such as injection molding and micro-thermoforming.
The centrifugo-pneumatic multi-liquid aliquoting can be positioned on a wide range of radial positions. Especially noteworthy is that the aliquoting principle enables aliquoting from an outer array of metering chambers to an inner array of collection chambers using centrifugo-dynamic inward pumping.20 The aliquoting of multiple liquids on a single structured side significantly reduces the complexity of fabrication in comparison to state-of-the-art aliquoting. Furthermore, the aliquoting allows for simultaneous combination of liquids in an array of collection chambers, which is especially useful for reactions with fast reaction kinetics. These distinct advantages are useful for many applications, e.g. nucleic acid analysis of multiple samples with a shared master mix or multiple colorimetric assays with a shared sample material. We demonstrate the combination of liquid aliquots in joint reservoirs between an inner and an outer aliquoting structure within only one single fluidic layer. The generation of 5 μl aliquots for two implementations of the aliquoting structure is characterized in detail. We investigate the system's robustness by evaluating the influence of over- and underfilling of the disk and the dependency on liquid properties using three significantly different liquids. Furthermore, we give design rules that can be used for the layout of the centrifugo-pneumatic multi-liquid aliquoting.
Fig. 1 The process chain of centrifugo-pneumatic multi-liquid aliquoting consists of the unit operations inner and outer aliquoting. Each aliquoting structure comprises an inlet and multiple metering chambers connected to the inlet. Each aliquot from the outer aliquoting is combined with a complimentary aliquot from the inner aliquoting within a collection chamber. The principle of the metering structures to be implemented in centrifugal microfluidics is depicted in Fig. 2. |
An aliquoting structure can be realized by connecting one inlet with multiple of the described metering structures (see Fig. 3). In order to meter and transfer two liquids in a shared collection chamber, the aliquoting principle can be implemented as an inner and an outer aliquoting structure. For the outer aliquoting structure, the transfer channel connects the metering chamber to a collection chamber positioned radially inwards. For the inner aliquoting, this transfer channel is implemented as a siphon, connecting the metering chamber with the collection chamber positioned radially outwards. The implementation, as qualified in this manuscript, is shown in Fig. 3.
Vali = Vmeter + Vout − Vbf | (1) |
(2) |
Another critical parameter for centrifugo-pneumatic multi-liquid aliquoting is the size of the pneumatic chamber. In order to prevent any liquid transfer to the collection chamber during the metering step, the pneumatic overpressure in the pneumatic chamber during metering phase is balanced by the applied centrifugal pressure. The pneumatic overpressure in the pneumatic chamber during metering is:
(3) |
(4) |
(5) |
Disks were processed in a prototype LabDisk Player (Qiagen Lake Constance GmbH, Germany), which was modified to include a stroboscopic setup31 for monitoring of the microfluidic disk under rotation (BioFluidix GmbH). All reported aliquots were quantified using a custom programmed Matlab (MathWorks GmbH) based image recognition algorithm. To use this algorithm, every second collection chamber was equipped with an additional chamber designed for precise quantification of the aliquoted liquid volume (see Fig. 5). By detecting the two circular structures and the liquid meniscus the program quantifies volumes with a precision of approximately ±20 nl. A similar concept was recently reported in detail by Kazarine et al.32
Fig. 3 Microfluidic disk with 10 inner aliquoting structures and two outer aliquoting structures. The microfluidic disk (upper left) includes two outer aliquoting structures with twenty aliquots each and ten inner aliquoting structures with four aliquots each. The fluidic operation of the inner and outer aliquoting is described step by step for the marked inset. Labels A)–D) describe the fluidic state of the inner and outer aliquoting as detailed in the caption of Fig. 2. Green arrows indicate liquid flow at selected locations. A detailed description of the implemented channels and chambers is given in ESI† Fig. S3. An experimental run of the implemented design using colored dye can be seen in Fig. 4. |
Fig. 4 Experiment showing principle of centrifugo-pneumatic multi-liquid aliquoting with stained sample material. Labels A)–D) describe the fluidic state of the inner and outer aliquoting as detailed in the caption of Fig. 2. Every second collection chamber is combined with a quantification structure (a) for measurement of aliquoted volumes. The quantification structure is described in detail in Fig. 5. |
Liquid | Viscosity at 25 °C in mPa s | Density in kg m−3 | Advancing contact angle | Receding contact angle |
---|---|---|---|---|
Di water (0.1% Tween 20 v/v) | 0.89 | 1000 | 62.5° ± 4.8° | 12.0° ± 3.2° |
Ethanol | 1.1 | 789 | 42.7° ± 7.1° | 7.1° ± 1.5° |
Fluorinert FC-40 | 4.1 | 1855 | 19.0° ± 1.0° | 0° |
We quantify variations as overall variations, intra-run variations and inter-run variations. The overall variation is the coefficient of variation of all measurements of either the inner or outer aliquoting for a given liquid and input volume. For the quantification of variations between different runs it is useful to look at the inter-run CV, which is the coefficient of variation between the mean of individual runs. Variations within individual runs can be quantified by intra-run CVs, which is the coefficient of variation within one individual run. All underlying formulas are given in the ESI.† We define one run as the generation of 20 aliquots. Thus one run for outer aliquoting consists of 20 aliquots from one outer aliquoting structure. One run for inner aliquoting consists of 20 aliquots from 5 inner aliquoting structures, corresponding to one outer aliquoting structure.
The overall variation for inner aliquoting (outer aliquoting) was found to be between 0.5% and 1.1% (0.9% and 1.4%) for the tested liquids. The mean of the aliquoted volumes was increased by 1.2% to 2.5% (1.5% to 2.0%) from the targeted 5 μl for the inner aliquoting (outer aliquoting). This means both the inner aliquoting and the outer aliquoting structure satisfy the ISO 8655-5:2002 standard for pipettes in the same volume range for all tested liquids. Detailed results for all conditions, including the intra-run and inter-run variations, can be found in Table 2.
Input volume | Mean volume in μl | Systematic error | Overall CV | Intra-run CV | Inter-run CV | ||
---|---|---|---|---|---|---|---|
Inner aliquoting | 0.1% Tween 20 | Default | 5.08 | 1.6% | 0.6% | 0.6% | 1.1% |
+10% | 5.09 | 1.9% | 0.5% | 0.5% | — | ||
−10% | 5.06 | 1.1% | 0.3% | 0.3% | — | ||
Ethanol | Default | 5.06 | 1.2% | 1.1% | 1.0% | 1.2% | |
FC-40 | Default | 5.13 | 2.5% | 0.5% | 0.4% | 0.7% | |
Outer aliquoting | 0.1% Tween 20 | Default | 5.07 | 1.5% | 1.4% | 0.9% | 1.1% |
+10% | 5.09 | 1.8% | 0.6% | 0.6% | — | ||
−10% | 5.11 | 2.1% | 0.5% | 0.5% | — | ||
Ethanol | Default | 5.08 | 1.6% | 0.9% | 0.6% | 0.9% | |
FC-40 | Default | 5.10 | 2.0% | 1.0% | 0.8% | 0.5% |
Since a user can never fill the inlet with 100% accuracy, an aliquoting structure needs to tolerate varying input volumes. The maximum permissible variation for a calibrated pipette is 1% of systematic deviation and 0.5% for random variations in the range of 20–200 μl (ISO 8655-5:2002). In order for a microfluidic aliquoting structure to be robust, it should also tolerate additional variations for more complex liquid samples or mishandling by the operator. As a worst case scenario, we tested the aliquoting structures for variation of input volumes by ±10%. The mean aliquoted volume did not change within errors for the inner aliquoting 5.09 ± 0.03 μl (underfilling) and 5.06 ± 0.01 μl (overfilling) and for outer aliquoting 5.09 ± 0.03 μl (underfilling) and 5.11 ± 0.02 μl (overfilling). Furthermore, the coefficient of variation for aliquoted volumes did not increase for over & underfilling (see Table 2). Thus, the aliquoting principle tolerates variation of input volumes of at least ±10%. We determined the maximum permissible variation in input volume via a network simulation based approach.26,33 For inner aliquoting the maximum permissible variation is 20% for underfilling and 40% for overfilling. For outer aliquoting the maximum permissible variation is 25% for both underfilling and overfilling. In the presented design the maximum volume is limited by the size of the waste reservoirs. For inner aliquoting the minimum volume is limited by the 5 μl target volume. For outer aliquoting some liquid volume is required in the waste chamber in order to push all liquid out of the transfer channel during aliquoting. The outer aliquoting fails if the input volume is less than 5.5 μl per aliquot.
A total of 238 aliquots were tested in 24 runs. Out of 238 aliquots, 237 aliquoted volumes were found to be between 4.75–5.25 μl. One aliquot failed (Fig. 6, run no. 14) because of particles associated with the prototyping environment that blocked one channel. One inner aliquoting structure was excluded in an optical quality control prior to experiments. The corresponding run (Fig. 5, run no. 9) shows results of the 8 remaining aliquots. Fabrication with high precision steel tools, automated sealing and clean room conditions is expected to further improve performance of the centrifugo-pneumatic multi-liquid aliquoting (as observed by Mark et al.14 when comparing aliquoting CVs for prototyping and injection molding).
Fig. 6 Performance of centrifugo-pneumatic multi-liquid aliquoting for the inner and outer aliquoting structures with three different liquids. One run corresponds to 20 aliquots. Overall variation, inter-run variation and intra-run variation are given in Table 2. Inner aliquoting (upper row) and outer aliquoting (lower row) was performed for three different liquids (see Table 1) of varying contact angles, densities and viscosities. Overfilling by +10% volume and underfilling by −10% volume confirms robustness of the aliquoting against different input volumes. Error bars represent one standard deviation. Individual aliquots are represented by black dots. One aliquot of run no. 14 (Tween 20, outer aliquoting) was excluded from evaluation. The filling channel of this aliquot was completely clogged due to an error during fabrication. |
The centrifugo-pneumatic multi-liquid aliquoting is compatible to standard fabrication technologies and does not require sharp corners or surface modifications. Furthermore, we showed that the aliquoting principle is compatible to a variety of liquids and robust against changes in density and viscosity by more than a factor of two and four, respectively. The aliquoting was demonstrated for advancing contact angles between 20–60°. Such highly wetting liquids are very challenging to aliquot and cannot be processed with most of the available aliquoting principles. Lastly, the presented structures tolerate variations of input volumes by at least ±10%. For all tested liquids and volumes, the aliquoting meets the requirements in both accuracy and precision for calibrated pipettes in the same volume range as defined by ISO 8655-5:2002.
In the future, the aliquoting principle can be easily combined with microfluidic timers for timed sequential addition of metered reagents.33 Reagent pre-storage can be included by use of miniature-stick-packs,35 which can be introduced directly in the inlets to further reduce the number of required pipetting steps. Together with centrifugal step-emulsification the aliquoting could be used to combine multiple RPA master-mixes with a shared Mg2+ solution, prior to droplet generation for digital droplet RPA.36 We expect the new aliquoting to be useful for automation of applications, where one sample needs to be combined with multiple liquid reagents, or one liquid reagent needs to be combined with multiple samples. Possible applications range from PCR reactions of multiple samples and a shared master-mix, to multiparameter analysis of a single sample with different liquid reagents.
Footnote |
† Electronic supplementary information (ESI) available: Supporting figures and underlying formulas. See DOI: 10.1039/c5lc00513b |
This journal is © The Royal Society of Chemistry 2015 |