PIXL Pinning Pressure Report

Tom Drake, Singer Instruments, Roadwater, UK.
01/03/2019

Summary

This report covers the measurement and evaluation of the pinning pressure achieved by the PIXL Precision Colony Picker, produced by Singer Instruments.

The pinning pressure was measured using a bespoke test procedure, which replicated the standard pinning functionality of the PIXL. Over the span of 80 separate pinning events the PIXL achieved an average pressure of 6.97 ± 0.74 g/mm2, with a maximum recorded pressure of 8.531 g/mm2.

Introduction

When Singer Instruments were asked by their customers to develop a colony picker, one of the main requirements was to outperform the achievable precision and contamination rates of pneumatic firing pins. There are serious concerns that accelerated pin firing damages and/or misses important samples and can contribute to contamination. When designing PIXL, one of the primary considerations was the maximum speed of approach to ensure no chance of microbial material becoming airborne and causing contamination; cell splashing. Meticulous analysis identified the maximum speed of approach to be 19mm/s to ensure no cell splashing, and therefore this is the default approach speed for PIXL.

PIXL uses a novel polymer (PickupLine™) to pick microbial colonies and deposit them on a variety of target plates. The pressure applied to agar plates (and colonies) is vital to ensure agar is not pierced, this guarantees that picking is repeatable and that colonies are not damaged. PIXL was designed to have a gentle picking force and therefore pressure, to never pierce agar. We have minimised the chance of contamination through our regulated speed of approach and pressure. A high speed of approach could induce contamination if cells are forcibly hit by a firing pin causing cell splashing. Regulating contact pressure on every pick is vital to ensure that agar depth can vary across plates and that contamination is minimised whilst protecting microbial samples.

Method

To measure the pinning pressure delivered by PIXL, a laboratory scale was used. This was positioned at the source bay location replicating the location of a source plate. The height was representative of an agar filled PlusPlate.

For the testing, a routine was produced that would hold the applied weight to allow a reading to be taken from the scale. Every applied parameter in this routine was set to default to represent a typical picking run. The applied weights were recorded for eighty consecutive pinning events to give a representative sample.
The recorded weights were then converted to pressure, accounting for the diameter of PickupLine as 1mm.

Results

The data from eighty consecutive pinning events is presented in Figure 1, with the raw data and calculated pressure shown in Table 1 (below).

Figure 1 – Calculated pressure values for all pinning events. All eighty calculated pressure values are shown with their corresponding pin number. An average is shown and suggests there is no increase over time.

Each data point recorded is detailed below in Table 1. To account for the diameter of the PickupLine, a surface area of 0.785 mm2 was used to calculate surface pressure.


A mean pressure of 6.97 g/mm2 was calculated, with a standard deviation of ±0.74 g/mm2. The maximum pressure observed was 8.79 g/mm2 and the minimum pressure observed was 5.22 g/mm2.

Table 1 – Complete raw weight and calculated pressure for all recorded pinning events. All eighty data points are shown in full below, with corresponding ID, raw weight recorded, and calculated pressure values.

IDWeight [g]Pressure [g/mm2]  IDWeight [g]Pressure [g/mm2]
15.46.875  416.68.403
26.98.785  426.78.531
35.36.748  435.77.257
44.86.112  445.87.385
54.25.348  456.17.767
64.86.112  465.57.003
75.16.494  475.26.621
85.06.366  484.96.239
95.06.366  495.26.621
104.86.112  505.16.494
114.96.239  515.57.003
125.36.748  525.67.130
134.96.239  535.36.748
146.17.767  544.75.984
156.17.767  555.26.621
166.58.276  565.36.748
174.15.220  575.67.130
185.36.748  585.26.621
194.86.112  594.75.984
204.65.857  604.55.730
215.46.875  615.97.512
225.16.494  625.57.003
235.16.494  635.16.494
245.77.257  645.46.875
255.67.130  655.67.130
265.77.257  665.06.366
275.06.366  675.36.748
285.36.748  685.97.512
294.96.239  695.87.385
305.87.385  705.67.130
315.77.257  716.38.021
324.75.984  725.87.385
335.06.366  735.87.385
345.36.748  746.07.639
356.27.894  755.77.257
366.58.276  765.87.385
376.27.894  775.77.257
386.17.767  785.97.512
396.48.149  795.57.003
406.58.276  805.97.512

Conclusion

PIXL has demonstrated a consistent picking pressure of under 9g/mm2, with an average pressure of approximately 7g/mm2. This has been attained through regulated contact pressure on every pick, ensuring protection of both the agar surface and microbial sample, whilst minimising the risk of contamination.