Working principle and relevant physical properties of the Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS) evaluation

Abstract

Purpose: To describe and evaluate relevant physical properties of the Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS) for ocular surface sensitivity measurement.

Methods: Characteristics of Liquid Jet (LJ) droplets (consisting of isotonic saline solution) were analysed: vertical and horizontal displacement and speed of LJ droplets were recorded with the aid of the High Speed Photron FASTCAM NOVA S6 camera (stimulus duration: 40 ms). Stimulus mass was assessed for 20 sets of 10 LJs with aid of a microbalance (pressure range of 100-1500 mbar).

Results: Because continuous flow LJ disintegrated into droplets in the lower pressure range (<700 mbar), pulsed stimuli were applied in order to obtain similar stimulus characteristics across the applied pressure range. For all measurements, very little variability was observed. Vertical and horizontal displacement did not exceed 0.13 mm in either direction. The mass per shot showed an unexpected cubic dependency on pressure. Up to approximately 700 mbar, LJ speed showed an almost linear relationship. For the pressure range of >700-1500 mbar, variability increased and speed decreased compared to the expected in a linear manner. However, this may be caused by the difficulty of identifying pattern changes of LJ droplets from one high speed image frame to the next with increasing stimulus speed, when determining LJ speed via pixel count.

Conclusions: Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity was shown to deliver fine droplets with a pulsed stimulus mode, in a repeatable manner with precise localisation to the ocular surface. Very little variability was observed in LJ speed and mass for the typical pressure range required for clinical sensitivity measurements.

Keywords: cornea; corneal sensitivity; liquid jet aesthesiometry.

FIGURE 1

Diagram of the Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS)

FIGURE 2

Close‐up diagram of the Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS)

FIGURE 3

Lateral view of Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS) in front of the subject's eye

FIGURE 4

Liquid jet arriving at the corneal surface

FIGURE 5

Example for determination of a threshold with stimulus intensity [dB] on the y‐axis and the number of stimuli presented to the ocular surface on the x‐axis. Green dots represent ‘yes’ responses and red dots represent ‘no’ answers. The dotted grey lines represent the upper and lower stimulus range limits, while the dotted turquoise line represents estimated thresholds and the blue line the true threshold

FIGURE 6

Jet images with a time difference of ten time frames: first frame in the top picture and ten frames later in the bottom picture; the patterns are similar but not identical. There is a very small shift of the overall pattern towards the right, as the droplets travel from left to right. The scale is marked in mm

FIGURE 7

Continuous Liquid Jet at pressures of (a) 200, (b) 250, (c) 350 and (d) 500 mbar (top four figures, from top to down), disintegrating into droplets at a distance of 5 mm from the exit valve. No disintegration of the continuous Liquid Jet at a pressure of 1000 mbar was seen in the bottom figure (e). The scale is marked in mm

FIGURE 8

Pulsed Liquid Jet at pressures of (a) 200, (b) 300, (c) 500, (d) 800 and (e) 1000 mbar (40 intervals, 1000 µs each, with a valve opening duration of 500 µs per interval; total stimulus duration: 40 ms). The scale indicates mm and applies to both horizontal and vertical directions

FIGURE 9

Mass per shot [mg] versus pressure [mbar] with a cubic fit. Error bars indicate the standard error means

FIGURE 10

Relationship between pressure [mbar] and Liquid Jet speed [m/s]. Error bars indicate the standard error means