Nanobubbles Form at Active Hydrophobic Spots on the Luminal Aspect of Blood Vessels: Consequences for Decompression Illness in Diving and Possible Implications for Autoimmune Disease-An Overview

Abstract

Decompression illness (DCI) occurs following a reduction in ambient pressure. Decompression bubbles can expand and develop only from pre-existing gas micronuclei. The different hypotheses hitherto proposed regarding the nucleation and stabilization of gas micronuclei have never been validated. It is known that nanobubbles form spontaneously when a smooth hydrophobic surface is submerged in water containing dissolved gas. These nanobubbles may be the long sought-after gas micronuclei underlying decompression bubbles and DCI. We exposed hydrophobic and hydrophilic silicon wafers under water to hyperbaric pressure. After decompression, bubbles appeared on the hydrophobic but not the hydrophilic wafers. In a further series of experiments, we placed large ovine blood vessels in a cooled high pressure chamber at 1,000 kPa for about 20 h. Bubbles evolved at definite spots in all the types of blood vessels. These bubble-producing spots stained positive for lipids, and were henceforth termed "active hydrophobic spots" (AHS). The lung surfactant dipalmitoylphosphatidylcholine (DPPC), was found both in the plasma of the sheep and at the AHS. Bubbles detached from the blood vessel in pulsatile flow after reaching a mean diameter of ~1.0 mm. Bubble expansion was bi-phasic-a slow initiation phase which peaked 45 min after decompression, followed by fast diffusion-controlled growth. Many features of decompression from diving correlate with this finding of AHS on the blood vessels. (1) Variability between bubblers and non-bubblers. (2) An age-related effect and adaptation. (3) The increased risk of DCI on a second dive. (4) Symptoms of neurologic decompression sickness. (5) Preconditioning before a dive. (6) A bi-phasic mechanism of bubble expansion. (7) Increased bubble formation with depth. (8) Endothelial injury. (9) The presence of endothelial microparticles. Finally, constant contact between nanobubbles and plasma may result in distortion of proteins and their transformation into autoantigens.

Keywords: decompression illness; endothel; nucleation; sheep; stabilization; surfactant.

Figure 1

Evolution of nanobubbles on a hydrophobic surface. Nanobubbles which evolved from dissolved gas on a hydrophobic surface observed using atomic force microscopy (taken from Tyrrell and Attard, with permission).

Figure 2

Expansion of decompression bubbles on a hydrophobic surface. Six hydrophobic wafers on the periphery and one hydrophilic wafer in the center, photographed 2.5 h after decompression from 300 kPa (20 m sea water) (taken from Arieli and Marmur, with permission).

Figure 3

Phospholipids on the capillary. Electron microscope of the luminal aspect of a cerebral capillary from a sheep. An oligolamellar lining of phospholipids can be seen (taken from Hills, with permission).

Figure 4

Hydrophobicity on blood vessels. Hydrophobicity was demonstrated at specific locations using a small drop of saline and a large contact angle (taken from Arieli and Marmur, with permission).

Figure 5

Bubble-producing spots. An example of defined spots, as revealed by bubble development from a pulmonary vein and aorta. Some spots produced just one bubble at a time, whereas others produced several bubbles at once.

Figure 6

Active spots are hydrophobic. Samples of blood vessels photographed under saline, revealing the AHS at which bubbles nucleate and expand. The same sample after staining for lipids (dark red) is presented above its previous presentation. Arrows indicate the bubbles on the AHS and the corresponding red staining for lipids.

Figure 7

DPPC on blood vessels. The amount of DPPC (mean ± SD) in AHS which produced more than four bubbles over 30 min, compared with those which produced <4 bubbles and control samples (adapted from Arieli et al., with permission).

Figure 8

Size on detachment. The distribution of bubble diameters on detachment, in pulsatile flow and in stable conditions.

Figure 9

Removal of small sized AHS. (Left) Distribution of AHS (black columns) and the distribution of AHS staining positive for lipids (empty columns), as a function of their bubble production. (Right) The odds ratio value of having a higher bubble score over 4 consecutive days of diving compared with day 1 (adapted from Zanchi et al., with permission).

Figure 10

Dispersed initiation of AHS. Frequency of initiation of active hydrophobic spots (AHS). Initiation is defined as the moment the first bubble from an AHS reaches a diameter of 0.1 mm, as a function of time from decompression. The dotted line is a suggested smooth function (taken from Arieli and Marmur, with permission).

Figure 11

Distribution of bubblers—non bubblers. (A) In blood vessels of sheep. (B) (data compiled from Lambrechts et al., 2013) and (C) (data compiled from Cialoni et al., with permission), in divers.

Figure 12

Activation of AHS. (Left) Time intervals between bubble detachments from the AHS as a function of the sequence of detachment from the same AHS. The first value is the time from decompression (adapted from Arieli and Marmur, with permission). (Right) Percentage of high bubble grades after the first and second dives on the same day (adapted from Dunford et al., with permission).

Figure 13

Effect of exposure pressure on bubble formation. (A) Diameter of bubbles which detached from an ovine blood vessel plotted against time from decompression. The first period before pump operation (gas tension in the proximity of the vessel 228 kPa), and the second after pump operation (gas tension in the proximity of the vessel 600 kPa). (B) Density of decompression bubbles on a hydrophobic silicon wafer as a function of exposure pressure (taken from Arieli and Marmur, with permission). (C) Bubble score in a diver after decompression from various pressures (adapted from Eckenhoff et al., with permission). (D) Bubble score after decompression from various depths (adapted from Cameron et al., with permission).