Can 1 gram hyaluronic acid really hold 6L of water?

According to this study (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3970829/), they make an (uncited) claim about hyaluronic acid: "One of its most important properties is that it can attach and hold large amounts of moisture; approximately 6L of water in just lg ."

Now, the MW of HA is $776.651$ g/mol, so there are about $\pu{0.00128865979}$ moles of HA in a gram. And about $\pu{7.757732e20}$ molecules.

Using google, 6L of water has $\pu{2.007e26}$ molecules.

So this is essentially telling me that roughly 1 million water molecules are able to hydrogen bond with 1 molecule of HA...

Firstly, is this fact true? And if so, how is this possible??

• I agree, that claim is a stretch. My guess is there was a units error, and they meant to say that you can hold $6~\mathrm L$ of water in $1~\mathbf{kg}$ of HA. – hBy2Py Feb 17 '17 at 23:50
• Bear in mind that hyaluronic acid is a polymer, and one molecule may have up to ~25k monomers, so your 1 million water molecules figure may not be that far off. (of course, its MW is 776 Da/unit, so the polymer may be 5 million Da, which changes your calculation) I'm not sure what "attach and hold" is supposed to mean, but perusing the literature, people seem to be making hyaluronic acid hydrogels at 1–2 wt. %. Not sure how low you can go and still have something that appears gel-ish. – Michael DM Dryden Feb 19 '17 at 8:26
• Note that the abstract at the original journal site here has not the typo which you copied, ‘1g’ → ‘lg’. – mykhal Oct 15 '18 at 17:12

Various figures can be found “on the internet”. E.g. ‘Hyaluronic Hydro Gel Has 1000 Times Water Holding Capacity Perfect for Hair to Heel’ (commercial link intentionally omitted).

Gel liquid phase absorption ability measure seems to be a ‘swelling capacity’ (ratio).

For hyaluronic acid (HA), it can be increased by additional crosslinking. In one paper from 1992, usual swelling ratio was found to be in the order of magnitude 1 to 10, the maximum figure was 120 (for crosslinking ratio much higher than 2.1; 25–30 for the ratio 2.1).[1]

In another paper, another guys are using biphasic nanoparticles (some composite of crosslinked and non-crosslinked HA), they state they have achieved for two different particle types ‘… swelling ratio 1270, 1000 %, respectively’[2] (note the percent sign). Which is, in both cases, approximately, not 1000× capacity, but rather 10× capacity, I'd say.

However, there are polymers with the claimed property – superabsorbents:[3]

3.27 superabsorbent polymer

Polymer that can absorb and retain extremely large amounts of a liquid relative to its own mass.

Note 1: The liquid absorbed can be water or an organic liquid.
Note 2: The swelling ratio of a superabsorbent polymer can reach the order of 1000:1.
Note 3: Superabsorbent polymers for water are frequently polyelectrolytes.

References:

1. Shah, Chirag B., and Stanley M. Barnett. "Swelling behavior of hyaluronic acid gels." Journal of applied polymer science 45.2 (1992): 293-298.
2. Chun, Cheolbyong, et al. "Effect of molecular weight of hyaluronic acid (HA) on viscoelasticity and particle texturing feel of HA dermal biphasic fillers." Biomaterials research 20.1 (2016): 24.
3. Compendium of Polymer Terminology and Nomenclature (IUPAC Recommendations 2008)

Attach and Hold - a bit of a misnomer; it forms a 3 D Lattice matrix containing water; given its inherent viscosity and inherent elasticity, it can be compressed and upon release of pressure spring back to its original dimensions thus the visco-elastic/shock absorbing properties advantages medically (knees) and cosmetically (skin fulfillment/youthful appearance etc).