Uric acid crystal study reveals the effect of water on mechanical properties

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A large percentage of molecular compounds – materials ranging from explosives to pharmaceuticals – can crystallize in different hydration states. This makes understanding how water affects their physical properties such as solubility, thermal stability, and an important research focus.

The uric acid crystals that make up kidney stones can exist in both hydrated and anhydrous (water-free) forms. Both forms share a similar two-dimensional layer structure, making uric acid a particularly useful crystal system for examining the contribution of water molecules to overall mechanical properties.

A team from Georgetown University, Finishing Manufacturing Science (Sigma-2), and the Center for Integrated Nanotechnologies (MPA-CINT) used nanoindentation and atomic force microscopy (AFM) to assess how these two crystal forms responded to localized surface stresses. The presence or absence of water between the layers imparts these crystal forms with dramatically different mechanical properties. Their results showed that uric acid is substantially harder and more brittle than hydrated uric acid. The journal Chemistry of Materials published their findings.

The study relied on the Laboratory’s expertise in atomic force microscopy (AFM) and in nanoindentation, which involves indentation hardness tests applied to small volumes. Post-indent imaging revealed slip planes in preferred crystallographic directions and oriented crack formation at higher load forces. By contrast, the hydrated forms were much softer and had substantial creep in response to indentation. Time-lapsed images of hydrated crystal indents revealed that some amount of “self-healing” on the surface may be possible at shallow indentation depths of ∼100 nm.

Figure. Postindent images of anhydrous uric acid (100) resulting from cono-spherical indents at loads of (A) 2000 micronewtons and (B) 5000 micronewtons.

These findings have significance to understanding fundamental materials science and treating kidney stones. Mechanical perturbation is a current method of treating kidney stones. Therefore, determining and controlling the mechanical properties of these surrogate stones could provide insight into the development of better treatments.

Reference: “Mechanical Properties of Anhydrous and Hydrated Uric Acid Crystals,” Chemistry of Materials 30, 3798 (2018); doi: 10.1021/acs.chemmater.8b00939. https://pubs.acs.org/doi/10.1021/acs.chemmater.8b00939 Authors: Fan Liu and Jennifer A. Swift (Georgetown University), Daniel E. Hooks (Finishing Manufacturing Science, Sigma-2), Nan Li and Nathan A. Mara (Center for Integrated Nanotechnologies, MPA-CINT).

The work supports the Laboratory’s Nuclear Deterrence and Global Security mission areas and its Materials for the Future science pillar by expanding the understanding of defects and interfaces and improving controlled functionality. NNSA funded the Los Alamos portion of the research, which was performed, in part, at the Center for Integrated Nanotechnologies (CINT), a DOE Office of Science Basic Energy Sciences user facility jointly operated by Sandia National Laboratories and Los Alamos National Laboratory. Technical contact: Daniel Hooks

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Caption 1:

Figure. Postindent images of anhydrous uric acid (100) resulting from cono-spherical indents at loads of (A) 2000 micronewtons and (B) 5000 micronewtons.