In Situ TEM Shows Why Polymer Nanocapsules Deform

In Situ TEM Shows Why Polymer Nanocapsules Deform

Nanocapsules are widely used in various industries such as drug delivery systems. The recent study published in Langmuir focuses on the analysis of nanocapsule buckling and collapse using real-time transmission electron microscopy (TEM). The results are useful for understanding the mechanism of buckling and can help improve the design aspect of such structures.

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Study: Dynamics of buckling and collapse of polymer nanocapsules revealed by in situ liquid phase TEM. Image Credit: Employee of Love/Shutterstock.com

What are nanocapsules?

Nanocapsules consist of hollow nanoparticles with a diameter of 10 to 200 nm that enclose fluids in a spherical shell. The structures are made of chemical, organic and polymeric materials that have been created to discharge the encapsulated components with precise spatial and temporal regulation.

The ability to separate and then discharge chemicals with precise precision has many applications including drug delivery, agricultural studies, cosmetology, pressure sensors, physical controllers, cell anodes, and sequestration of carbon.

Prediction of buckling behavior

The buckling of geometric shapes, especially spherical nanocapsules, is a very complex concept. A shell can bend into a variety of morphologies in terms of size, structure, defects, and frequency of cargo discharge, ranging from bowl-shaped colloidal particles with solitary grooves to nanocapsules with numerous protrusions, topologies wrinkled or flattened disks.

Many paths for the slicing process are known, including those that result in the same edge topologies.

How do nanocapsules work?

Changes in acidity or osmolarity, as well as exposure to chemicals, temperature, radiation, electromagnetic fields, shear, vibration, or mechanical forces can all promote cargo unloading. Such activation is based on the principle that its thin walls make them susceptible to bending and breaking under compressive stress.

Buckling is caused by the in-plane energy transfer necessary to expand or compress the exterior of the shell in out-of-plane momentum. Even a small in-plane surface displacement can cause substantial and sudden changes in the shape of the nanocapsule.

Which capsule geometries are susceptible to buckling?

As the pressure differential across the surface approaches a predefined value of critical pressure, buckling occurs, exchanging unnecessary in-plane strain potential for bending energy. This critical differential differential needed to print or bend the shells is exactly proportional to the h/R ratio.

This ratio is closely related to the design methodology because h represents the thickness of the nanocapsule shell. The radius of the capsule is denoted R, in accordance with the conventional theory of shell buckling. Thinner walls and larger diameters of nanocapsules, indicating low h/R, are more likely to deform.

Advantages of the buckling study

A precise and instantaneous resolution of the intermittent bending phases would help us to better understand this complicated event and its reliability on the structural parameters.

These results could help researchers develop capsules that are resistant to sudden discharges and equipped with discharge processes adapted to certain uses. In theory, real-time imaging of nanocapsule degradation could show correlations between bending and critical physical factors such as shell defects.

The insights gained could be used to develop buckling action selection techniques useful for achieving desirable non-spherical geometries, stimulating cellular uptake, and facilitating interfacial aggregation.

Advantages of TEM examinations over conventional methods

Structural penetration has been used to explore in situ TEM examinations of the dry hollow shell from buckling, but many implementations require enclosures that enclose or operate in liquid.

The kinematics of nanocapsules and other fragile colloidal particles can now be studied with excellent temporal and spatial precision thanks to recent advances in liquid-phase TEM. TEM could be used to capture the whole mechanics of nanocapsule disintegration.

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The researchers demonstrated that h/R, the Föpplvon Karman number γ and the compaction frequency δV are the most important factors in determining buckling trajectories. Nanocapsules with solitary protrusions, the lower energy structure, result in a low γ value as well as a lower δV value as well. Large values ​​of γ or δV, on the other hand, create many thermodynamically confined markings.

The thickness to radius ratio is reduced resulting in deep indentations. Due to the increased consistency of the finished product, the collapse of the solitarily protruding bowl geometry, which can be achieved at low γ and δV, is preferred. Nanocapsules with reduced elasticity are more durable and less likely to deform, while those with high elasticity deform with modest pressure.

Quantitative cargo unloading speed is generally comparable to nanocapsule capacity, which is another key design guideline.

The new nanoscale method could replace complicated compressors and regulators in liquid reservoirs, enabling high-throughput microscopic electron imaging of various individual events (such as nanocrystal formation) on a specific substrate.

Reference

Alam, SB et. Al. (2022). Dynamics of buckling and collapse of polymer nanocapsules revealed by in situ liquid phase TEM. Langmuir. Available at: https://pubs.acs.org/doi/10.1021/acs.langmuir.2c00432

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