Applications

AFM of biomaterials

Biomaterials are materials that are designed to interact with biological systems, or materials made from biological sources. The complexity of advanced materials and biological systems creates challenges in micro- and nanoscale characterization for R&D or QC projects. Atomic force microscopy (AFM) allows us to approach this subject from both a materials and biology perspective.

Imaging the interface of biology and materials

Biomaterials are materials that are designed to interact with biological systems, or materials made from biological sources. The complexity of advanced materials and biological systems creates challenges in micro- and nanoscale characterization for R&D or QC projects. Atomic force microscopy (AFM) allows us to approach this subject from both a materials and biology perspective.

AFM is unique in measuring quantitative three-dimensional specimen surface topography and mechanical data in a single tool and it can operate in the specific gas or liquid environment where the biomaterial fulfills its function. The AFM also differentiates itself from transmission and scanning electron microscopy (TEM/SEM) by simpler, nontoxic specimen preparation, ease of use, and substantially lower cost. The key benefits of AFM are in its ability to easily extract statistical quantities of the specimen topography (e.g. roughness) and to measure mechanical and other parameters through the direct interaction with the sample surface (e.g. phase imaging and force-distance curves).

Filtration membranes fall into the biomaterials category and are ubiquitous in water and air filtration both in everyday life and in laboratory applications. Despite standard labels listing pore sizes and solvent compatibility, the underlying structure can vary dramatically and cause differences in performance among “identical” membranes. Atomic force microscopy (AFM) is the ideal tool to compare and control micro- and nanoscale features before, during, and after use.

Topography imaging

Choosing a membrane can seem confusing. There are many membrane types and materials to choose from, with varying chemical compatibility to specific solvents and supposedly identical pore size. Simple AFM topography scans of dry membranes illustrate substantial differences in the membrane structure and can help with the selection of the appropriate type. The simplicity and low cost make AFM the ideal tool for this task. A membrane can be punched, mounted on an AFM disc, and scanned down to the nanoscale in minutes without any further sample preparation. The quantitative, three-dimensional data is easily extracted and can be used for quality control of pore distribution and pore size.

Nanoscale arrangement of cellulose fibers in a protein dialysis membrane with 14 kDa cutoff.

Microscale characteristics of polyethersulfone (PES) membrane with 0.22 µm nominal pore size.

Microscale characteristics of polycarbonate membrane with 0.8 µm nominal pore size.

Pore distribution in a different polycarbonate membrane with 0.8 µm nominal pore size. Notice the high pore density with areas where individual pores have merged.

Line profiles showing the consistent diameter of individual pores in comparison
to a 0.8 µm distance (vertical lines).

Membrane performance analysis

AFM can also be used to perform failure analysis of the membrane after use and to characterize both the filter cake and residual particulates in the filtrate.

A flat membrane such as the polycarbonate membrane above can be used to concentrate particulates in the filter cake. A readily available biomaterial to demonstrate this is milk, a colloid suspension.1

Analysis of the filtrate is as relevant, for example when studying the micro and nanoparticulates in drinking water.2 In cases of such particle mixtures, the phase information can be used to differentiate individual particles by their mechanical properties. Even contaminants consisting of very soft components can still be imaged but create image artifacts (oscillations and streaking).

The same 0.8 µm polycarbonate membrane deliberately clogged by filtering 2% milk
(topography and phase).

Spherical milk components, presumably casein micelles, immobilized as filter cake on the membrane.3

Various types of nano to microparticulate residue from a home water filter appear similar
in topography (red) but can be differentiated in phase (blue).

Sub-nanometer particulate residue from drying distilled water on mica.

Why AFM and not SEM?

The scanning electron microscope (SEM) is a good choice to analyze biomaterials but typically requires metal coating of the fully dried, nonconductive polymers to get to high resolution with sufficient contrast. Once prepared and loaded into the vacuum chamber, the SEM can quickly collect many images in a short period of time and collect elemental distribution data with an EDS system. This requires access to equipment with >$300k purchase price and >$20k in annual maintenance fees that is only accessible to users of high-end facilities.

The AFM does not require any sample processing because the probe tip directly interacts with the sample surface. A polymer membrane is mounted directly onto a disc with double-sided tape and the first image is collected within minutes. The scans are not instantaneous but take a few minutes to record depending on the selected parameters and AFM cannot scan surfaces with large topography >15 µm. But AFM delivers quantitative 3D data, can image surfaces in liquid and measure or exert forces through the direct interaction with the specimen. The AFM system used here delivers this data at a 10x lower purchasing cost without the need for costly maintenance contracts.

References

  • 1
    Goff, H. D.; Hill, A.; Ferrer, M. A. Dairy Science and Technology eBook.
  • 2
    Cherian, A. G.; Liu, Z.; McKie, M. J.; Almuhtaram, H.; Andrews, R. C. Microplastic Removal from Drinking Water Using Point-of-Use Devices. Polymers 2023, 15 (6), 1331. https://doi.org/10.3390/polym15061331.
  • 3
    Dalgleish, D. G.; Spagnuolo, P. A.; Douglas Goff, H. A Possible Structure of the Casein Micelle Based on High-Resolution Field-Emission Scanning Electron Microscopy. Int. Dairy J. 2004, 14 (12), 1025–1031. https://doi.org/10.1016/j.idairyj.2004.04.008.

Additional information

The scanning electron microscope (SEM) is a good choice to analyze biomaterials but typically requires metal coating of the fully dried, nonconductive polymers to get to high resolution with sufficient contrast. Once prepared and loaded into the vacuum chamber, the SEM can quickly collect many images in a short period of time and collect elemental distribution data with an EDS system. This requires access to equipment with >$300k purchase price and >$20k in annual maintenance fees that is only accessible to users of high-end facilities.

The AFM does not require any sample processing because the probe tip directly interacts with the sample surface. A polymer membrane is mounted directly onto a disc with double-sided tape and the first image is collected within minutes. The scans are not instantaneous but take a few minutes to record depending on the selected parameters and AFM cannot scan surfaces with large topography >15 µm. But AFM delivers quantitative 3D data, can image surfaces in liquid and measure or exert forces through the direct interaction with the specimen. The AFM system used here delivers this data at a 10x lower purchasing cost without the need for costly maintenance contracts.