Interfaces and Mechanical Properties of Nanomaterials and Biological Systems with AFM

For the past decade, scientists and engineers have been gaining increasing control over the properties of matter at the nanometer scale - measuring, predicting and constructing nanoparticles and nanostructures. Novel applications have been created which have the potential to transform everything from manufacturing to energy production and access to clean water; more effective pollution reduction and prevention; stronger, lighter, cheaper materials. An area where nanotechnology could make one of the most substantial impacts is medicine.

Why Nano in Medicine?

The fundamental building blocks of life - DNA, proteins, lipids - are nano-sized systems. At the molecular level a lot of biology happens at the nm-scale. DNA (diameter ~ 2nm) and proteins (typically ~ 3 - tens of nm), are effectively complex nanomachines fine-tuned by evolution, and their function, their movements, their mechanics and their interactions with each other in health and disease can be studied, and targeted, with nanotechnology tools.

Figure 1. Size comparison: nanoparticles and biological systems

This convergence of nanotechnology and biology has led to the emergence of nanomedicine. Nanomedicine is the use of nanotechnology to create radically improved research, diagnosis and treatment of disease that can reach the single-molecule level. Nanotechnology is helping to create a revolution, a paradigm shift in the way we treat and diagnose disease; current research focuses on areas such as new targeted drug-delivery systems, nanomaterials to restore damaged tissues, and extremely accurate biosensing devices. Nanomedicine offers hope for treating e.g. spinal cord injuries, diabetes, heart disease, Parkinson’s disease and cancer.

Multidisiciplinarity of Nanomedicine

Nanomedicine arises from the convergence of different sciences at the nm scale: materials science, physics, chemistry, biology, engineering, etc. This leads to scientists with different backgrounds, and different technical and intellectual skills, trying to tackle medical problems using nanotechnology, such as "what is the best way to target a tumour with a nanoparticle?". The challenge of nanomedicine is to integrate the knowledge of chemists, biologists and physicists to reach the optimal answer.

One important contribution to this field comes from physics. Physicists try to identify and to quantify the basic interactions of nanomaterials with biological systems - the molecular forces that drive the interaction (electrostatics, van der Waals, dynamical complex phenomena), the thermodynamics, the interface of the nanoparticle with the liquid, the role of mechanical properties (stiffness, elasticity, adhesion). The objective is to understand the basic phenomena so that rational design of nanoparticles for a specific medical application becomes possible.

From a physicist's point of view, biological systems exploit the physical chemistry of nm-sized biomolecules (proteins, DNA...) to create complex functional, dynamic structures with detailed nanomechanical properties (adhesion, stiffness, elasticity), tailored interfaces and a functional hierarchical organization (from nm to micron to mm scales). Interfaces and mechanical properties modulate the functional structures that enable biological function, from the selectivity of a membrane channel, to the binding of a protein to DNA, to cell division and morphogenesis and the organization of tissues and organs. All of these functions are altered by disease or trauma.

Additionally, biological systems such as proteins and DNA will create interfaces with the surrounding fluids that will govern their interactions with nanomaterials; cells will react to manmade nanomaterials through interactions at their interfaces that will be modulated by mechanical properties (e.g. adhesion, elasticity) of both the cell and the material, as cells dynamically react to chemical, as well as to mechanical cues (mechanotransduction). Understanding these complex, dynamic structures and physical properties is one of the main challenges of modern science and constitutes the scientific background to modern nanomedicine, biomaterials and bioinspired/biomimetic systems.

Key physical parameters and concepts:

• interfaces
• molecular forces
• mechanical properties (adhesion, elasticity)
• dynamics
• nanochemistry
• nanomechanics
• mechanochemistry

Quantitative Measurement

There is little quantitative scientific knowledge of the basic processes that govern the nanomaterial/biological medium interactions. With currently available techniques, it is very challenging to obtain the necessary quantitative information of all the relevant parameters, from nm and sub-nm resolution of structures and their dynamics in physiological fluids, to mapping of mechanical properties of cells, biomolecules and nanomaterials at the nm scale, and the properties of the interfaces that complex biological and nanostructured materials establish with biological fluids.

Our Work

Figure 2. Sonia Contera uses AFM based techniques for quantitative measurement of mechanical properties and interfaces of biological systems in physiological conditions

In the last few years we have developed techniques based on the atomic force microscope (AFM) that have enabled us to quantitatively measure the interfaces of biological molecules and structures with physiological fluids.

Using AFM with a novel small-amplitude method in which a microcantilever is oscillated just with ~ 1 Å amplitude at the interface of the surface and the liquid, we’ve been able to measure the solid-liquid adhesion energy with sub-nm resolution ¹. Using this technique we quantified the complex electrostatics of membrane proteins (bacteriorhodopsin, a 3-nm sized light activated proton pump) measuring ionic effects on the water structure at the interface ². Using the AFM tip as a very precise nanoindenter we have quantified the stiffness of a single membrane protein ³. Furthermore we have been able to show that the elasticity of a membrane protein is related to its interface properties ². Using a high-speed AFM technique (developed by Toshio ando and colleagues at Kanazawa Univeristy) that combines sub-nm resolution with speeds up to 50 frames/s, we have studied the dynamics of bacteriorhodopsin during pumping of proteins ^{4, 5} and shown how protein function within the membrane involves the coupling with neighboring proteins ⁵. We have increased the resolution of AFM in solution to resolve single atoms in solution ¹, an ion binding to a membrane protein and have been able to resolve the DNA double-helix. More recently, using state-of-the-art multifrequency AFM we have been able to quantitatively map the nanomechanical properties of living cells with unprecedented speed and accuracy ⁶; this will make it possible to study the fundamental mechanisms that determine cell nanomechanical response in different contexts. We have shown the relevance of these properties for the interaction of biomolecules and cells with surfaces and we have shown that interfaces, dynamics and mechanical properties are indeed interrelated ².

Application of Basic Physics to Nanomedicine

Currently we are exploiting this knowledge and techniques to design nanostructures (nanostructure-based drug-delivery systems, and nanocomposites for tissue regeneration) that enable selectivity and biocompatibility by controlling interfaces and mechanical properties.

1. It has been shown that mechanics matters in cancer: for example nanoparticles can reach tumours using the differential mechanical properties of the surrounding blood vessels (the so-called EPR effect ⁷). Our aim is to design nanoparticles that not only have the right chemistry but also the right mechanical properties, using our ability to quantify mechanical properties at the nm-scale.
2. There is a growing interest in using nanotechnology in bio materials applications such as implants to repair bone tissue. Bioinspired nanomaterials and nanocomposites can promote healing and tissue regeneration because they can be used to provide a good structural and mechanical matching to that of real tissue, can provide nanoscale electrical conductivity (important in e.g. heart and spinal cord tissues), improve the implant adhesive and micro/nanoenvironment- defining moieties, and improve the ability of cells to self-assemble in 3D tissues.

Figure 3. SEM image of a 3D scaffold created using a nanocomposite of chitosan and carbon nanotubes, by L Bugnicourt, S. Trigueros and S Contera, unpublished.

We are particularly interested in carbon nanotubes. For example, carbon nanotubes show viscoelastic behaviour similar to that observed in soft-tissue membranes, so they can be used to increase the Young's modulus and tensile strength of hybrid biomaterials.

Carbon nanotubes have been shown to support the cultivation of neurons. Conjugation of these nanotubes to different substrates can affect cell behaviour and promote attachment, growth, differentiation and long-term survival of neurons, as neurons seem to need a conductive nanostructure to be able to survive. Despite the advantages of carbon nanotubes they have shown some biocompatibility issues. We are developing strategies for creating nanocomposite networks of carbon nanotubes and biopolymers, with controlled structural and mechanical properties. We could ensure that the nanocomposites are biocompatible and electrically active by using the self-assembly properties and biocompatibility of e.g. chitosan ⁸.

References

1. Voitchovsky,K., JJ Kuna, SA Contera, E Tosatti, F Stellacci, Direct mapping of the solid-liquid adhesion energy with subnanometre resolution. Nature Nanotechnology, 2010. 5(6): p. 401-405.
2. Contera*, S.A., K. Voitchovsky, and J.F. Ryan, Controlled ionic condensation at the surface of a native extremophile membrane. Nanoscale, 2010. 2(2): p. 222-229.
3. Voitchovsky, K., S. A. Contera, et al. (2007). "Electrostatic and steric interactions determine bacteriorhodopsin single-molecule biomechanics." Biophysical Journal 93(6): 2024-2037.
4. Yamashita, H., K. Voitchovsky, et al. (2009). "Dynamics of bacteriorhodopsin 2D crystal observed by high-speed atomic force microscopy." Journal of Structural Biology 167(2): 153-158.
5. Voitchovsky, K., S. A. Contera, et al. (2009). "Lateral coupling and cooperative dynamics in the function of the native membrane protein bacteriorhodopsin." Soft Matter 5(24): 4899-4904.
6. Raman, A., S. Trigueros, et al. (2011). "Mapping nanomechanical properties of live cells using multi-harmonic atomic force microscopy." Nature Nanotechnology 6(12): 809-814.
7. Matsumura, Y. and H. Maeda (1986). "A new concept for macromolecular therapeutics in cancer chemotherapy: mechanism of tumoritropic accumulation of proteins and the antitumor agent smancs." Cancer Res 46(12 Pt 1): 6387-6392.
8. Bugnicourt, L., S. Trigueros, SA Contera "Engineering Biocompatibility and Assembly in Carbon Nanotube Electrodes Using the Physicochemical Properties of Chitosan" Solid State Devices and Materials 2011, proceedings, no. 5372.