Collagen fibrils are one of the main building blocks of the human body at the nanoscale. They are present in many different tissues such as lungs, cartilage, bones or tendons. They are of a great biomechanical importance providing structural and mechanical stability and constitute the framework in which cells reside and function. In many tissues, if not all, collagen fibrils co-exist with proteoglycans, which are located in the interfibrillar space of the extra cellular matrix and bear high negative charges at physiological pH. These negative charges create a cation (salt ions) concentration gradient and as a result exert osmotic pressure on collagen fibrils. Although, proteoglycans seem to play a significant role in tissues such as tendon and cartilage, the effect of osmotic pressure on the structural and mechanical properties of individual collagen fibril is largely unknown. The aim of this master thesis project is thus to investigate the effects of increasing osmotic pressure on individual collagen fibrils. Atomic force microscopy (AFM) allows for direct evaluation of both the mechanics and the structure of individual collagen fibrils at the nanoscale level. In this study, AFM was used to image, indent and pull individual collagen fibrils. Firstly, imaging and cantilever-based nanoindentation experiments on individual collagen fibrils in air, phosphate buffered saline (PBS) and solutions of increasing concentration of polyethylene glycol (PEG) were performed. Solutions of increasing concentration of PEG were used to create varying osmotic pressures, in situ, during measurements. Then a protocol was developed to perform nanotensile experiments on individual collagen fibrils using the AFM. Finally, these experiments were performed on individual collagen fibrils in PBS and solutions of increasing concentration of PEG. These experiments aimed to generate a relationship linking osmotic pressure to collagen fibril mechanics. Collagen fibrils shrink and become stiffer with increasing osmotic pressure. The fibril diameter reduces up to 15%-20% at the highest PEG concentration used (solution in 3.5M PEG). Nanoindentation experiments provide information about the transverse elastic modulus which increases 44 times from PBS to a solution of 3.5M PEG in PBS. The longitudinal elastic modulus is obtained from the nanotensile experiments and increases by a factor of 3 at 3.5M PEG. Mechanical conditioning of an individual collagen fibril was highlighted with softening of the fibril under cycling loading. It is demonstrated all along this study that osmotic pressure is a major contributor to the function of collagen fibrils. The main take home message being that the structural and mechanical properties of individual collagen fibrils can finely be tuned to a high range using osmotic pressure. Further experiments and analysis are required to fully understand the consequences of osmotic pressure on individual collagen fibril. Influence of the osmotic pressure on the mechanical conditioning is also not well determined. Understanding how, from similar main building blocks (i.e. the collagen fibrils), the properties of tissues can be adapted to assume numerous distinct functions is one of the futures challenges. For example, cells might be reducing or increasing the amount of proteoglycans to tune the mechanical properties of their microenvironment i.e. of near by collagen fibrils.