Mechanical testing tissue microstructure is becoming an increasingly active area of research, especially in oncology. It has been shown that changes in this microenvironment are key in cancerous growth, namely an increased tissue stiffness. External mechanical forces can directly influence cell signaling regulating and influencing growth. Investigating the mechanical properties of the microenvironment is, therefore, key to creating a biomechanical model of tumor growth.
Tumour cell samples are available in collagen scaffolds which are 150μm thick. Mechanical testing of such thin samples is a difficult problem. High sampling micro-indentation using the ElectroPuls™ E3000 was able to extract mechanical properties of these very small samples.
Indentation testing is becoming an increasing field for mechanical characterization of thin film tissues due to its ability to view response over a range of forces, shows a bulk modulus value and localized testing can be obtained.
Testing the Thin Films
The Instron ElectroPuls E3000 was fitted with a 5N load cell. 150μm thick collagen gels were placed into a fluid-filled petri dish container. Initially, a simple waveform was created with a 0.05μms-1 indentation rate.
Since it was difficult to accurately place the indenter a few μm above the surface of the gel it meant that tests ran for a very long time. A more complex waveform was created that would indent the gel up to 0.5N then raise the indenter 150μm to just above the gel surface. This reduced the indentation times by a factor of 4 while still maintaining 0.05μm indentation rates.
This ultra-slow indentation allowed for precise mapping of the force-indentation profile of the thin gel from which stiffness can be estimated.
The results were processed using a low pass filter to further reduce noise as well as a downsample filter to reduce the file size. An algorithm was used that uses linear modeling analysis to find the start point of the test as well as the point where the plastic beneath became heavily involved
From the load – depth profile the modulus of the collagen gels was taken using the modeling formula:
Where F is the load applied, E is the elastic modulus, R is the radius of the indenter, δ is the indentation depth and ν is Poisson’s ratio of the material.