Strain gages have long been used to measure the forces imposed on structures comprised of a variety of materials including metals, concrete and plastics. Another application involves a different type of structure: the skeletal system. Researchers mount gages on actual bone samples to study such things as joint interaction, effects of replacement joints, spinal loading conditions and overall strength. The data is used for the development of prosthetic devices and surgical techniques, as well as a better understanding of the biomechanics of the body.
Mounting a strain gage to bone is very similar in practice as done for other materials. It requires a thorough, disciplined process of cleaning the area, abrading, neutralizing, bonding the gage to the surface and protection of the gage. The main difference when working with bones is they are typically kept moist before, during and after testing to maintain their structural integrity. Degreasing agents and conditioners normally used in the gage bonding process can cause a local drying of bone, which will also affect its physical properties, resulting in invalid data.
- Start with a clean work area and tools, with everything arranged for easy access. Gloves are typically worn for handling the specimen but should be powder-free and not contain silicone, which can contaminate the bonding area. If the specimen has an irregular shape it may be helpful to have a fixture or an assistant available to hold things in place.
- Working in a well-ventilated area, scrape the area to be gaged with a scalpel held at 90-degrees to the surface, maintaining the strokes in one direction. Use ether or denatured alcohol to clean an area approximately 2” square, for the gage placement. Note: Isopropyl alcohol can be used, but the drug-store variety will contain more water that that found from a lab supplier.
- Scrub, using cotton-tipped applicators saturated with alcohol, and then clean the surface by using a folded gauze sponge to wipe in one direction so contaminants are not dragged back onto the cleaned area.
Now proceed with the bonding of the gage to the cleaned and prepped area. M-Bond 200 is typically used for this application and can maintain a bond line up to one year under proper conditions. For longer term applications or specimens with a porous or irregular surface, M-Bond AE-10 is a better choice.
Attach the gage lead wires, remove the residual flux and apply a protective coating. Selecting a gage with pre-attached wires removes the gage wire installation and flux removal from the process but care is needed to avoid getting tangled up or having the excess weight of the wire bundle tugging on the gage during installation and bonding process.
For more information on the actual surface prep and gage bonding process, please see Micro-Measurements Instruction Bulletins B-129 “Surface Preparation for Strain Gage Bonding” B-127 “Strain Gage Installations with M-Bond 200 Adhesive” and B-137 Strain Gage Applications with M-Bond AE-10, AE-15 and GA-2 Adhesive Systems” for further details.