Rohit Ramnath, Master Bond
Designers of medical electronics assemblies have many options to choose from when selecting one or more compounds for bonding, coating, potting and encapsulating components.
While manufacturers can control many of the properties of these compounds, the underlying polymer chemistry is a critical factor to consider. Each family of compounds – epoxies, silicones and UV/LED light curables – offers a different set of performance parameters and processing requirements.
Epoxies are among the most versatile polymer compounds used in medical electronics. They offer excellent cohesion and resistance to chemicals, adhere well to a variety of materials and some specialty systems can operate over a wide range of temperatures from cryogenic (4K) to more than 550°F. Because they are 100% reactive, epoxies produce no volatiles during cure and exhibit little or no shrinkage during polymerization.
Epoxies are also showing promise in the rapidly growing field of brain stimulation.
Magnetic coils for brain stimulation
Transcranial magnetic stimulation (TMS) is a non-invasive way to stimulate the motor cortex and other parts of the brain. A coil energized by a pulse generator is placed near the head of a human or animal, creating a pulsed magnetic field which induces small electric currents in the part of the brain just under the coil. By observing the resultant motor activity of the patient or subject, medical professionals can assess the damage from a brain injury or disorder, such as a stroke or multiple sclerosis.
Scientists at israel’s Bar-Ilan University designed a study to explore exactly how magnetic stimulation acts on nerve cells in the brain (their work is described in the June 3, 2014 issue of Frontiers in Cellular Neuroscience). Thin slices of the somatosensory cortex of rats’ brains were prepared for use in the study. A common-used procedure for studying the electrical activity of neurons, known as the patch-clamp technique, was modified to facilitate magnetic stimulation of individual neurons. The main component of the modification was a custom-made magnetic coil.
Standard lacquer-coated copper wire was used to make the coil, consisting of 14 turns of wire in each of two layers. The coil was constructed using a wet-winding technique, in which the coil is impregnated with an epoxy compound during the winding process. The researchers selected low-viscosity Master Bond EP29SPLP epoxy for the wet-winding process. The epoxy was mixed with 25 μm alumina particles to enhance heat transfer, increase electrical insulation and strengthen the coil. The magnetic coil was positioned below the neuron under test during the experiment, which gave researchers important insights into how TMS affects neurons.
In an earlier study – published in 2011 in volume 194 of the Journal of Neuroscience Methods – researchers at the same university fabricated a custom-made mini coil for use in a TMS experiment on an awake monkey. In this case, the coil was immersed in a saline solution and placed inside a chamber designed to record brain activity via multiple micro-electrodes attached to various regions of the monkey’s brain. A wet-winding technique was used to build the coil, which included 32 turns of standard copper wire. Again, the coil was impregnated with Master Bond EP29LPSP epoxy mixed with 25 μm alumina particles during the winding process. For this application, electrical insulation of the coil and its windings was especially important in order to minimize the risk of electric breakdown. The insulated mini-coil was tested to voltage levels up to 1200V.
The studies out of Israel once again demonstrate that the success of any engineered product depends on the performance of all its parts – and that includes any chemical compound/epoxy used to join or protect one or more parts.
Rohit Ramnath is a senior product engineer for Master Bond (Hackensack, N.J.), a custom formulated adhesives manufacturer.