Bed-of-Nails effect: Unraveling the insertion behavior of aerosol jet 3D printed microneedle array in soft tissue

Implantable biomedical devices often contain rigid components such as microneedles, Si chips, and sensors, that can frequently come in physical contact with soft biological tissue. Brain-computer-interfaces (or BCIs) are an example of such devices where an array of microelectrodes is inserted in the brain to record neuronal activity, stimulate neurons for neuro-prosthetics, and treat neurodegenerative diseases. Recently, CMU Array, a customizable ultra-high-density aerosol jet (AJ) 3D nanoprinted BCI platform was developed by the authors to record action potentials from throughout the 3D volume of the brain. Although the mechanics of insertion of a single sharp needle in biological tissue has been studied, the behavior of an array is still not fully understood. In this paper, we develop a linear elastic model for insertion of multiple microneedles in close proximity with each other and determine the severity of the bed-of-nails effect, when interacting strain fields from neighboring needles fail to cause clean needle insertion into the tissue. We then carry out experiments where an array of 3D-printed and sintered microneedles (80–90 µm diameter, 1 mm long, tip radius of the order of 10 µm) are inserted in agarose, that acts as a phantom brain. We show that our model can predict the experimentally measured peak force, agarose displacement, and energy absorbed during insertion for arrays with microneedles at increasing distance from one another. We show that for our system, the microneedles in the array act completely independent of each other when they are roughly 8–10 needle diameters apart, consistent with the model predictions. This work is fundamental to the understanding of the insertion mechanics and related deformation/damage caused by rigid microscale objects implanted in various soft biological tissue.