Credit: Unsplash/CC0 public domainMost people are familiar with the sensation of a needle puncturing skin, particularly as COVID-19 vaccines gain momentum. What happens when a needle is inserted into skin? A new paper in the Journal of the Mechanics and Physics of Solids reveals the answer in a recent paper.Mattia Bacca is an assistant professor at University of British Columbia. He often turns to nature for solutions when faced with a problem in mechanical engineering. For example, a gecko can stick to a surface using its pads, while an ant can tear through a leaf several times its size.Dr. Bacca and Stefano Fregonese (Ph.D. candidate) used bioinspired engineering to solve the question of how piercing works with soft materials like skin.Bacca says that cutting is a common part of our daily survival and daily life. To make food digestible, we use cutting tissue when we chew. Nearly all species of the animal kingdom are able to cut tissue to feed or defend themselves. They have remarkable morphological, physical and morphological features that allow them to do this efficiently.They developed a mechanical theory that would determine the critical force needed for needle insertion, which is what causes puncture. They created a semi-analytical model that describes the process from dimensional arguments to finite elements analysis.Engineering has been studying the mechanisms involved in cutting soft tissues for the past several decades. This was initially due to research into rubber's properties. Previous methods determined the force required to insert a needle into tissue following its initial puncture. However, physical experiments were not able to measure the complex failure mechanisms and deformations involved in breaking through soft materials' surfaces.Fregonese & Bacca's new model can predict puncture force and confirm it with past experiments. The needle insertion force is related to tissue toughness and scales are inversely proportional to the radius of the needle. This means that thinner needles require less force. These observations, while intuitive, can be used to predict the future. Contrary to popular belief, material rigidity plays a key role in this process. Tissue rigidity is inversely proportional to puncture force. Softer tissue requires higher force (at the same toughness). To "deepen" the understanding of this problem's physics, the UBC team is performing further experiments and refinements to their models.Their results have been based on a variety of inquiries into animal solutions. Fregonese first joined Dr. Bacca’s Micro & Nano Mechanics Lab to work on a project about the mechanics and adhesion of animals such as geckos. They began exploring overlaps in this area and the problem of slicing. With an international collaboration, they studied leafcutter ants together with Dr. David Labonte (Imperial College) and Dr. Natalie Holt, University of California muscle physiology expert. To study the interaction of ballistics with gels, they also worked closely with Dr. Kevin Golovin from UBC Okanagan and Dr. Gwynn Elfring, a mechanical engineer colleague.Engineers may be able to use their new theoretical model to develop various applications, including protective equipment, automated processes that involve food and emerging technologies like robotic surgery.This could also affect how people receive injections in the future. This is something that should be considered by people who have recently waited to get their COVID-19 vaccine. Future technology may offer self-administered disposables equipped with microneedles, such as the ones developed by Dr. Boris Stoeber at UBC. These are designed to penetrate skin at the right depth with the right force.Continue reading to learn more about a new method for planning preoperative needle insertion.More information: Stefano Fregonese et. al., Piercing soft substances: A mechanical theory to aid needle insertion. Journal of the Mechanics and Physics of Solids (2021). Stefano Fregonese and colleagues, Piercing soft substances: A mechanical theory to needle insertion. (2021). DOI: 10.1016/j.jmps.2021.104497
