Session: 07-07-03: Computational Modeling in Biomedical Applications III

Paper Number: 172895

Foreign Body Response Influences Mass Release From Implanted Medical Devices 

Implanted medical devices can support bodily functions that are impaired or absent due to injury, disease, or congenital conditions. However, when an object is implanted into the body, it can induce an immune response, often referred to as the “foreign body response”. The intensity of the response is related to biocompatibility which is influenced by a combination of factors including the implanted material, the location of implantation, and individual sensitivity. When the implant is identified as “not – self” it can initiate a cascade of events including acute inflammation, chronic inflammation, and fibrous capsule formation in the tissue surrounding the implant. Fibrous capsules have different material properties than the original tissue which influences diffusion through the tissue. In this research a 1D model with planar diffusion outward from the implant surface, and a 2D model that also models depth below the surface are used to characterize the diffusion of potentially hazardous materials from implanted medical devices. Phase field modeling is used to characterize the transformation of tissue from “normal” upon implantation to “capsule” as a function of position and time. This modeling approach has two major advantages over other methods that could have been used for this application. First, because the system is governed by a field equation that is used to control how the tissue transforms, it allows cells within the model to change their material properties during the simulation. It also uses smooth transitions which are more realistic for a biological system effectively allowing the model to capture gradual tissue transformations and tissue transformation gradients within the system. The system is modeled as a coupled model where one phase field equation controls the evolution of the phase, and another phase field model is used to quantify mass transfer by diffusion based on the phase of the material in each cell. Results of the 1D model show the mass transfer to be dramatically reduced following the formation of the capsule. The capsule is modeled as a barrier tissue having a diffusion coefficient about 100x smaller than that of typical normal tissue (~10^-8 vs. ~10^-6 – see Polymer-interface-tissue model to estimate leachable release from medical devices - https://doi.org/10.1093/imammb/dqae020).  Because the 1D model represents planar diffusion away from the implant surface it is only able to capture a capsule of uniform thickness that fully surrounds the implanted medical device. The more complex 2D model is also able to capture capsule formation that differs based on depth below the skin surface, for example. Thus, a heterogeneous capsule that exists in some regions but not in others is possible. In cases where the capsule only partially impairs diffusion, formation of the capsule has a much smaller influence on the overall mass transfer because mass diffuses through the high diffusion constant materials effectively flowing around the partially formed capsules. However, when enough nucleation sites exist to initiate capsule formation in multiple locations, these capsules can fuse together forming a continuous barrier. When this occurs, the simulation results are similar to the 1D model.  It is important to note that these models were developed when the author was working at the US Food and Drug Administration (FDA), so the models apply to a generic/any implanted medical device, not a specific device being examined.

Presenting Author: Martin Tanaka Western Carolina University

Presenting Author Biography: As a faculty member at WCU, I spend my time teaching courses, conducting research, and serving the university and the community. I usually have a few graduate students and some students doing undergraduate research. My research interests include developing innovative biomedical devices, computational modeling, neuromuscular control, and industry-academic partnerships. Most of our research projects have design, build, test, and optimization components stemming from my experience designing products in industry. I am a licensed professional engineer and enjoy working with students and local companies on their projects. My degrees are in mechanical engineering (NCSU), engineering mechanics (VT), and biomedical engineering from Virginia Tech and Wake Forest University.

Authors:

Martin Tanaka Western Carolina University