Wireless Mechanical Stimulation Upregulates the Foreign Body Response to Biomedical Implants Through Immune Cell Pathways

Introduction: : Implantable biomedical devices are a hundred-billion-dollar industry and include devices such as pacemakers, joint prosthetics, or breast implants [1]. However, these devices often fail after implantation due to the foreign body response (FBR), in which the body's immune reaction to an implanted device creates a fibrotic capsule leading to device failure. We have previously published that increasing extrinsic tissue mechanical forces activates RAC2+ mechanoresponsive macrophages to drive severe FBR in mice and humans (Fig. 1) [2]. However, we utilized a bulky, wired device to upregulate mechanical stress and did not explore the temporal aspects of FBR development.

Materials and

Methods: : We engineered a novel, wireless mechanically stimulating implant (MSI) to increase extrinsic tissue mechanical forces and promote human-like FBR in a murine small animal model. The device consists of a silicon-encapsulated circuit and motor. The device is powered wirelessly and battery-free at 13.56 MHz via a Neurolux radiofrequency system, in which the circuit receives the frequency while the mice are placed in a wire-wrapped cage. 46 wild type (WT) and 12 RAC2+ knockout (KO) mice were implanted with either no-stimulation (NS) or MSI devices, and fibrotic capsules were collected at either postoperative day (POD) 10 (n=15), 28 (n=40), or 90 (n=3). In addition, we injected a RAC2 inhibitor in 6 MSI WT mice everyday for 10 days and then explanted at POD10. We also performed single cell RNA sequencing (scRNA-seq) on human Baker grade I, mild FBR (n=3) and grade IV, severe FBR (n=3) breast implant capsule tissue collected from patients with IRB STUDY00003033. We analyzed these samples using Seurat package in R and compared them to previously published murine data.

Results, Conclusions, and Discussions:: Mechanical stimulation in WT mice at POD10 caused a 110.5% increase in capsule thickness from 60.9 um to 128.2 um (p = 0.0004) between NS and MSI (Fig. 1). At POD28, mechanical stimulation in WT mice caused capsule thickness to increase 57.2% between NS and MSI, from 120.3 um to 189.1 um (p = 0.03) (Fig. 2). Even in the presence of mechanical stimulation, knockout of RAC2 KO decreased the capsule thickness by 36.5% compared to in WT mice (p = 0.01). The RAC2 inhibitor injections significantly decreased the capsule thickness compared to MSI mice by 51.2% (Fig 3).

In humans, severe Baker IV FBR capsules increased the number of myeloid cells and upregulated inflammatory and fibrotic pathways with genes such as RNF149 (inflammatory), PLAUR (inflammatory), CD44 (increases fibroblast formation) and RAC2 (inflammatory and reactive oxygenation stress mediation). Mechanically stimulated murine capsules also demonstrated upregulation of myeloid cells and similar pathways and genes.

Our results demonstrate that wireless mechanical stimulation activates mechanosensitive immune pathways and exacerbates FBR. By engineering the novel, fully wireless implant, we achieved precise delivery of mechanical stimulation at controlled frequencies in mice. The MSI device significantly increased capsule thickness in wild-type mice, demonstrating that we are able to replicate human FBR in a murine model. Moreover, the lack of capsule in RAC2 KO mice with mechanical stimulation suggests fibrotic capsule formation is dependent on RAC2+ signaling. The injection of the RAC2 inhibitor targeted this pathway and caused a decrease of fibrotic capsule, even though it was paired with mechanical stimulation of the implant. Additionally, human and murine scRNA-seq comparison revealed the important presence of mechanoresponsive immune cells in severe fibrotic Baker IV and MSI capsules.

By uncovering the important genes and pathways that trigger FBR, we open new avenues for treating a preventing this reaction to biomedical implantable devices. Future work will be performed to further investigate the specificity of the RAC2 inhibitor. By targeting this pathway, we aim to prevent biomedical implant failures, reduce the need for revision surgeries, and save billions of dollars in healthcare costs and resources.

Acknowledgements and/or References (Optional):: [1] Dolan, E. B. et al. "An actuatable soft reservoir modulates host foreign body response," Sci. Robot. https://doi.org/10.1126/scirobotics.aax7043 (2019).

[2] J. Padmanabhan et al., “Allometrically scaling tissue forces drive pathological foreign-body responses to implants via Rac2-activated myeloid cells,” Nature Biomedical Engineering, pp. 1–18, doi: https://doi.org/10.1038/s41551-023-01091-5. (2023)