## 2 Applications of RIM
### Solid-Fluid Interaction in Biological Engineering  
As researchers continue to investigate the potential applications of the RIM method, several biologically compatible materials are being employed in biological engineering. The fundamental principles underlying RIM remain consistent, yet particular attention must be paid to the materials utilized. Biofilms are complex communities of microorganisms that adhere to surfaces and are enclosed in a matrix of extracellular polymeric substances they produce. They play a crucial role in various environmental and medical processes due to their unique properties, such as increased resistance to antibiotics and the ability to facilitate nutrient exchange. [PERSON] et al. (2018) introduced a non-destructive method for observing microbial biofilms using an optically transparent porous medium made of amorphous fluoropolymers. The biofilms were cultivated in flow cells filled with Nafion granules, which permitted visualization due to the matching of the refractive index between the solid fluoropolymer grains and the aqueous immersion medium. This method represents a significant advance in microbiological research, as it offers a novel approach to examining biofilms in their natural state. The application of confocal microscopy has enabled the real-time, three-dimensional imaging of biofilms, providing insight into the complex structure and dynamic processes within these communities at previously unattainable depths. This technique presents novel opportunities to gain a deeper understanding of the behavior and ecological significance of microbial communities in diverse environments. In the field of biomedical fluid dynamics, flow phantoms are artificial models designed to mimic the physiological conditions of blood flow in the human body. They are used to study hemodynamics, test medical devices, and improve diagnostic and treatment methods for vascular diseases. [PERSON] et al. [69] presents a new method for creating flow phantoms for PIV analysis of cerebral aneurysms. The researchers utilized desktop 3D printing technology, specifically Stereolithography (SLA), to fabricate a rigid refractive-index-matched flow phantom. The team printed a cerebral aneurysm's idealized geometry using FLGPCLXX photopolymer resin. The resin was refractive-matched with a working fluid composed of deionized water and NH4 SCN. This approach overcomes the limitations of traditional investment casting techniques, providing a more efficient and cost-effective alternative. This advancement has significant implications for the study of hemodynamics in vascular diseases, with the potential to lead to improved diagnostic and treatment strategies for conditions such as cerebral aneurysms. In the field of sidewall aneurysm hemodynamics, the formation of unsteady vortices was visualized by [PERSON] et al. [70], in which a transparent acrylic replica was manufactured to simulate the aneurysm. A solution of sodium iodide, glycerin, and water was matched to the RI of this solid by specific weight percentages. The outcomes of the physical experiments and CFD numerical simulations were found to be highly concordant, demonstrating great experimental potential for flow field visualization in other medically relevant problems. [PERSON] et al. [71] reported an imaging platform that enables long-term, high-resolution imaging of biofilms for the study of cellular-level dynamics within bacterial biofilms. This platform employs a thin MY133-V2000 polymer film that is glued across the 3D-printed channel, effectively eliminating optical aberrations. The application of RIM in biological engineering facilitates the study of complex solid-fluid interactions, such as microbial biofilm dynamics and blood flow in medical models. The use of biocompatible materials with tailored refractive indices enables non-invasive, high-resolution imaging, providing unprecedented insights into cellular processes and hemodynamics.