Structural, Mechanical, and Dielectric Properties of Polydimethylsiloxane and Silicone Elastomer for the Fabrication of Clinical-Grade Kidney Phantom  
[PERSON]  
1 Department of Radiology, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia,  
Seri Kembangan 43400, Selangor, Malaysia; [EMAIL_ADDRESS]
2 Department of Medical Imaging, School of Health Sciences, KJP Healthcare University College,  
Nilai 71800, N. Sembilar, Malaysia; zuz  
A wide selection of materials have been studied to find an optimal outcome that best suits certain criteria for phantom development [5]. This has resulted in the fabrication of various materials, such as agar, silicone, polyvinyl alcohol (PVA), and polyacrylamide (PAA), to mimic human tissues [6]. The results have shown that agar is easy to prepare but does not have an excellent long-term stability compared to silicone, which can be considered as the material of choice for stable phantoms. A study comprehensively reviewed four different types of polymer-based materials for constructing medical phantoms: carrageenanan-based polymer gel, polymer cross-linked aerogels, UV-curable silicone, and self-healing polymer materials [7]. The findings revealed that even though water-based silica and cellulose aerogel phantoms meet the imaging properties of human tissue, their volume may shrink and the fabrication is complex for larger-scale production. Nonetheless, a multimodal phantom has been successfully innovated with the use of polydimethylsiloxane (PDMS) and hydrophilic silicone [8; 9]. The results demonstrated that the addition of hydrophilic silicone can improve the mechanical and imaging properties of a phantom. PDMS, or dimethicone, is a polymer that has been widely used for the fabrication and prototyping of microfluidic chips. The empirical formula of PDMS is (C\({}_{2}\)H\({}_{6}\)OSi)n and its fragmented formula is CH\({}_{3}\)[Si(CH\({}_{3}\))2O]nSi(CH\({}_{3}\))3, where n represents the number of monomer repetitions. PDMS has an elastic modulus of 360-870 kPa, which is larger than the elastic modulus of a real kidney, with 180.32 \(\pm\) 11.11 kPa (mean \(\pm\) SD) and 95.64 \(\pm\) 9.39 kPa under axial and transversal loadings, respectively [8]. Elastomers are required to undergo vulcanization, which is the formation of a cross-link between long polymer chains. This feature controls the ability of the elastomer to reconfigure its structure when stress is applied, indicating the elasticity. The covalent cross-linkages ensure that the elastomer returns to its original configuration when the stress is removed and result in permanent deformation. Cross-links can be introduced into polysiloxanes by a range of methods, including (i) the use of a tri- or tetrafunctional siloxane comonomer during polymerization; (ii) the incorporation of a thermal initiator, which attracts hydrogen atoms from PDMS, thus resulting in cross-linking; or (iii) the exposure of PDMS to high-energy irradiation. Figure 1 shows the method of PDMS cross-linking proposed by a previous author [9]. Cross-linking reactions can also be applied with the intention to improve the surface of a cured PDMS sample using hyperthermal hydrogen-induced cross-linking (HHIC) [10]. In this paper, we also discuss the group of polymers known as silicone elastomers (SEs). SE is typically cross-linked using hydrosilylation (usually by Pt catalysis), condensation (usually by Sn catalysis), and radical reaction [11]. In Figure 2, the two most commonly used curing chemistries are illustrated. SE is widely used due to its favorable properties, namely flexibility and durable dielectric insulation, and its exceptional barrier properties  
Figure 1: Cross-linking of polydimethylsiloxane (PDMS). against environmental contaminants. SE is superior to PDMS in terms of simpler fabrication (it can be cured at room temperature without the use of an oven) because the premixes have low viscosity. The amount of curing agent required is robust and not as sensitive to PDMS formulations.