## Special Issue on \"Active Materials for Medical Applications\"  
[PERSON],* and [PERSON]  
This Special Issue was proposed by engineers, physicists, medical doctors, researchers and scientists. We intend to analyze and discuss different topics on active materials for medical applications. There is great potential in the application of active or smart materials (metallic, polymer or ceramic) for the progression of applications in the medical domain of MEMS, actuators, sensors or functional systems. Active or \"smart\" materials have the ability to respond to different physical or chemical stimuli in a specific, repeatable mode. The actual activity in the domain, however, presents problems connected to obtaining and processing, characterizing, modeling and simulating or prototyping technologies. This Special Issue of _Applied Sciences_ focuses on the most recent advances in obtaining and thermal and mechanical processing active materials used in the medical field with enhanced performances. In this volume, two papers deal with biodegradable metallic materials based on Mg and Zn (Microstructural, Electrochemical and In Vitro Analysis of Mg\({}_{0.5}\)Ca\({}_{x}\)Gd Biodegradable Alloys [1] and In Vitro Corrosion Behavior of Zn\({}_{3}\)Mg\({}_{0.7}\)Y Biodegradable Alloy in Simulated Body Fluid (SBF) [2]), presenting findings in this new important field with new chemical systems, degradation rate determination and main physical, chemical and mechanical properties. Biodegradable metallic materials represent a new class of biocompatible materials for medical applications based on numerous advantages. Among them, those based on zinc have a rate of degradation close to the healing period required by many clinical problems, which makes them more suitable than those based on magnesium or iron. The poor mechanical properties of Zn can be significantly improved by the addition of Mg and Y. In this research study, we analyze the electro-chemical and mechanical behavior of a new alloy based on Zn\({}_{3}\)Mg\({}_{0.7}\)Y compared with pure Zn and Zn\({}_{3}\)Mg materials [2]. Microstructure and chemical compositions were investigated by electron microscopy and energy dispersive spectroscopy. Electrochemical corrosion was analyzed by linear polarization (LP), cyclic polarization (CP) and electrochemical impedance spectroscopy (EIS). For hardness and scratch resistance, a microhardness tester and a scratch module were used. Findings revealed that the mechanical properties of Zn improved with the addition of Mg and Y. Zn, Zn-Mg and Zn-Mg-Y alloys in this study showed highly active behaviors in SBF with uniform corrosion [2]. Zinc metals and their alloys with magnesium and yttrium showed a moderate degradation rate and can be considered as promising biodegradable materials for orthopedic application. Two other papers are based on the Finite element method (FEM) of investigation with exceptional findings in the field of mandibular anterior teeth with healthy but reduced periodontium and inlay-retained dental bridges [3,4]. Finite element analysis studies have been of interest in the field of orthodontics, and this is due to the ability to study stress in the bone, periodontal ligament (PDL), teeth and the displacement in the bone by using this method. Our study aimed to present a method that determines the effect of applying orthodontic forces in a bodily direction on a healthy and reduced periodontitis and to demonstrate the utility of finite element analysis. Using the cone-beam computed tomography (CBCT) of a patient with a healthy and reduced periodontitis, we modeled the geometric construction of the contour of elements necessary for the study [4]. Afterwards, we applied a force of 1 N and a force of 0.8 N in order to achieve bodily movements and to analyze the stress in the bone, in the periodontal ligament and in absolute displacement. The analysis of the applied forces showed that a minimal ligament thickness is correlated with the highest value of the maximum stress in the PDL and decreased displacement [4].