## 3 Primary or Polymer Metallization  
Since most polymers are electrically nonconductive, their surface should be made conductive for electrodeposition through a process called Polymer metallization (PM), which can be made possible through many ways of depositing a metallic layer on it. The most common methods of PM are physical vapor deposition (PVD), electroless plating, thermal spray methods, and direct polymer-metal bonding methods [33; 34; 35; 36; 37]. The advantages and disadvantages of each technique are summarized in Table 1.  
Electroless plating followed by electroplating is the most prolifically used PM technique [38]. Electroless deposition is a chemical process similar to electrodeposition but without the need for an external power source. Instead, the process relies on a redox reaction between a reducing agent in the plating solution and metal ions in the electrolyte [39]. This process is autocatalytic, meaning that once the initial layer of metal is deposited, it \begin{table}
\begin{tabular}{c c c} \hline
**PM Method** & **Advantages** & **Disadvantages** \\ \hline Physical Vapor Deposition (PVD) & High precision, thin coatings, uniform coverage, high-quality finish & Expensive, slow deposition rates, not suitable for complex geometries \\ \hline Electroless Plating & Uniform coating, no electrical current required, suitable for complex geometries & Limited material compatibility, slow deposition rates, less durable \\ \hline Thermal Spray Methods & High deposition rates, one step process, superior coating adhesion & Metals compatibility is limited, special resolution is low, coatings are porous \\ \hline Direct Polymer-Metal Bonding & Strong bonding, versatility in method selection, customizable surface finish & Temperature limitations on polymers, weak mechanical properties, time intensive \\ \hline \end{tabular}
\end{table} 
Table 1: Common polymer metallization techniques and their advantages and disadvantages. catalyzes further deposition [40]. Electroless deposition is a multistep, time consuming process [38]. The various steps involved in the electroless deposition are schematically shown in Figure 3. The pre-treatment creates a clean, pH-balanced surface so that later catalytic chemicals can infiltrate the surface uniformly. The catalytic stage deposits nanoscale \"seeds\" that start the autocatalytic reduction. The overall chemical deposition stage builds the final coating with a thickness controlled only by immersion and bath chemistry with no applied current [39]. Although electroless deposition is the most usual form of PM, many difficulties arise during this process. The chemicals used in surface pre-treatments and the chemical deposition stage can interact adversely with certain polymers, leading to the degradation or deformation of the substrate [41; 42]. Achieving a uniform deposit can be difficult due to surface irregularities and the potential for uneven catalyst distribution. Furthermore, many of the chemicals used in electroless deposition, such as chromic acid, formaldehyde, and sodium hypophosphite, among others, are harmful to the environment [43]. Another effect that must be considered is how common PM techniques, including electroless plating, modify the interface of the polymer (as the substrate) and the plated metal [41]. For example, as depicted in Figure 4, the etching process in electroless plating roughens the surface of the polymer to allow for the deposition of a primer metal, e.g., palladium, on the surface and, thus, to facilitate the autocatalytic process. It should be noted that for electrodeposition on 3D-printed polymers, PM techniques, such as electroless plating, alter the as-printed surface finish of the polymeric material and therefore roughen the interior surface of the plated metal if the polymer is removed. This roughening is desirable for some processes because it adds adhesive strength and encourages metal-polymer diffusion [43].