Owing to their porous three-dimensional (3D) interconnected structure, polymeric foams possess unique properties. They are light weight, flexible, compressible, and possess a high surface area. Moreover, they are chemically inert, biocompatible, and easy to mass fabricate at low cost. Therefore, polymeric foams have traditionally been used for mattresses, seat cushions in homes, offices, aircraft, automobiles, and trains, and as insulators against heat, electricity, and noise [1,2]. Recent advancements and demand for modern materials have expanded the application of polymeric foams in various advanced technologies. However, proper modification of foams is a prerequisite for their use in advanced applications. Polymeric foams are modified by (i) incorporating fillers during fabrication or (ii) surface coating (post-treatment). In the former technique, fillers are usually physically mixed with the foam components during foaming, and high filler loading is required to achieve the desired functionality [3]. This drastically affects foamability, making it challenging to fully incorporate all the unique properties of polymeric foams, that is, density, flexibility, softness, and compressibility. Conversely, the surface coating strategy primarily alters the surface properties of polymeric foams with minimal impact on the bulk properties [2]. Additionally, the coating layer is thin and deposited on the surface of the foam, precisely where it is required, because most advanced applications involve surface phenomena. To date, several coating techniques such as dip coating [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]], layer-by-layer (LbL) assembly [[17], [18], [19], [20]], polyelectrolyte complex coating [2,21], hydrothermal treatment[[22], [23], [24], [25], [26]], in situ polymerization [[27], [28], [29], [30]], polymer-assisted electroless plating [31,32], sol-gel process [33], carbonization [34,35], chemical vapor deposition [36], plasma treatment [37,38], and a combination of two or more methods [[39], [40], [41], [42], [43], [44], [45]] have been employed. Surface coating of polymeric foams imparts several functionalities such as flame retardancy, hydrophobicity, electrical conductivity, thermal conductivity, catalytic activity, and photothermal effects. Consequently, modified polymeric foams have been used in advanced technologies, such as flame sensors, piezoresistors/pressure sensors, electromagnetic wave interference (EMI) shielding, supercapacitors, batteries, oil-water separation, steam generation, catalysis, and triboelectric energy harvesting.

Among the large family of polymeric foams, polyurethane foam (PUF), melamine foam (MF), and natural rubber latex foam (NRLF) have garnered increasing interest in many research and application fields over the past few years. These foams have attracted significant research attention due to their remarkable properties such as their porous structure, excellent mechanical properties, and sustainability and biodegradability (mainly NRLF). The development of functional foams is currently an appealing topic, and advances have been made to identify and address critical and scientific issues related to research and application in this field. Reviews of advanced applications of polymeric foams are summarized in Table 1, with a major focus on flame retardancy and oil-water separation. However, these previous review articles lack a comprehensive discussion of the coating techniques, especially these emerging surface coating methods, i.e., in situ polymerization, and polymer-assisted electroless plating. Additionally, none of these reviews provides an inclusive progress report regarding the application of polymeric sponges. These emerging applications of polymeric foams in metal recovery, catalysis, steam generation, desalination, supercapacitors, solar/electric-to-thermal energy conversion and storage, as well as renewable triboelectric energy harvesting have not been thoroughly reviewed to effectively assess the progress and future perspectives. Most of these previous review papers are also concentrated on the surface engineering and applications of single type polymeric foam (either PUF or MF).

Considering this gap in the literature, we provide a comprehensive overview of achievements in the development of functional polymeric foams. This review covers the surface engineering and applications of the widely known foams (PUF and MF) as well as NRLF, which is recently gaining an increasing attention owing to its renewability and biodegradability. First, a brief introduction on the general properties and synthesis of polymeric foams is provided. Next, state-of-the-art research in all methods and processes employed for the coating of polymeric foams are thoroughly discussed, with an emphasis on their advantages and disadvantages, as well as future directions to address the drawbacks of each surface coating method. Then, potential applications in several advanced technologies, including these emerging fields, i.e., steam generation and water desalination, phase change materials for solar/electric-to-thermal energy conversion and storage, and renewable triboelectric energy harvesting for self-powered sensing applications are demonstrated using select examples. In this section, emphasis is given in the main innovative and emerging state-of-the-art strategies used to address common limitations of current practices in advanced applications of polymeric foams. Finally, more research is recommended to tackle existing challenges regarding coating material durability, scalability of the coating processes, and environmental friendliness, and to explore the feasibility of commercial applications of polymeric foams.