It is almost unavoidable to come across some sort of polymer foam in daily life in the modern world. The excellent characteristics such as superior acoustic absorption, strength-to-weight ratio, cost-effectiveness, and ease of being processed in various forms make polymeric foams an attractive material to be found virtually everywhere. High-density polymer foams are found to replace traditional materials in transportation and building constructions for weight reduction, while low-density foams are commonly used in shock absorption and rigid packaging. The flexible and soft polymer foams provide comfort when used for furniture and bedding. The low heat transfer and sound transmission inherited from the porous structures make them optimal thermal and sound insulators. New insights and innovations continue to drive the field of polymer foams forward and are expected to witness increasing attentions (Fig. 1).

Various industries are on a constant search for these lighter structures. One of the major factors that drive the effort to reduce the weight of structures is the increase in fuel prices. In particular, the transportation industries, i.e., automotive, marine, and aerospace industry, spend a significant amount of money to fund their R&D activities with the aim to reduce the weight of cars, boats, and aircraft, respectively. The progresses made on polymer nanocomposite laid a solid foundation for nanocomposite foams fabrication. These foams take advantage of the size compatibility of the nanofillers to achieve microscopic reinforcements where conventional fillers failed to do so. Novel porous materials are fabricated by incorporating functional nanofillers and the applications are extended to fields of biomedical, tissue engineering, electronics and areas require high temperature resistance.
The technologies for polymer foam fabrication are constantly steered by environmental considerations and cost-effectiveness. Recent research is focused on using more environmentally friendly chemicals for foaming and less stringent conditions such as lower temperature or pressure. Microcellular foams attracted much attention since the invention in the 1980s. Common gas releasing agents such as CFCs were replaced by inert gases such as supercritical CO2 and N2 which almost cause zero environmental impacts.
Syntactic foams represent a special kind of composite materials comprising of a matrix and a hollow filler. The filler is often also referred to as hollow spheres with diameter ranging from a couple of nanometers to millimeters and can be made of various types of materials, including polymers, glass, metals, and carbon. Glass microspheres are most frequently used due to their mechanical strength, smoothness, regularity of the surface, good wetting characteristics, and low viscosity of the resulting foam. The extremely low density of the hollow spheres effectively reduced the density without severe mechanical performance deteriorations. The inclusion of spherical particles also eliminates the problem posed by the anisotropic bubbles in conventional foams and acted as a good moisture barrier due to the highly closed-cell nature.