Foams are one of the forms of the polymer constructs intensively developing in material engineering. The main factors responsible for their popularity are low weight, low density, and reduced costs of products based on them. This means that they are used in a wide range of functional materials. Currently, the polymer foam market is dominated by the conventional polymer foams made mostly of polystyrene (PS) and polypropylene (PP). However, in specialized industries such as the tissue engineering, it is necessary to use materials that, in addition to standard mechanical properties, have additional functions. These functions are often associated with the activation of growth of the bone tissue, cartilage, ligaments, skin, blood vessels, nerves, and muscles [
1]. Spongy materials are also used as the carriers for the controlled drugs release [
2]. Due to the origin of the raw material, spongy materials are divided into synthetic, natural, and ceramic materials and their combinations. Clearly, the natural polymers are seen to have the greatest potential in tissue engineering include collagen, the protein that forms the majority of the extracellular matrix, alginate-a plant polymer derived from the algae, and chitosan obtained from chitin and present mainly in protective shells of crustaceans [
3,
4]. The advantage of the synthetic materials is better control of chemical, physical, and mechanical properties. The most popular polymers in this group are linear aliphatic polyesters, which include polyglycolic acid (PGA), polylactic acid (PLA), and their copolymers (PLGA). Their degradation involves random hydrolysis of ester bonds, e.g., PLA breaks down into the lactic acid, which is present in the human body [
5]. The spongy materials can also be created by combining synthetic and natural materials [
6]. Natural fillers such as chitosan [
7] and cellulose [
8,
9] can be used to improve the properties of PLA-based foams. Chitosan is obtained from chitin in a deacetylation process. Chitin is the second most common polysaccharide in nature after cellulose and is obtained from crustacean processing waste. Chitin, chitosan, and materials obtained from them are bioavailable, biocompatible, biodegradable, and biofunctional. Moreover, they do not have antigenic properties and are non-toxic and environmentally friendly [
10,
11]. The presence of two functional groups in the molecule of chitosan, i.e., hydroxyl and amine, makes it possible to carry out many chemical and enzymatic modifications. Thus, it is often used in the design and construction of systems for the immobilization and release of therapeutic compounds, as well as for obtaining water-soluble derivatives of chitosan [
12,
13]. The specific feature of chitosan is its antibacterial and antifungal activity [
14]. These properties exhibit acidic chitosan solutions, hydrogel forms, films, and dry sponges [
15,
16,
17]. The antimicrobial activity of chitosan depends on many physicochemical factors: the molecular weight of the polymer, its degree of deacetylation, the pH of the environment, and the changes caused by the modification. Scientists have not strictly defined the mechanism of chitosan antibacterial activity. According to one theory, the antimicrobial activity of chitosan is associated with its polycationic character and interactions with the negatively charged bacterial cell membrane. These interactions can lead to changes in the permeability of the cell membrane, thus causing an osmotic imbalance inside the cell, and consequently inhibiting the growth of microorganisms [
18]. They may also be responsible for the hydrolysis of peptidoglycan in the bacterial cell wall, which leads to leakage of intracellular electrolytes [
19,
20]. Chitosan demonstration of antibacterial properties can probably also be caused by its ability to chelate metal ions necessary for the growth of microorganisms [
18]. Additional modifications, e.g., with N-propyl phosphonic anhydride, also affect the intensification of antibacterial properties [
13]. It can be assumed that the increased antibacterial activity of the modified chitosan is mainly associated with the solubility of the polymer in a neutral pH environment, its polycationic character, as well as the presence of phosphate groups that are responsible for chelation of cations. This promotes the formation of intermolecular and intramolecular hydrogen bonds, which, by the way, produce hydrophobic micro-spaces within the polymer chain. The local division of the polymer area into hydrophobic and hydrophilic fragments promotes, in structural terms, the affinity between the bacterial cell wall and the chitosan derivative [
13,
18]. One of the main limitations of combining chitosan as a carrier of antimicrobial activity with synthetic polymers such as PLA or PGA is its hydrophilic character. It is possible to chemically modify chitosan in such a way that it exhibits hydrophobic properties and interacts more strongly with PLA, giving new properties to the obtained biocomposites [
21]. Another solution may be to use the technique of emulsification and lyophilization of polymer solutions of different chemical nature, which enables, for example, the obtaining of chitosan/collagen/PLA biocomposites [
22]. Most techniques for obtaining PLA/CS composites are based on the use of a chitosan solution dissolved in diluted organic acids, which entails the use of additional methods associated with the removal or neutralization of acid residues in the finished material [
23,
24].
An alternative way may be to use the method of producing material in the following work by mixing a solution of PLA in chloroform, and chitosan precipitate dissolved in water saturated with CO
2, and polyethylene glycol and freeze-drying to remove solvents: water and volatile chloroform. In the scientific literature are known the methods of using carbon dioxide for the preparation of similar materials. Hijazi et al. presented two supercritical CO
2-assisted processes aimed at generating chitosan nanoparticles for modification of the PLA films [
25]. The work of Kazimierczak et al. presents method of foaming materials by using chemically produced CO
2 directly in the chitosan/agarose/nanohydroxyapatite scaffolds [
26]. The novelty of our work is the use of carbonic acid in the construction of foamed PLA/CS composites, which by using lyophilization allows the obtaining of foamed materials in a simple way. The use of the CO
2 saturation technique of chitosan has not yet been presented in the design of this type of functional materials. The obtained foams were characterized by a lack of cytotoxicity toward L929 fibroblast cells, low density, and high porosity and hardness, and possess antimicrobial activity, the effectiveness of which depends on the share of chitosan in the composite.