Polyurethanes as Biomaterials in Medicine: Advanced Applications, Infection Challenges, and Innovative Surface Modification Methods

PUs stand out as one of the most versatile classes of polymer materials. Their versatility is primarily due to their segmented structure, including hard and soft segments (Rusu et al. 2020). The soft segments in polyurethanes are responsible for the material’s elastomeric properties. Their chemical structure is based on polyols, such as polyethers and polyesters. The low glass transition temperature of the soft segments allows for flexibility and influences the material’s hydrophilicity, which determines the degree of water diffusion into the polymer. As a result, the degradation rate of soft segments is related to hydrophilicity and the presence of labile groups. Polyethers are commonly used to impart flexibility and increase stability to the material. Another class of soft segments includes A-B-A tri-block polyols, which are used in producing resorbable polyurethanes due to their versatile properties (Mirhosseini et al. 2019; Zhang et al. 2019).

The hard segments in polyurethanes provide increased mechanical resistance. They consist of isocyanates and chain extenders. Their high transition temperature and pronounced crystallinity contribute to the material’s mechanical strength. The content and chemistry of the hard segments have a direct impact on tensile strength and modulus. The amount of isocyanates and chain extenders determines the physical characteristics of polyurethane, and their type (aromatic or aliphatic) affects reactions with nucleophilic reagents and the material’s toxicity. Aromatic isocyanates are highly reactive but can lead to side effects in carcinogenic degradation products, whereas aliphatic isocyanates have a lower toxicity potential (Somdee et al. 2019; Rogulska 2023).

They are valued for their excellent biocompatibility, exceptional resistance to hydrolytic processes, high abrasion resistance and outstanding mechanical durability, including resistance to bending. Their diversity manifests in adhesives, coatings, sealants or foams, available in a wide range of Shore hardnesses (Shin et al. 2018). Thanks to such broad properties, it is possible to use PUs to create short- and long-term biomaterials.

According to the U.S. Food and Drug Administration (FDA) and the European MDCG 2021–24, implants made of biomaterials are devices placed inside the body or on the body’s surface (EC 2021; FDA 2023). A distinction can be made between long-term implants, which have a lifespan of 30 days or more in contact with a tissue before biodegradation or removal to promote healing to the tissue remodeling phase. In contrast, short-term implants have less than 30 days of lifespan in contact with living tissues. Finally, non-implantable devices are materials designed to be placed on the body’s surface (EC 2021; FDA 2023). The properties mentioned above render polyurethanes ideal materials for various medical device components (Fig. 1).

Medical-grade PU-based polymers can be found under several trade names, such as Carbothane™, Pellethane® or Tecoflex™ from Lubrizol (USA) and Carbosil® and Bionate® from DSM (Netherlands) (Wendels and Avérous 2021).

The insertion of respiratory biomaterials, such as tracheostomy tubes, is often essential in treating patients requiring long-term respiratory support. The tracheostomy procedure is an alternative to prolonged endotracheal intubation, offering better control of the patient’s airway and reducing the risk of nosocomial infections. Nevertheless, using such devices may be associated with complications, such as granuloma formation, which can lead to bleeding, airway obstruction or scab formation. In addition, tracheostomy tubes may provide a site for colonization by microorganisms, promoting infection development (Cheung and Napolitano 2014).

These include different bacterial species such as Klebsiella pneumoniae, Pseudomonas. aeruginosa, Staphylococcus aureus, and Acinetobacter baumannii (Restrepo et al. 2013; Huang et al. 2018; Ścibik et al. 2020; Thirumurthi et al. 2021; Raveendra et al. 2022) and Strep-tococcus pneumoniae (Thirumurthi et al. 2021). Bacterial colonization on tracheostomy tubes can further lead to ventilator-associated pneumonia (VAP) (Restrepo et al. 2013; Ferro et al. 2021), which can be caused by a single pathogen or have a polymicrobial origin (Ścibik et al. 2020).

A study conducted in twenty-seven intensive care units across nine European countries reported a VAP rate of 18.3 cases per 1,000 days of mechanical ventilation. The predominant pathogens were S. aureus, P. aeruginosa, and Acinetobacter spp. (Koulenti et al. 2016). Additionally, it was shown that patients with SARS-CoV-2 infection had a higher risk of developing VAP (50.5%) compared to patients without viral infections (25.3%) (Rouzé et al. 2021). On the other hand, a meta-analysis that included 8,282 cases from twenty provinces in China revealed that the cumulative incidence of VAP in mainland China was 23.8% between 2006 and 2014 (Ding et al. 2017).

In addition to colonizing the tracheostomy tube, the natural microbiome colonizing patients is also essential. It is estimated that approximately 20% of healthy individuals are chronically colonized by S. aureus (Pickens and Wunderink 2022). Studies indicate that between 15.2% and 22.1% of VAP cases are caused by MRSA strains (Feeney et al. 2018; Pasha et al. 2020). Furthermore, a systematic review and meta-analysis suggest that MRSA colonization status may be helpful as an indicator of the risk of developing a VAP infection, which may support decision-making about the use of empiric therapy (Butler-Laporte et al. 2018).

In addition, patients colonized by MRSA tend to have more prolonged mechanical ventilation and poorer clinical outcomes, highlighting the need for strict infection control and targeted therapies (Feeney et al. 2018)

As a result, it can lead to patient death. Scientific sources report that VAP mortality rates range from 20–30% (Restrepo et al. 2013), with some studies indicating that the mortality rate can be as high as 49.2% (Tamaya et al. 2012). In patients who have undergone heart surgery, VAP was identified as the most significant independent risk factor for in-hospital mortality. The mortality rate in patients with VAP was 49.2%, compared to 2% among those without VAP (Tamaya et al. 2012).

The use of medical devices, including pacemakers, defibrillators, stents, and heart valves, has increased dramatically over the past fifty years. Today, more than 1.7 million cardiovascular devices and over one million other medical devices are implanted worldwide annually (Scialla et al. 2021).

Infective endocarditis (IE) associated with implantable devices, such as artificial valves, is referred to as prosthetic valve endocarditis (PVE). PVE is a severe complication of heart valve replacement surgery, with a reported mortality rate ranging from 20% to 80% (Angelina et al. 2016). The pathophysiology of PVE varies depending on the time elapsed since the operation, allowing for a distinction between two types of PVE: early and late. Early PVE is diagnosed within the first year after the implantation of an artificial valve, while late PVE is diagnosed after this period (Ramos-Martínez et al. 2023).

In early PVE, the lack of endothelialization of the suture ring and adjacent tissue plays a key role. When combined with the presence of adhesion proteins such as fibrinogen and fibronectin, it facilitates the development of infection. Additionally, early PVE often results from accidental contamination during surgery or hematogenous spread occurring postoperatively within the first hours to months. In contrast, during late PVE, the heart valve structures are fully covered by endothelium, and the pathogenesis of the disease more closely resembles that of native valve endocarditis (NVE) (Galar et al. 2019). A large cohort study involving 1,354 cases of PVE demonstrated that early PVE is more commonly associated with nosocomial pathogens, whereas late PVE is characterized by greater microbiological diversity. The hospital mortality rate for PVE was reported to be 32.6% (Ramos-Martínez et al. 2023; Ivanovic et al. 2019). In contrast, other studies suggest that the most common microorganisms responsible for early PVE (within two months of implantation) are S. aureus (36%), coagulase-negative staphylococci (17%), and fungi. For PVE occurring later, the incidence of S. aureus and coagulase-negative staphylococci decreases to 18–20%, with a corresponding increase in infections caused by enterococci and streptococci (10–13%) (Ivanovic et al. 2019). Patients with S. aureus-induced PVE represent a unique subgroup characterized by an increased risk of complications and a higher mortality rate (Tan et al. 2015). The causative factors in late PVE are similar to those in native valve endocarditis. It is usually caused by bacteria, such as α-haemolytic streptococci and CoNS (Coagulase Negative Staphylococci), which colonize various human body surfaces (Ivanovic et al. 2019; Berisha et al. 2022). Patients with S. aureus-induced PVE represent a distinct subgroup characterized by an increased risk of complications and a higher mortality rate (Rivoisy et al. 2018).

Other examples of biomaterial-related infections include infections of synthetic blood vessels, which typically result from direct contamination during surgery (Hasse et al. 2013). These infections affect approximately 1–6% of patients, with a higher risk (around 6%) for prostheses placed in the groin region (Wilson et al. 2016). Treatment usually involves antibiotic therapy and surgical prosthesis removal, a particularly challenging process due to the biofilm’s presence and the infection’s depth (Leroy et al. 2012). Gram-positive bacteria, such as S. aureus and coagulase-negative staphylococci, and less commonly Gram-negative bacteria like Escherichia coli, are most frequently responsible for these infections. The biofilms produced by these bacteria significantly complicate the elimination of infections (Gharamti and Kanafan 2018). However, a recent retrospective study revealed that vascular prosthesis infections have evolved, with a notable increase in infections caused by Gram-negative bacteria exhibiting high antibiotic resistance, particularly in early infections (Gouveia e Melo et al. 2021).

Another group of biomaterials, some made from polyurethanes, includes implantable cardiac electronic devices (ICEDs). This group comprises implantable cardiac defibrillators (ICDs), cardiac resynchronization therapy devices (CRTDs), and permanent pacemakers (PPMs) (Armaganijan and Healey 2011). In 2010, approximately 40,000 ICEDs were implanted in the UK (Sandoe et al. 2014).

Studies indicate that the infection rate associated with ICEDs is relatively low, ranging from 0.5% to 2.2%. However, the mortality rate of these infections is as high as 35% (Sandoe et al. 2014; Sławiński et al. 2019). In Poland, the infection rate is reported to be 1.2% (Sławiński et al. 2019). More than eighteen scientific reports, including at least 100 patients each, were analyzed in recent guidelines on diagnosing, preventing, and treating infections associated with implantable cardiac electronic devices (ICEDs). The etiological agents were found to be highly recurrent. Gram-positive bacteria were the predominant group, isolated in 67.5% to 92.5% of cases, with CoNS and S. aureus being the most common pathogens. Gram-negative bacilli accounted for 6% to 10.6% of all isolates. Polymicrobial infections were reported in seven studies, ranging from 2% to 24.5%. Fungal infections, in contrast, were rare, with a prevalence of less than 2% (Sandoe et al. 2014).

Central venous catheters (CVCs) are essential in managing critically ill patients, particularly those in intensive care units (ICUs). They facilitate intravenous access, enabling the administration of drugs and fluids, as well as the collection of blood for laboratory analysis (Gomes Resende de Souza da Silva et al. 2021). However, their use requires proper preparation and precise insertion techniques to minimize the risk of procedural complications. Central line-associated bloodstream infections (CLABSIs) are among the most common infections in ICUs. In developing countries, incidence rates range from 1.7 to 44.6 cases per 1,000 catheter days (Fontela et al. 2012). These infections are caused by various species, with CoNS (e.g., Staphylococcus epidermidis) being predominant, particularly in patients using long-term central catheters (Özalp Gerçeker et al. 2019; Akaishi et al. 2023). Other common pathogens include S. aureus (Sellamuthu et al. 2023), Enterococcus faecium and Enterococcus faecalis (Belloni et al. 2022). Gram-negative bacteria, such as Klebsiella spp. and Pseudomonas spp., are responsible for many infections, particularly in pediatric and oncology wards (Tomar et al. 2015). This group also includes Gram-negative bacteria, such as Klebsiella spp. and Pseudomonas spp., responsible for many infections, particularly in pediatric and oncology wards (Tomar et al. 2015). Furthermore, the emerging phenomenon of antimicrobial resistance among clinically significant Gram-positive and Gram-negative bacteria, particularly those in the ESKAPE group (E. faecium, S. aureus, K. pneumoniae, A. baumannii, P. aeruginosa, and Enterobacter spp.), poses a significant challenge for the prevention and treatment of infections (De Oliveira et al. 2020).

Another infection associated with biomaterials is catheter-associated urinary tract infection (CAUTI). CAUTI occurs when bacteria enter the urinary tract due to using a urinary catheter, a tube inserted into the bladder through the urethra to drain urine when a patient cannot urinate independently.

Catheter-associated urinary tract infections are significant due to their high prevalence, as they are one of the most common healthcare-acquired infections, accounting for up to 40% of hospital-acquired infections (Rubi et al. 2022). Moreover, 15–25% of hospitalized patients use urinary catheters, and approximately 75% of UTIs developing in hospitals are associated with catheter use (CDC 2024). Some studies indicate that the costs per patient with CAUTI range from $876 when a patient requires hospitalization, additional diagnostic tests, and medications to as much as $10,197 when a patient is hospitalized in the ICU (Hollenbreak and Schilling 2018). Other data suggest that reducing the incidence of CAUTIs can lower healthcare costs by $4,501 for every 1,000 catheterized patient days (Sutherland et al. 2015).

The most commonly identified microorganisms in the biofilms of long-term catheterized patients include Proteus mirabilis(Melo et al. 2016; Yuan et al. 2021; Herout et al. 2023), E. coli, P. aeruginosa, K. pneumoniae, Proteus stuartii, Morganella morganii, and E. faecalis (Lassek et al. 2015; Puspitasari et al. 2021). Less frequently, S. aureus has been found to be responsible for 0.5% to 2% of all CAUTIs (Walkera et al. 2017).

One technique for processing polyurethanes is laser surface texturing (LST). This method modifies the surfaces of polymer biomaterials using a laser beam. LST offers numerous advantages, such as altering surface roughness and chemistry in a single step without using toxic substances. Laser surface texturing can also alter polymer surfaces on the macro-, micro, and nanoscale with high spatial and temporal resolution (Lippert 2004) (Fig. 4a).

Fig. 4.

Methods applied for the surface treatment of polyurethane polymers: a) Laser Surface Texturing, b) Nanoparticle Grafting, c) Cold spraying metallization, d) Ion Implantation, e) Low-temperature plasma, f) Biodegradable and biocompatible polyurethanes.

Laser processing technology enables the creation of submicrometer structures on polyurethane surfaces, first documented for components around 250 nm in size. Studies have shown that periodic line-like patterns with spatial periods of up to 500 nm can be achieved using this technique. The depth of these structures ranges from 0.88 to 1.25 μm for periods larger than 2.0 μm. It reaches up to 270 nm for periods between 500 nm and 1.0 μm, demonstrating precise control over the parameters of the textures obtained (Estevam-Alves et al. 2016).

In comparison, the average size of most bacteria falls within the 1–2 μm range. As a result, surfaces with submicron topographies have been found to exhibit antimicrobial properties (Siddiquie et al. 2020). These properties are particularly relevant in the context of biomaterial-associated infections (BAI), where bacterial adhesion to the biomaterial surface is a critical first step in biofilm formation (Arciola et al. 2018). Submicron surface structures reduce bacterial adhesion by inducing stress on bacterial cell walls when the spacing between protrusions is less than 1.5 μm and by trapping air to decrease the apparent solid surface area available for bacterial attachment. While this does not equate to bacteriostatic or bactericidal activity, limiting bacterial adhesion is an important indicator of antimicrobial efficacy in preventing biofilm formation and subsequent infections (Siddiquie et al. 2020).

Nanoparticle grafting involves attaching nanoparticles to the surface of a polymer, often through covalent bonds, to enhance surface properties. This process allows for the modification of both the physical and chemical properties of the polymer and the nanoparticles, thereby expanding their application potential across various technological fields. Grafting nanoparticles onto polymers is typically achieved using chemical methods such as ‘grafting to’ and ‘grafting from.’ This technique is gaining popularity due to its ability to improve nanoparticles’ dispersion, chemical reactivity, and stability within polymer matrices, making them more effective in advanced material applications (Kango et al. 2013) (Fig. 4b).

Various chemical techniques, including silane coupling agents, atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (RAFT) polymerization, are employed to graft polymers onto nanoparticles. These techniques allow fine-tuning of the surface properties of nanoparticles and their compatibility with polymer matrices, making them more versatile and practical in a wide range of applications (Kango et al. 2013).

One example of nanoparticle grafting is the creation of polyurethane nanofibers with incorporated ZnAg composite nanoparticles for use in antibacterial wound dressings. The grafting process involved synthesizing ZnAg composite nanoparticles (silver and zinc oxide) and incorporating them into polyurethane nanofibers (PUZnAg) through electrospinning. The resulting PUZnAg nanofibers demonstrated excellent antibacterial properties. Specifically, the PUZnAg₂ sample, containing 8% ZnAg by weight, exhibited 100% antibacterial efficacy against E. coli, S. aureus, and Bacillus subtilis, completely inhibiting bacterial growth. Furthermore, the nanofibers effectively prevented bacterial growth in the medium for up to 72 hours (Jatoi et al. 2020).

A similar technique was used for the single-phase synthesis of silver (Ag) nanoparticles embedded in polyurethane (PU) nanofibers. These PU/Ag nanofibers showed antibacterial efficacy against E. coli and S. aureus, forming inhibition zones of 11.4 mm and 10.8 mm in diameter, respectively. Additionally, the PU/Ag nanofibers were shown to be biocompatible, promoting fibroblast proliferation making them promising materials for medical applications, such as wound dressings (Pant et al. 2019).

Another potential application of polyurethanes is in flexible nanocomposite foams based on biocompatible thermoplastic polyurethane (TPU) and ZnO nanoparticles, which have potential uses as wound dressings. The TPU/ZnO foams were produced using the thermally induced phase separation (TIPS) method. This resulted in a highly porous structure with pore sizes ranging from 10 to 60 μm, allowing water vapor transport up to 8.9 mg/cm²-h. The TPU/ZnO foams exhibited strong antibacterial activity against Gram-positive bacteria, such as E. faecalis and S. aureus, and Gram-negative bacteria, such as E. coli and P. aeruginosa. The highest reduction in bacterial numbers, by up to 10³ Colony Forming Units (CFU), was observed in foams containing 10% ZnO (Bužarovskaa et al. 2019).

Ion implantation is a process in which ions of a specific element are accelerated and introduced into the surface of a polymer, leading to the formation of metallic nanoparticles within the polymer matrix. This technique allows for the modification of both the chemical and physical properties of the surface without altering the bulk material’s properties. Ion implantation enables the creation of advanced metal-polymer nanocomposites with enhanced mechanical properties, such as increased strength, wear resistance, and improved electrical conductivity. This method is widely applied in medical implants, biomaterials, and electronics (Popok et al. 2014). Ion implantation also provides precise control over the depth and distribution of nanoparticles within the polymer, influencing properties such as surface conductivity and roughness. This level of control is crucial for applications like surface plasmon resonance, which plays a key role in photonics and sensor technology (Salvadori et al. 2014) (Fig. 4d). PUs are being modified to enhance their antimicrobial properties, which are essential for reducing the risk of infections associated with medical implants.

Recent studies have demonstrated significant microbiological effects of modifying polyurethane (PU) surfaces using nitrogen ion implantation (N₂⁺). This treatment substantially reduced the viability of S. epidermidis colonies, decreasing their survival by 3 to 5 times compared to unmodified PU. Additionally, a significant reduction in the total number of bacteria adhering to the surface was observed. These effects remained stable over time, with antibacterial properties persisting for up to 11 months post-treatment. The reduction in bacterial adhesion and viability was closely associated with structural changes in the surface, including increased roughness, the development of an undulating morphology, and enhanced surface hydrophilicity (Morozov et al. 2019).

The plasma ion implantation (PIII) method also shows great promise in improving the antimicrobial properties of polymeric biomaterials, such as polyurethane (PU) and polyethylene terephthalate (PET). This technique significantly reduces bacterial adhesion on modified materials, crucial for preventing infections related to medical implants. For instance, PIII-modified PET surfaces treated with acetylene (C₂H₂) exhibited a marked decrease in the adhesion of bacteria, including S. aureus, S. epidermidis, E. coli, and P. aeruginosa. Structural changes induced by the modification, such as increased roughness and enhanced hydrophilicity, were key factors contributing to the reduction in bacterial growth. These findings highlight the potential of the PIII method as an effective tool for creating antimicrobial biomaterials that reduce infection risks and enhance the safety of medical implants (Huang et al. 2004).

Cold spraying, also known as cold dynamic gas spray (CGDS) technology, is an advanced method of applying coatings and depositing powder materials to various surfaces. The process involves accelerating powder particles to supersonic speeds (300–1200 m/s) using a gas jet (e.g., nitrogen, helium or air) passed through a special nozzle. After hitting the substrate, the particles experience intense plastic deformation phenomena, which enables effective adhesion without melting the material (Dai et al. 2024).

Cold spraying is emerging as a promising technique for biomedical applications, particularly for improving the properties of PUs. The process involves the deposition of various materials, including metals, ceramics, and composites, onto the surface of polymers without the risk of thermal degradation, which is essential for medical materials. One of the key application areas for this technique is to improve biomaterials’ biocompatibility and antimicrobial properties, which is particularly important in medical devices such as prostheses and internal fixation systems. With cold spraying coatings, these biomaterials can achieve improved cell adhesion, reduced cytotoxicity, and improved mechanical properties, making them ideal for implant applications (Vilardell et al. 2015; Dosta et al. 2018) (Fig. 4c).

Another example of using cold spray metallization techniques is the production of copper coatings on carbon steel. Silva et al. (2019) demonstrated that these coatings, characterized by a dense microstructure and low porosity, exhibit strong antibacterial properties against S. aureus, achieving complete bacterial mortality (100%) within 10 minutes of direct contact under controlled experimental conditions. The antibacterial mechanism involves the release of copper ions, which damage bacterial cell membranes and disrupt protein structures. The high copper content on the surface is essential for antibacterial efficacy. Notably, the coating produced by this method is nearly oxide-free, ensuring direct contact between the copper and bacteria, which enhances its antimicrobial performance. These findings suggest that copper cold spraying coatings could be effective antibacterial surfaces, with potential applications in hospital equipment and touch surfaces in public spaces (Silva et al. 2019).

The integration of natural biopolymers into polyurethane (PU) matrices is a promising approach to enhance their antimicrobial properties and prevent biofilm formation. Modifying PUs with components like chitosan and alginate during synthesis makes it possible to create materials with improved biocompatibility, biodegradability, and antimicrobial properties (Uscátegui et al. 2019) (Fig. 4f). Composites are materials composed of two or more components with different physical, chemical or mechanical properties that remain separate but work together to produce unique properties (Biermann 2019)

Chitosan is a natural polysaccharide derived from crustaceans, making it a dedicated and renewable resource. Studies have shown that it is biocompatible, biodegradable, bioadhesive, non-toxic, and possesses antimicrobial properties (Confederat et al. 2021). Due to these attributes, chitosan is widely used in applications such as wound dressings, surgical sutures, and scaffolds in tissue engineering, among others (Wu et al. 2018). Similarly, alginate, a polysaccharide derived from brown algae, is valued for its biocompatibility and capacity to support tissue regeneration. Alginate-modified PUs form hydrogels that can trap bacteria, limiting their mobility and reducing biofilm growth. These composites have demonstrated antimicrobial activity by acting as physical barriers and delivering antimicrobial agents directly to the infection site (Zafar et al. 2022).

Building on this foundation, the research team led by Villani et al. developed a polyurethane composite incorporating chitosan (Chit) as a functional filler within a thermoplastic polyurethane (TPU) matrix. With its well-documented antimicrobial properties, chitosan was integrated into the TPU matrix to enhance biological performance. The resulting TPU-Chit composite exhibited significant antimicrobial activity, particularly against S. aureus. This antimicrobial effect was primarily attributed to the anti-adhesion properties of chitosan, which reduced bacterial attachment to the composite surface and inhibited biofilm formation. The effectiveness of the composite was selective, with S. aureus growth reduced by 20% to over 50%, depending on bacterial concentration, while no significant effect was observed against E. coli. Additionally, scanning electron microscopy (SEM) confirmed structural damage to S. aureus bacteria adhering to the composite surface, including cell wall deformation, indicating a reduced capacity for growth and adhesion (Villani et al. 2020).

The research conducted by Khan et al. focused on the development of polyurethane (PUR) membranes modified with sodium alginate (SAg) to enhance their antimicrobial properties. By incorporating sodium alginate as a filler, the team explored its impact on the structural and functional characteristics of the membranes. Antimicrobial testing of the PUR-alginate membranes was performed using Gram-negative (E. coli) and Gram-positive (Bacillus cereus) bacteria. The study revealed that the membranes effectively inhibited bacterial growth, with B. cereus demonstrating a higher sensitivity to the alginate content. Membranes with increased alginate concentrations produced larger bacterial growth inhibition zones, indicating a clear link between alginate levels and antimicrobial efficacy. In the case of E. coli, a gradual reduction in bacterial proliferation was observed, likely due to the enhanced hydrophilicity of the alginate-modified membranes. Khan et al. proposed that the antimicrobial mechanism arises from the interaction of alginate’s carboxyl groups (-COOH) with bacterial cells. The dissociation of protons from these groups lowers the local pH, compromising bacterial cell walls. Additionally, carboxylate ions can bind to positively charged components of bacterial cells, disrupting essential cellular processes. Overall, incorporating alginate into polyurethane membranes improved their hydrophilicity and significantly enhanced their antimicrobial performance, particularly against B. cereus (Khan et al. 2021).

In summary, incorporating natural biopolymers like chitosan and alginate into polyurethane matrices enhances their antimicrobial efficacy by disrupting bacterial adhesion and biofilm formation. These modifications address critical challenges in biomaterials engineering, particularly for medical devices prone to infection.

Plasma, often referred to as the fourth state of matter, is a powerful tool for surface modification of polymers, including polyurethanes. Plasma treatment involves the exposure of polyurethane surfaces to a gaseous environment containing ions, radicals, and photons (visible and near-UV), leading to physical and chemical surface modifications (Friedrich et al. 2012; Thakur and Vasudevan 2021). The concept of polymer functionalization is based on exposing the modified surface to plasma, which results in the attachment of atoms or fragments of dissociated plasma gas as functional groups through H-substitution in the polymer chain. Since many different fragments and atoms are present in plasma, a wide range of functional groups can be produced (Friedrich et al. 2012; Thakur and Vasudevan 2021) (Fig. 4e).

The interaction of plasma with polymer materials can be divided into three stages:

Surface cleaning: In this stage, organic impurities are mainly removed.

Chemical modification: In this stage, C-C and C=C bonds are broken, and fragments of dissociated plasma gas are incorporated as functional groups through H-substitution in the polymer chain.

Surface etching and nanotopography creation: For longer exposure times, plasma etches the surface and creates nanotopography.

However, a significant challenge lies in introducing plasma energy into polyurethanes. While this energy is necessary to sustain the plasma state, it also poses a risk of degrading the polymer material. Excessive energy delivery can break C-H and C-C chemical bonds, which are crucial for the structural stability of polymers (Friedrich et al. 2012; Thakur and Vasudevan 2021).

In biomaterials engineering, there are various surface treatment methods in which plasma plays a key role. Low-temperature plasma is one of the most effective techniques for surface modification of biomaterials, such as polyurethanes. Due to its unique properties and wide range of applications, this method allows selective surface modification in inert (e.g., argon) or reactive (e.g., oxygen) atmospheres, providing precise control over the chemical changes introduced. For polyurethanes, the modification must be tailored to the specific material, considering chemical composition, crystallinity, and thickness factors. It is essential to optimize parameters like the type of feed gas (oxidizing, reducing, inert), gas partial pressure, plasma generator power, and exposure time. Thanks to its flexibility, low-temperature plasma is particularly effective in enhancing the surface properties of polyurethanes without altering their bulk characteristics (Alves et al. 2011; Cvrček et al. 2019; Drożdż et al. 2024).