Functional Zwitterionic Polyurethanes: State-of-the-Art Review

Recent advancements in bioengineering and medical devices have been greatly influenced and dominated by synthetic polymers, particularly polyurethanes (PUs). PUs offer customizable mechanical properties and long-term stability, but their inherent hydrophobic nature poses challenges in practically biological application processes, such as interface high friction, strong protein adsorption, and thrombosis. To address these issues, surface modifications of PUs for generating functionally hydrophilic layers have received widespread attention, but the durability of generated surface functionality is poor due to irreversible mechanical wear or biodegradation. As a result, numerous researchers have investigated bulk modification techniques to incorporate zwitterionic polymers or groups onto the main or side chains of PUs, thereby improving their hydrophilicity and biocompatibility. This comprehensive review presents an extensive overview of notable zwitterionic PUs (ZPUs), including those based on phosphorylcholine, sulfobetaine, and carboxybetaine. The review explores their wide range of biomedical applications, from blood-contacting devices to antibacterial coatings, fouling-resistant marine coatings, separation membranes, lubricated surfaces, and shape memory and self-healing materials. Lastly, the review summarizes the challenges and future prospects of ZPUs in biological applications.

Application of Shape Memory and Self-Healable Polymers/Composites in the Biomedical Field: A Review

Shape memory-assisted self-healing polymers have drawn attention over the past few years owing to their interdisciplinary and wide range of applications. Self-healing and shape memory are two approaches used to improve the applicability of polymers in the biomedical field. Combining both these approaches in a polymer composite opens new possibilities for its use in biomedical applications, such as the "close then heal" concept, which uses the shape memory capabilities of polymers to bring injured sections together to promote autonomous healing. This review focuses on using shape memory-assisted self-healing approaches along with their respective affecting factors for biomedical applications such as tissue engineering, drug delivery, biomaterial-inks, and 4D printed scaffolds, soft actuators, wearable electronics, etc. In addition, quantification of self-healing and shape memory efficiency is also discussed. The challenges and prospects of these polymers for biomedical applications have been summarized.

Shape memory polyurethanes crosslinked with castor oil-based multifunctional polyols

As both the industry and academia become more focused on biomass-based smart materials, they are attracting a lot of attention. There has been a significant effort in the field of polyurethane (PU) synthesis to replace polyols used in synthesis with bio-derived polyols. Bio-derived polyols have limited application potential for bio-based PU due to their low functionality. Here, we reported castor oil (CO) based multifunctional polyols prepared by grafting thiols such as 1-mercaptoethanol or α-thioglycerol via a facile thiol-ene click reaction method (coded as COM and COT, respectively). Subsequently, bio-based shape memory polyurethanes (SMPU) crosslinked with prepared polyols were synthesized using a 2-step prepolymer method. By confirming the functionality of the synthesized polyols, it was determined that COT has an OH value of 380 mg KOH/g, which is approximately three times that of CO. The successful synthesis of SMPUs was confirmed through chemical structural analysis. It was also proved that the phase separation between the soft and hard segments was limited due to the increase in crosslinking density. As compared to SMPU crosslinked with CO, the mechanical strength of SMPU crosslinked with COT was improved by 80%, while the elongation was decreased by about 26%. As a result of shape memory behavior analysis, it was confirmed that the outstanding SMPU can be synthesized using CO-based multifunctional polyols.

Polyurethanes Modified by Ionic Liquids and Their Applications

Polyurethane (PU) refers to the polymer containing carbamate groups in its molecular structure, generally obtained by the reaction of isocyanate and alcohol. Because of its flexible formulation, diverse product forms, and excellent performance, it has been widely used in mechanical engineering, electronic equipment, biomedical applications, etc. Through physical or chemical methods, ionic groups are introduced into PU, which gives PU electrical conductivity, flame-retardant, and antistatic properties, thus expanding the application fields of PU, especially in flexible devices such as sensors, actuators, and functional membranes for batteries and gas absorption. In this review, we firstly introduced the characteristics of PU in chemical and microphase structures and their related physical and chemical performance. To improve the performance of PU, ionic liquids (ILs) were applied in the processing or synthesis of PU, resulting in a new type of PU called ionic PU. In the following part of this review, we mainly summarized the fabrication methods of IL-modified PUs via physical blending and the chemical copolymerization method. Then, we summarized the research progress of the applications for IL-modified PUs in different fields, including sensors, actuators, transistors, antistatic films, etc. Finally, we discussed the future development trends and challenges faced by IL-modified PUs.

Mechanically Strong, Wet Adhesive, and Self-Healing Polyurethane Ionogel Enhanced with a Semi-interpenetrating Network for Underwater Motion Detection

The gel-based sensors have developed rapidly in recent years toward multifunctionality. However, there are still some challenges that need to be solved, such as poor mechanical properties and inaccessibility to wet or water environments. To address these issues, we have developed an ionogel with a semi-interpenetrating network structure by adopting poly(vinylidene fluoride-co-hexafluoropropylene) as the linear non-cross-linked network, a double-bonded ionic liquid and double-bonded capped polyurethane as the cross-linked network, and an ionic liquid as the conductive media. The obtained ionogel exhibits tunable mechanical properties (3.67-8.76 MPa) and excellent sensing properties (IG-20, GF = 8.2). The superb environmental stability and self-healing properties of the ionogel were also demonstrated. Meanwhile, adhesion, self-healing, and sensing performance were guaranteed for underwater due to the presence of a large number of C-F bonds. We strongly believe that this ionogel with excellent mechanical properties and underwater communication is expected for monitoring the health of the human body and information transmission in the future.

Comprehensive Investigation of Stoichiometry–Structure–Performance Relationships in Flexible Polyurethane Foams

Polyurethane (PU) foams are versatile materials with a broad application range. Their performance is driven by the stoichiometry of polymerization reaction, which has been investigated in several works. However, the analysis was often limited only to selected properties and compared samples differing in apparent density, significantly influencing their performance. In the bigger picture, there is still a lack of comprehensive studies dealing with the stoichiometry impact on PU foams’ performance. Herein, flexible PU foams with a similar apparent density but differing in the isocyanate index (IIso) (from 0.80 to 1.20) were prepared. The stoichiometry–structure–performance relationships were investigated considering cellular and chemical structure, as well as the static and dynamic mechanical properties, thermal stability, thermal insulation, and acoustic performance. For IIso of 1.00, the biggest cell diameters of 274 µm were noted, which was 21–25% higher compared to 0.80 and 1.20 values. Increasing IIso reduced open cell content from 83.1 to 22.4%, which, combined with stiffening of structure (rise of modulus from 63 to 2787 kPa) resulting from crosslinking, limited the sound suppression ability around five times. On the other hand, it significantly strengthened the material, increasing tensile and compressive strength 4 and 13 times, respectively. Changes in the foams’ performance were also induced by the glass transition temperature shift from 6.1 to 31.7 °C, resulting from a greater extent of urethane groups’ generation and additional isocyanate reactions. Generally, the presented work provides important insights into preparing flexible PU foams and could be very useful for the future development of these materials.

Fibrous Scaffolds for Muscle Tissue Engineering Based on Touch-Spun Poly(Ester-Urethane) Elastomer

Development of fiber-spinning technologies and materials with proper mechanical properties is highly important for the manufacturing of aligned fibrous scaffolds mimicking structure of the muscle tissues. Here, the authors report touch spinning of a thermoplastic poly(1,4-butylene adipate)-based polyurethane elastomer, obtained via solvent-free polymerization. This polymer possesses a combination of important advantages such as 1) low elastic modulus in the range of a few MPa, 2) good recovery ratio and 3) resilience, 4) processability, 5) nontoxicity, 6) biocompatibility, and 7) biodegradability that makes it suitable for fabrication of structures mimicking extracellular matrix of muscle tissue. Touch spinning allows fast and precise deposition of highly aligned micro- and nano-fibers without use of high voltage. C2C12 myoblasts readily align along soft polymer fibers and demonstrate high viability as well as proliferation that make proposed combination of polymer and fabrication method highly suitable for engineering skeletal muscles.

Rationally Designed Eugenol-Based Chain Extender for Self-Healing Polyurethane Elastomers

Bio-based polyurethane (PU) has recently drawn our attention due to the increasing interest in sustainability and the risks involved with petroleum depletion. Herein, bio-based self-healing PU with a novel polyol, i.e., eugenol glycol dimer (EGD), was synthesized and characterized for the first time. EGD was designed to have pairs of primary, secondary, and aromatic alcohols, which all are able to be involved in urethane bond formation and to show self-healing and antioxidant effects. EGD was incorporated into a mixture of the prepolymer of polyol (tetramethylene ether glycol) and 4,4'-methylene diphenyl diisocyanate to synthesize PU. EGD-PU showed excellent self-healing properties (99.84%), and it maintained its high self-healing property (84.71%) even after three repeated tests. This dramatic self-healing was induced through transcarbamoylation by the pendant hydroxyl groups of EGD-PU. The excellent antioxidant effect of EGD-PU was confirmed by 2,2-diphenyl-1-picrylhydrazyl analysis. Eugenol-based EGD is a promising polyol chain extender that is required in the production of bio-based, self-healing, and recyclable polyurethane; therefore, EGD-PU can be applied to bio-based self-healable films or coating materials as a substitute for petroleum-based PU.

Biocompatibility studies of polyurethane electrospun membranes based on arginine as chain extender

Electrospun polymers are an example of multi-functional biomaterials that improve the material-cellular interaction and aimed at enhancing wound healing. The main objective of this work is to fabricate electrospun polyurethane membranes using arginine as chain extender (PUUR) in order to test the fibroblasts affinity and adhesion on the material and the polymer toxicity. Polyurethane membranes were prepared in two steps: (i) the polyurethane synthesis, and ii) the electrospinning process. The membranes were characterized by scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy, gel permeation chromatography, and differential scanning calorimetry techniques. The evaluation of PUUR as a scaffolding biomaterial for growing and developing of cells on the material was realized by LIVE/DEAD staining. The results show that the fluorescent surface area of human fibroblasts (hFB), was greater in control dense membranes made from Tecoflex than in electrospun and dense PUUR. From SEM analysis, the electrospun membranes show relatively uniform attachment of cells with a well-spread shape, while Tecoflex dense membranes show a non-proliferating round shape, which is attributed to the fiber's structure in electrospun membranes. The cell morphology and the cell attachment assay results reveal the well spreading of hFB cells on the surface of electrospun PUUR membranes which indicates a good response related to cell adhesion.

Soft Elastic Fibrous Scaffolds for Muscle Tissue Engineering by Touch Spinning

This paper reports an approach for the fabrication of highly aligned soft elastic fibrous scaffolds using touch spinning of thermoplastic polycaprolactone-polyurethane elastomers and demonstrates their potential for the engineering of muscle tissue. A family of polyester-polyurethane soft copolymers based on polycaprolactone with different molecular weights and three different chain extenders such as 1,4-butanediol and polyethylene glycols with different molecular weight was synthesized. By varying the molar ratio and molecular weights between the segments of the copolymer, different physicochemical and mechanical properties were obtained. The polymers possess elastic modulus in the range of a few megapascals and good reversibility of deformation after stretching. The combination of the selected materials and fabrication methods allows several essential advantages such as biocompatibility, biodegradability, suitable mechanical properties (elasticity and softness of the fibers), high recovery ratio, and high resilience mimicking properties of the extracellular matrix of muscle tissue. Myoblasts demonstrate high viability in contact with aligned fibrous scaffolds, where they align along the fibers, allowing efficient cell patterning on top of the structures. Altogether, the importance of this approach is the fabrication of highly oriented fiber constructs that can support the proliferation and alignment of muscle cells for muscle tissue engineering applications.

Self-healing elastomers based on conjugated diolefins: a review

The introduction of dynamic covalent and physical crosslinks into diolefin-based elastomers improves mechanical and self-healing properties. The presence of dynamic crosslinks also helps in the reprocessing of elastomers.

Preparation and Properties of SBR Composites Containing Graphene Nanoplatelets Modified with Pyridinium Derivative

The goal of this work was to study the effect of graphene nanoplatelets (GnPs) modified with ionic liquid (IL) on properties of styrene–butadiene rubber (SBR) composites. GnPs were decorated with IL or were modified in bulk directly during rubber mix preparation. The ionic liquid used was 1-butyl-4-methylpyridinium tetrafluoroborate (BMPFB). The textural properties were studied to confirm surface modification of GnPs with BMPFB. In these investigations, the impact of the concentration of GnPs and the effect of the method of GnPs’ modification with IL on elastomers properties are described. Some thermal measurements (e.g., differential scanning calorimetry and thermogravimetry) were conducted to characterize the thermal stability or the vulcanization process of the investigated samples. Complementary experimental techniques were used to investigate the properties of the obtained elastomers, specifically tensile testing, and electrical and barrier property measurements. The deposition of IL on the GnPs’ surface positively influenced the mechanical and barrier properties of elastomers. However, samples containing graphene nanoplatelets modified from solution were characterized by less electrical conductivity. SEM analysis was additionally performed to investigate GnPs’ dispersion within SBR composites.

Fabrication of silane-grafted graphene oxide and its effect on the structural, thermal, mechanical, and hysteretic behavior of polyurethane

Incorporation of nanofillers into polyurethane (PU) is a promising technique for enhancing its thermal and mechanical properties. Silane grafting has been used as a surface treatment for the functionalization of graphene oxide (GO) with numerous reactive sites dispersed on its basal plane and edge. In this study, amine-grafted GO was prepared using silanization of GO with (3-aminopropyl)triethoxysilane. The functionalized graphene oxide (fGO) was characterized by Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy. Next, it was introduced in PU fabricated using polycaprolactone diol, castor oil, and hexamethylene diisocyanate. The fGO-PU nanocomposites were in turn characterized by FT-IR, X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis, and a universal testing machine. The results obtained from these analyses showed changes in structural thermal properties, as well as improved thermal stability and mechanical properties because of the strong interfacial adhesion between the fGO and the PU matrix.

Fabrication of bio-based polyurethane nanofibers incorporated with a triclosan/cyclodextrin complex for antibacterial applications

A hybrid polyol consisting of a polycaprolactone diol/castor oil mixture was used to synthesize a biopolyurethane (BPU) that has a dendritic point but is soluble in organic solvents. The chemical structure of the obtained BPU was determined using Fourier transform infrared (FT-IR) spectroscopy and proton nuclear magnetic resonance spectroscopy. The mechanical properties of the electrospun BPU nanofiber were confirmed using a universal testing machine. To enhance the solubility of triclosan (TR), TR–cyclodextrin (CD) complexes were prepared. αCD, βCD, and γCD were used to study the formation of the TR–CD complexes using a coprecipitation technique. The results showed that TR did not form a complex with αCD, whereas it forms complexes partially with βCD and completely with γCD. These findings are supported by FT-IR, differential scanning calorimetry, and X-ray diffraction analyses. The electrospun BPU/TR–CD nanofibers were investigated in terms of morphology, releasing behavior, and antibacterial tests. The BPU/TR–γCD nanofiber shows better antibacterial activity than the others. The results obtained in this study are expected to broaden the range of biobased polyurethane applications where antibacterial properties are required.

Bio-polyurethane nanofibers containing triclosan–cyclodextrin complexes to enhance antibacterial properties were prepared using an electrospinning method.

A comprehensive review of the structures and properties of ionic polymeric materials

This review focuses on the mechanistic approach, the structure–property relationship and applications of ionic polymeric materials.

Synthesis and characterization of biodegradable linear shape memory polyurethanes with high mechanical performance by incorporating novel long chain diisocyanates

Novel long chain diisocyanates were developed for synthesis of biodegradable linear shape memory polyurethanes demonstrating high mechanical performance.

Imidazolium-based ionic polyurethanes with high toughness, tunable healing efficiency and antibacterial activities

Ionic polyurethanes (PUs) with high toughness, fast self-healing ability, antibacterial activity and shape memory behaviors are synthesized.