Stretching-induced nanostructures on shape memory polyurethane films and their regulation to osteoblasts morphology

A shape memory polynorbornene-based dynamic cell culture platform capable of providing pure mechanical strains

Shape memory polymers (SMPs) can actively generate recovery strain during shape recovery. This characteristic endows SMPs with the potential for dynamic cell culture platforms capable of providing cells with strain stimulation. However, a critical challenge encountered by current SMP-based platforms is that, besides providing recovery strain, shape recovery can alter surface topography and chemistry. Given that both surface topography and chemistry influence cell behavior, decoupling the recovery strain from the alterations in surface topography and chemistry becomes greatly important. In this work, polynorbornene (PNB), an amorphous homopolymer with a shape memory temperature close to the body temperature, was fabricated into films. We found that the shape recovery of the PNB films does not alter the surface chemistry and topography but produces pure recovery strain. The recovery strain is featured with a compressive strain (up to ∼50 %) along and a tensile strain (up to ∼40 %) perpendicular to the stretching direction, which combine to facilitate the orientation of rat mesenchymal stem cells via both passive and active modes related to the F-actin cytoskeletons. These findings confirm that PNB-based films can act as an effective dynamic cell culture platform, which adds a new alternative for smart mechanical loading in biomedical fields.

Recent Advancements in Polyurethane-based Tissue Engineering

In tissue engineering, polyurethane-based implants have gained significant traction because of their high compatibility and inertness. The implants therefore show fewer side effects and lasts longer. Also, the mechanical properties can be tuned and morphed into a particular shape, owing to which polyurethanes show immense versatility. In the last 3 years, scientists have devised methods to enhance the strength of and induce dynamic properties in polyurethanes, and these developments offer an immense opportunity to use them in tissue engineering. The focus of this review is on applications of polyurethane implants for biomedical application with detailed analysis of hard tissue implants like bone tissues and soft tissues like cartilage, muscles, skeletal tissues, and blood vessels. The synthetic routes for the preparation of scaffolds have been discussed to gain a better understanding of the issues that arise regarding toxicity. The focus here is also on concerns regarding the biocompatibility of the implants, given that the precursors and byproducts are poisonous.

Shape-memory responses compared between random and aligned electrospun fibrous mats

Significant progress has been made in the design of smart fibers toward achieving improved efficacy in tissue regeneration. While electrospun fibers can be engineered with shape memory capability, both the fiber structure and applied shape-programming parameters are the determinants of final performance in applications. Herein, we report a comparison study on the shape memory responses compared between electrospun random and aligned fibers by varying the programming temperature T _prog and the deforming strain ε _deform . A PLLA-PHBV (6:4 mass ratio) polymer blend was first electrospun into random and aligned fibrous mat forms; thereafter, the effects of applying specific T _prog (37°C and 46°C) and ε _deform (30%, 50%, and 100%) on the morphological change, shape recovery efficiency, and switching temperature T _sw of the two types of fibrous structures were examined under stress-free condition, while the maximum recovery stress σ _max was determined under constrained recovery condition. It was identified that the applied T _prog had less impact on fiber morphology, but increasing ε _deform gave rise to attenuation in fiber diameters and bettering in fiber orientation, especially for random fibers. The efficiency of shape recovery was found to correlate with both the applied T _prog and ε _deform , with the aligned fibers exhibiting relatively higher recovery ability than the random counterpart. Moreover, T _sw was found to be close to T _prog , thereby revealing a temperature memory effect in the PLLA-PHBV fibers, with the aligned fibers showing more proximity, while the σ _max generated was ε _deform -dependent and 2.1-3.4 folds stronger for the aligned one in comparison with the random counterpart. Overall, the aligned fibers generally demonstrated better shape memory properties, which can be attributed to the macroscopic structural orderliness and increased molecular orientation and crystallinity imparted during the shape-programming process. Finally, the feasibility of using the shape memory effect to enable a mechanoactive fibrous substrate for regulating osteogenic differentiation of stem cells was demonstrated with the use of aligned fibers.

Biobased polyurethanes for biomedical applications

Polyurethanes (PUs) are a major family of polymers displaying a wide spectrum of physico-chemical, mechanical and structural properties for a large range of fields. They have shown suitable for biomedical applications and are used in this domain since decades. The current variety of biomass available has extended the diversity of starting materials for the elaboration of new biobased macromolecular architectures, allowing the development of biobased PUs with advanced properties such as controlled biotic and abiotic degradation. In this frame, new tunable biomedical devices have been successfully designed. PU structures with precise tissue biomimicking can be obtained and are adequate for adhesion, proliferation and differentiation of many cell's types. Moreover, new smart shape-memory PUs with adjustable shape-recovery properties have demonstrated promising results for biomedical applications such as wound healing. The fossil-based starting materials substitution for biomedical implants is slowly improving, nonetheless better renewable contents need to be achieved for most PUs to obtain biobased certifications. After a presentation of some PU generalities and an understanding of a biomaterial structure-biocompatibility relationship, recent developments of biobased PUs for non-implantable devices as well as short- and long-term implants are described in detail in this review and compared to more conventional PU structures.

Polyurethane/chitosan/hyaluronic acid scaffolds: providing an optimum environment for fibroblast growth

OBJECTIVE

Biodegradable polymers can replace damaged tissue components using tissue engineering techniques. The objective of this study is to determine an optimum environment for polymer scaffolds to improve the proliferation of fibroblast cells capable of wound repair.

METHOD

In this study, the addition of polysaccharides, such as chitosan (CH) or hyaluronic acid (HA), to a polyurethane (PU) polymer was evaluated using different methods to determine if they affect scaffold morphology and cell activity of fibroblasts prepared from human foreskin tissues. Mechanical properties, such as tensile strength, contact angle and swelling test, were used to check the physical and mechanical properties of the scaffold. Fibroblast growth was also measured at 24, 48 and 72 hours.

RESULTS

Scanning electron microscopy (SEM) determined that a 3:1 ratio of PU/CH scaffold, developed by electrospinning, allowed the formation of a uniform structure in scaffold fibres. Physical mechanical tests showed that PU electrospun scaffolds were not modified by the addition of CH. The mean stretch and mean water absorption increased significantly using the PU/CH scaffold, compared with the PU scaffold. However, the mean tensile strength and the mean contact angle, used to study space and porosity, did not differ between scaffolds. Fourier transform infrared spectroscopy confirmed the functional groups (-OH, -NH and -C=O) in the PU/CH scaffold, compared with PU or CH chemical structures alone. HA was then added to CH and PU/CH scaffolds to evaluate the growth of fibroblast cells. Results showed that cell viability and the number of cells, using MTT and trypan blue exclusion assay, respectively, increased significantly at 24, 48 and 72 hours of culture in PU/CH/HA scaffold compared to HA, CH/HA, and PU/HA. Moreover, PU/HA at 48 and 72 hours also increased cell viability and cell numbers compared to HA and CH/HA scaffolds. However, scaffolds at 72 hours had limited space for cell growth. Moreover, SEM data demonstrated that fibroblasts were able to proliferate, penetrate, migrate and survive on PU/HA and PU/CH/HA three-dimensional scaffolds, especially during the first 48 hours. Furthermore, 4',6-diamidino-2-phenylindole (DAPI) staining confirmed that fibroblasts could penetrate PU scaffolds and showed higher cell viability and lower cellular damage in PU/CH/HA, compared to CH/HA and PU/HA scaffolds. Finally, flow cytometry using CD90 and CD105 surface markers revealed that >90% of cells isolated from the human dermis were fibroblasts.

CONCLUSION

In summary, PU/HA and PU/CH/HA scaffolds were found to be biocompatible and provided a suitable environment for the growth and proliferation of fibroblasts, which filled and covered all pores between the fibres. The new scaffold used in this study, made of synthetic and natural polymers, is a good candidate for applications in tissue engineering. It is therefore recommended to use PU in grafts or in wound dressing.

The Difference in Molecular Orientation and Interphase Structure of SiO2/Shape Memory Polyurethane in Original, Programmed and Recovered States during Shape Memory Process

In order to further understand the shape memory mechanism of a silicon dioxide/shape memory polyurethane (SiO₂/SMPU) composite, the thermodynamic properties and shape memory behaviors of prepared SiO₂/SMPU were characterized. Dynamic changes in the molecular orientation and interphase structures of SiO₂/SMPU during a shape memory cycle were then discussed according to the small angle X-ray scattering theory, Guinier’s law, Porod approximation, and fractal dimension theorem. In this paper, a dynamic mechanical analyzer (DMA) helped to determine the glass transition start temperature (T_g) by taking the onset point of the sigmoidal change in the storage modulus, while transition temperature (T_trans) was defined by the peak of tan δ, then the test and the calculated results indicated that the T_g of SiO₂/SMPU was 50.4 °C, and the T_trans of SiO₂/SMPU was 72.18 °C. SiO₂/SMPU showed good shape memory performance. The programmed SiO₂/SMPU showed quite obvious microphase separation and molecular orientation. Large-size sheets and long-period structures were formed in the programmed SiO₂/SMPU, which increases the electron density difference. Furthermore, some hard segments had been rearranged, and their gyration radii decreased. In addition, several defects formed at the interfaces of SiO₂/SMPU, which caused the generation of space charges, thus leading to local electron density fluctuations. The blurred interphase structure and the intermediate layer formed in the programmed SiO₂/SMPU and there was evident crystal damage and chemical bond breakage in the recovered SiO₂/SMPU. Finally, the original and recovered SiO₂/SMPU samples belong to the surface fractal system, but the programmed sample belongs to the mass fractal and reforms two-phase structures. This study provides an insight into the shape memory mechanism of the SiO₂/SMPU composite.

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.

Shape recovery strain and nanostructures on recovered polyurethane films and their regulation to osteoblasts morphology

Shape memory polyurethanes (SMPUs) have emerged as novel dynamic substrates to regulate cell alignment, in which recovery-induced change in substrates topography has been described as the major contributor. This work, for the first time, confirmed the pivotal roles of recovery strain and phase-separated nanostructures of SMPUs in regulating cell morphology. SMPU films with different stretching ratios (0%, 50%, 100%, and 200%) were found to produce an average recovery strain from 19.41% to 34.04% within 2 h in dulbecco's modified eagle medium (DMEM). Meanwhile, the assembly of hard domains was enhanced during shape recovery, leading to the reorientation of fibrillar apophyses (i.e., nanostructures). Further observation of osteoblast morphology revealed that recovery strain resulted in perpendicular orientation of osteoblasts to strain direction. With the extension of incubation time (24 h), however, the perpendicular orientation was transformed to follow the nanostructures on recovered films, suggesting that the nanostructures might become the determinant of the long-term cell orientation. This study provides a biomechanics-based perspective to understand the dynamic interactions between SMPU and cells, which can help to guide the design of SMPU for specific biomedical applications.

Preparation and characterization of titanium-segmented polyurethane composites for bone tissue engineering

Segmented polyurethanes were prepared with polycaprolactone diol as soft segment and 4,4-methylene-bis cyclohexyl diisocyanate and l-glutamine as the rigid segment. These polyurethanes were filled with 1 wt.% to 5 wt.% titanium particles (Ti), physicochemically characterized and their biocompatibility assessed using human dental pulp stem cells and mice osteoblasts. Physicochemical characterization showed that composites retained the properties of the semicrystalline polyurethane as they exhibited a glass transition temperature (T_g) between -35°C and -45°C, melting temperature (T_m) at 52°C and crystallinity close to 40% as determined by differential scanning calorimetry. In agreement with this, X-ray diffraction showed reflections at 21.3° and 23.6° for polycaprolactone diol and reflections at 35.1°, 38.4°, and 40.2° for Ti particles suggesting that these particles are not acting as nucleating sites. The addition of up to 5 wt.% of Ti reduced both, tensile strength and maximum strain from 1.9 MPa to 1.2 MPa, and from 670% to 172% for pristine and filled polyurethane, respectively. Although there were differences between composites at low strain rates, no significant differences in mechanical behavior were observed at higher strain rate where a tensile stress of 8.5 MPa and strain of 223% were observed for 5 wt.% composites. The addition to titanium particles had a beneficial effect on both human dental pulp stem cells and osteoblasts viability, as it increased with the amount of titanium in composites up to 10 days of incubation.

The interfacial pH of acidic degradable polymeric biomaterials and its effects on osteoblast behavior

A weak alkaline environment is established to facilitate the growth of osteoblasts. Unfortunately, this is inconsistent with the application of biodegradable polymer in bone regeneration, as the degradation products are usually acidic. In this study, the variation of the interfacial pH of poly (D, L-lactide) and piperazine-based polyurethane ureas (P-PUUs), as the representations of acidic degradable materials, and the behavior of osteoblasts on these substrates with tunable interfacial pH were investigated in vitro. These results revealed that the release of degraded products caused a rapid decrease in the interfacial pH, and this could be relieved by the introduction of alkaline segments. On the contrary, when culturing with osteoblasts, the variation of the interfacial pH revealed an upward tendency, indicating that cell could construct the microenvironment by secreting cellular metabolites to satisfy its own survival. In addition, the behavior of osteoblasts on substrates exhibited that P-PUUs with the most PP units were better for cell growth and osteogenic differentiation of cells. This is due to the hydrophilic surface and the moderate N% in P-PUUs, key factors in the promotion of the early stages of cellular responses, and the interfacial pH contributing to the enhanced effect on osteogenic differentiation.

Incorporating isosorbide as the chain extender improves mechanical properties of linear biodegradable polyurethanes as potential bone regeneration materials

Novel linear biodegradable polyurethanes based on poly (d,l-lactic acid) as soft segments and isosorbide as chain extender were exhibited with high molecular weight and appropriate mechanical performances, promising as the scaffold materials for bone regeneration.