Porous organic polymers (POPs) have emerged as a new class of multifunctional porous materials and received tremendous research attention from both academia and industry. Most POPs are constructed from versatile organic small molecules with diverse linkages through strong covalent bonds. Owing to their high surface area and porosity, low density, high stability, tunable pores and skeletons, and ease of functionalization, POPs have been extensively studied for gas storage and separation, heterogeneous catalysis, biomedicine, sensing, optoelectronics, energy storage and conversion, etc. Particularly, POPs are excellent platforms with exciting opportunities for biomedical applications. Consequently, considerable efforts have been devoted to preparing POPs with an emphasis on their biomedical applications. In this review, first, we briefly describe the different subclasses of POPs and their synthetic strategies and functionalization approaches. Then, we highlight the state-of-the-art progress in POPs for a variety of biomedical applications such as drug delivery, biomacromolecule immobilization, photodynamic and photothermal therapy, biosensing, bioimaging, antibacterial, bioseparation, etc. Finally, we provide our thoughts on the fundamental challenges and future directions of this emerging field. 

Emerging porous organic polymers for biomedical applications

Carotid artery dissections can be triggered by several factors. The underlying biomechanical phenomena and properties are unclear. This study investigates the dissection properties of 62 human carotid bifurcations using two experimental methods: direct tension and peeling tests. Direct tension tests study the mechanical strength of the tissue components in radial direction, while peeling tests quantify the fracture energy required to propagate a dissection in a tissue. Results show that the interface between the healthy adventitia and media has the highest radial failure stress (132 ± 20 kPa, mean ± SD, n = 25), whereas the lowest value occurs between the diseased intima and media (104 ± 24 kPa, n = 18). The radial tissue strength at the bifurcation is the highest compared with locations that are away from the central region of the bifurcation. Force/width values required to separate the individual layers and to dissect the media in the circumferential direction are always lower than related values in the axial direction, suggesting anisotropic dissection properties. Dissection energies per reference area generated during the peeling tests are also lower for strips in the circumferential direction than for axial strips, and they vary significantly with the location, as shown for the media. Histological investigations demonstrate that interfacial ruptures mainly occur in the media in both types of tests and are 2-5 elastic lamellae away from the external and internal elastic laminae. A remarkably “rougher” dissection surface is generated during axial peeling tests when compared with tests performed in the circumferential direction.

Dissection properties and mechanical strength of tissue components in human carotid bifurcations

Large amplitude oscillatory shear (LAOS) for nonlinear rheological behavior of heterogeneous emulsion gels made from natural supramolecular gelators

A robust regenerated cellulose-based dual stimuli-responsive hydrogel as an intelligent switch for controlled drug delivery.

Hydrogels as drug carriers for delivery of antibiotics are rarely derived from natural molecules and often require complicated synthesis. Puerarin is an herbal natural product that can self-assemble to form pH-sensitive hydrogels without any structural modification. However, Puerarin hydrogels have poor thermostability and weak mechanical strength. In this work, we prepared N-(9-fluorenylmethoxycarbonyl)-l-phenylalanine (Fmoc–Phe–OH) nanofiber-reinforced Puerarin hydrogels with good mechanical and antibacterial properties for suitable drug carriers. The phase transition temperature of Puerarin hydrogels can be increased from 35 to 55 °C by adding only 6 mg/mL of Fmoc–Phe–OH. Rheological study indicated that the presence of Fmoc–Phe–OH not only increased the storage modulus of Puerarin hydrogels by five times, but also did not affect their pH sensitivity. SEM study and pore analysis indicated that Fmoc–Phe–OH nanofibers intertwined with the Puerarin assembly, leading to enhancement of mechanical property of hydrogel. Antibacterial test indicated that the reinforced Puerarin hydrogels have antibacterial property compared with Puerarin hydrogels. Based on these features, a controllable release of loaded model drug (berberine hydrochloride) can be realized from the reinforced Puerarin hydrogels, showing a good synergistic antimicrobial effect on the S. aureus.

Supramolecular nanofiber-reinforced Puerarin hydrogels as drug carriers with synergistic controlled release and antibacterial properties

Viscoelastic characterization of pH-sensitive 2-hydroxyethyl methacrylate (2-dimethylamino) ethyl methacrylate HEMA-DMAEMA hydrogels

Structure–Processing–Property Relationships of 3D Printed Porous Polymeric Materials

Aortic dissection occurs predominantly in the thoracic aorta and the mechanisms for the initiation and propagation of the tear in aortic dissection are not well understood. We study the tearing characteristics of the porcine thoracic aorta using a peeling test and we estimate the peeling energy per unit area in the ascending and the descending segments. The stretch and the peel force per unit width undergone by the peeled halves of a rectangular specimen are measured. We find that there can be significant variation in the stretch within the specimen and the stretch between the markers in the specimen varies with the dynamics of peeling. We found that in our experiment the stretch achieved in the peeled halves was such that it was in the range of the stretch at which the stress-stretch curve for the uniaxial experiment starts deviating from linearity. Higher peeling energy per unit area is required in the ascending aorta compared to the descending aorta. Longitudinal specimens required higher peeling energy per unit area when compared to the circumferential specimens.

Segmental Variations in the Peel Characteristics of the Porcine Thoracic Aorta

Energy dissipation in pH-sensitive hydrogels subjected to large amplitude oscillatory shear

Three-dimensional printing (3DP) of functional materials is increasingly important for advanced applications requiring objects with complex or custom geometries or prints with gradients or zones with different properties. A common 3DP technique is direct ink writing (DIW), in which printable inks are comprised of a fluid matrix filled with solid particles, the latter of which can serve a dual purpose of rheology modifiers to enable extrusion and functional fillers for performance-related properties. Although the relationship between filler loading and viscosity has been described for many polymeric systems, a thorough description of the rheological properties of three-dimensional (3D) printable composites is needed to expedite the creation of new materials. In this manuscript, the relationship between filler loading and printability is studied using model paraffin/photopolymer composite inks containing between 0 and 73 vol. % paraffin microbeads. The liquid photopolymer resin is a Newtonian fluid, and incorporating paraffin microbeads increases the ink viscosity and imparts shear-thinning behavior, viscoelasticity, and thixotropy, as established by parallel plate rheometry experiments. Using Einstein and Batchelor's work on colloidal suspension rheology, models were developed to describe the thixotropic behavior of inks, having good agreement with experimental results. Each of these properties contributes to the printability of highly filled ([Formula: see text]43 vol. % paraffin) paraffin/photopolymer composite inks. Through this work, the ability to quantify the ideal rheological properties of a DIW ink and to selectively control and predict its rheological performance will facilitate the development of 3D printed materials with tunable functionalities, thus, advancing 3DP technology beyond current capabilities.

Chandler C. Benjamin

Papers

Viscoelastic characterization of pH-sensitive 2-hydroxyethyl methacrylate (2-dimethylamino) ethyl methacrylate HEMA-DMAEMA hydrogels

Structure–Processing–Property Relationships of 3D Printed Porous Polymeric Materials

Segmental Variations in the Peel Characteristics of the Porcine Thoracic Aorta

Energy dissipation in pH-sensitive hydrogels subjected to large amplitude oscillatory shear

Viscoelastic and thixotropic characterization of paraffin/photopolymer composites for extrusion-based printing