Matthew B. Baker

Consequences of chirality on the dynamics of a water-soluble supramolecular polymer

The rational design of supramolecular polymers in water is imperative for their widespread use, but the design principles for these systems are not well understood. Herein, we employ a multi-scale (spatial and temporal) approach to differentiate two analogous water-soluble supramolecular polymers: one with and one without a stereogenic methyl. Initially aiming simply to understand the molecular behaviour of these systems in water, we find that while the fibres may look identical, the introduction of homochirality imparts a higher level of internal order to the supramolecular polymer. Although this increased order does not seem to affect the basic dimensions of the supramolecular fibres, the equilibrium dynamics of the polymers differ by almost an order of magnitude. This report represents the first observation of a structure/property relationship with regard to equilibrium dynamics in water-soluble supramolecular polymers.

Effect of H-Bonding on Order Amplification in the Growth of a Supramolecular Polymer in Water

While a great deal of knowledge on the roles of hydrogen bonding and hydrophobicity in proteins has resulted in the creation of rationally designed and functional peptidic structures, the roles of these forces on purely synthetic supramolecular architectures in water have proven difficult to ascertain. Focusing on a 1,3,5-benzenetricarboxamide (BTA)-based supramolecular polymer, we have designed a molecular modeling strategy to dissect the energetic contributions involved in the self-assembly (electrostatic, hydrophobic, etc.) upon growth of both ordered BTA stacks and random BTA aggregates. Utilizing this set of simulations, we have unraveled the cooperative mechanism for polymer growth, where a critical size must be reached in the aggregates before emergence and amplification of order into the experimentally observed fibers. Furthermore, we have found that the formation of ordered fibers is favored over disordered aggregates solely on the basis of electrostatic interactions. Detailed analysis of the simulation data suggests that H-bonding is a major source of this stabilization energy. Experimental and computational comparison with a newly synthesized 1,3,5-benzenetricarboxyester (BTE) derivative, lacking the ability to form the H-bonding network, demonstrated that this BTE variant is also capable of fiber formation, albeit at a reduced persistence length. This work provides unambiguous evidence for the key 1D driving force of hydrogen bonding in enhancing the persistency of monomer stacking and amplifying the level of order into the growing supramolecular polymer in water. Our computational approach provides an important relationship directly linking the structure of the monomer to the structure and properties of the supramolecular polymer.

From supramolecular polymers to hydrogel materials

Supramolecular hydrogels formed by decorating benzene-1,3,5-tricarboxamide (BTA) units with amphiphilic ethylene glycol-based side chains are presented; careful selection of the substituents of the BTAs allows for the tuning of the self-assembly behaviour and hence the mechanical properties of the resultant hydrogel.

Hydrogels that listen to cells: a review of cell-responsive strategies in biomaterial design for tissue regeneration

The past decade has seen a decided move from static and passive biomaterials to biodegradable, dynamic, and stimuli responsive materials in the laboratory and the clinic. Recent advances towards the rational design of synthetic cell-responsive hydrogels—biomaterials that respond locally to cells or tissues without the input of an artificial stimulus—have provided new strategies and insights on the use of artificial environments for tissue engineering and regenerative medicine. These materials can often approximate responsive functions of a cells complex natural extracellular environment, and must respond to the small and specific stimuli provided within the vicinity of a cell or tissue. In the current literature, there are three main cell-based stimuli that can be harnessed to create responsive hydrogels: (1) enzymes (2) mechanical force and (3) metabolites/small molecules. Degradable bonds, dynamic covalent bonds, and non-covalent or supramolecular interactions are used to provide responsive architectures that enable features ranging from cell selective infiltration to control of stem-cell differentiation. The growing ability to spatio-temporally control the behavior of cells and tissue with rationally designed responsive materials has the ability to allow control and autonomy to future generations of materials for tissue regeneration, in addition to providing understanding and mimicry of the dynamic and complex cellular niche.

Dynamic diversity of synthetic supramolecular polymers in water as revealed by hydrogen/deuterium exchange.

Numerous self-assembling molecules have been synthesized aiming at mimicking both the structural and dynamic properties found in living systems. Here we show the application of hydrogen/deuterium exchange (HDX) mass spectrometry (MS) to unravel the nanoscale organization and the structural dynamics of synthetic supramolecular polymers in water. We select benzene-1,3,5-tricarboxamide (BTA) derivatives that self-assemble in H2O to illustrate the strength of this technique for supramolecular polymers. The BTA structure has six exchangeable hydrogen atoms and we follow their exchange as a function of time after diluting the H2O solution with a 100-fold excess of D2O. The kinetic H/D exchange profiles reveal that these supramolecular polymers in water are dynamically diverse; a notion that has previously not been observed using other techniques. In addition, we report that small changes in the molecular structure can be used to control the dynamics of synthetic supramolecular polymers in water.

Exposing Differences in Monomer Exchange Rates of Multicomponent Supramolecular Polymers in Water

The formation of multicomponent and bioactive supramolecular polymers is a promising strategy for the formation of biomaterials that match the dynamic and responsive nature of biological systems. In order to fully realize the potential of this strategy, knowledge of the location and behavior of bioactive components within the system is crucial. By employing synthetic strategies to create multifunctional monomers, coupled with FRET and STORM techniques, we have investigated the formation and behavior of a bioactive and multicomponent supramolecular polymer. By creating a peptide‐dye‐monomer conjugate, we were able to measure high degrees of monomer incorporation and to visualize the equal distribution of monomers within the supramolecular polymer. Furthermore, by tracking the movement of monomers, we uncovered small differences in the dynamics of the bioactive monomers.

Tailoring surface nanoroughness of electrospun scaffolds for skeletal tissue engineering

Electrospun scaffolds provide a promising approach for tissue engineering as they mimic the physical properties of extracellular matrix. Previous studies have demonstrated that electrospun scaffolds with porous features on the surface of single fibers, enhanced cellular attachment and proliferation. Yet, little is known about the effect of such topographical cues on cellular differentiation. Here, we aimed at investigating the influence of surface roughness of electrospun scaffolds on skeletal differentiation of human mesenchymal stromal cells (hMSCs). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis showed that the surface nanoroughness of fibers was successfully regulated via humidity control of the electrospinning environment. Gene expression analysis revealed that a higher surface roughness (roughness average (Ra)=71.0±11.0nm) supported more induction of osteogenic genes such as osteopontin (OPN), bone morphogenetic protein 2 (BMP2), and runt-related transcription factor 2 (RUNX2), while a lower surface roughness (Ra=14.3±2.5nm) demonstrated higher expression of other osteogenic genes including bone sialoprotein (BSP), collagen type I (COL1A1) and osteocalcin (OCN). Interestingly, a lower surface roughness (Ra=14.3±2.5nm) better supported chondrogenic gene expression of hMSCs at day 7 compared to higher surface roughness (Ra=71.0±11.0nm). Taken together, modulating surface roughness of 3D scaffolds appears to be a significant factor in scaffold design for the control of skeletal differentiation of hMSCs. STATEMENT OF SIGNIFICANCE Tissue engineering scaffolds having specific topographical cues offer exciting possibilities for stimulating cells differentiation and growth of new tissue. Although electrospun scaffolds have been extensively investigated in tissue engineering and regenerative medicine, little is known about the influence of introducing nanoroughness on their surface for cellular differentiation. The present study provides a method to engineer electrospun scaffolds with tailoring surface nanoroughness and investigates the effect of such topographical cues on the process of human mesenchymal stromal cells differentiation into osteoblasts and chondrocytes linages. This strategy may help the design of nanostructured scaffolds for skeletal tissue engineering.

Fragmentation of organic ions bearing fixed multiple charges observed in MALDI MS

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) was used to analyze a series of synthetic organic ions bearing fixed multiple charges. Despite the multiple intrinsic charges, only singly charged ions were recorded in each case. In addition to the pseudo-molecular ions formed by counterion adduction, deprotonation and electron capture, a number of fragment ions were also observed. Charge splitting by fragmentation was found to be a viable route for charge reduction leading to the formation of the observed singly charged fragment ions. Unlike multivalent metal ions, organic ions can rearrange and/or fragment during charge reduction. This fragmentation process will evidently complicate the interpretation of the MALDI MS spectrum. Because MALDI MS is usually considered as a soft ionization technique, the fragment ion peaks can easily be erroneously interpreted as impurities. Therefore, the awareness and understanding of the underlying MALDI-induced fragmentation pathways is essential for a proper interpretation of the corresponding mass spectra. Due to the fragment ions generated during charge reduction, special care should be taken in the MALDI MS analysis of multiply charged ions. In this work, the possible mechanisms by which the organic ions bearing fixed multiple charges fragment are investigated. With an improved understanding of the fragmentation mechanisms, MALDI TOF MS should still be a useful technique for the characterization of organic ions with fixed multiple charges.

Thiol–Ene Alginate Hydrogels as Versatile Bioinks for Bioprinting

Bioprinting is a powerful technique that allows precise and controlled 3D deposition of biomaterials in a predesigned, customizable, and reproducible manner. Cell-laden hydrogel (“bioink”) bioprinting is especially advantageous for tissue engineering applications as multiple cells and biomaterial compositions can be selectively dispensed to create spatially well-defined architectures. Despite this promise, few hydrogel systems are easily available and suitable as bioinks, with even fewer systems allowing for molecular design of mechanical and biological properties. In this study, we report the development of a norbornene functionalized alginate system as a cell-laden bioink for extrusion-based bioprinting, with a rapid UV-induced thiol–ene cross-linking mechanism that avoids acrylate kinetic chain formation. The mechanical and swelling properties of the hydrogels are tunable by varying the concentration, length, and structure of dithiol PEG cross-linkers and can be further modified by postprinting secondary cross-linking with divalent ions such as calcium. The low concentrations of alginate needed (<2 wt %), coupled with their rapid in situ gelation, allow both the maintenance of high cell viability and the ability to fabricate large multilayer or multibioink constructs with identical bioprinting conditions. The modularity of this bioink platform design enables not only the rational design of materials properties but also the gel’s biofunctionality (as shown via RGD attachment) for the expected tissue-engineering application. This modularity enables the creation of multizonal and multicellular constructs utilizing a chemically similar bioink platform. Such tailorable bioink platforms will enable increased complexity in 3D bioprinted constructs.