This review highlights the research on degradable polymeric tissue adhesives for surgery and tissue engineering. Included are a comprehensive listing of specific uses, advantages, and disadvantages of different adhesive groups. A critical evaluation of challenges affecting the development of next generation materials is also discussed, and insights into the outlook of the field are explored.

Poly(propylene fumarate) (PPF) has been highlighted as one of the most promising materials for bone regeneration. Despite the promising advantages of using polymer scaffolds for biomedical applications, their inherent lack of bioactivity has limited their clinical application. In this study, PPF was successfully functionalized with Bioglass and a novel catechol-bearing peptide bioconjugate containing bioactive short peptide sequences of basic fibroblast growth factor, bone morphogenetic protein 2, and osteogenic growth peptide. The binding affinity was assessed to be around 110 nmol/cm2 with the Bioglass content at 10 wt %. Fluorescence imaging studies show that the catechol-bearing modular peptide binds preferentially to the Bioglass. A 4 week in vitro cell study using human mesenchymal stem cells showed that cell adhesion, spreading, proliferation, and osteogenic differentiation at both gene and protein levels were all improved by the introduction of peptides, demonstrating the potential approach of dually functionalized polymers for bone regeneration.

The adhesive nature of mussels arises from the catechol moiety in the 3,4-dihydroxyphenylalanine (DOPA) amino acid, one of the many proteins that contribute to the unique adhesion properties of mussels. Inspired by these properties, many biomimetic adhesives have been developed over the past few years in an attempt to replace adhesives such as fibrin, cyanoacrylate, and epoxy glues. In the present work, we synthesized ethanol soluble but water insoluble catechol functionalized poly(ester urea) random copolymers that help facilitate delivery and adhesion in wet environments. Poly(propylene glycol) units incorporated into the polymer backbone impart ethanol solubility to these polymers, making them clinically relevant. A catechol to cross-linker ratio of 10:1 with a curing time of 4 h exceeded the performance of commercial fibrin glue (4.8 ± 1.4 kPa) with adhesion strength of 10.6 ± 2.1 kPa. These adhesion strengths are significant with the consideration that the adhesion studies were performed under wet conditions.Keywords: adhesion; biomimicry; catechol; DOPA; mussels; poly(ester urea); poly(propylene glycol);

Additive manufacturing has the potential to revolutionize regenerative medicine, but the harsh thermal or photochemical conditions during the 3D printing process limit the inclusion of drugs, growth factors and other biologics within the resulting scaffolds. Functionalization strategies that enable specific placement of bioactive species on the surface of 3D printed structures following the printing process afford a promising approach to sidestep the harsh conditions and incorporate these valuable bioactive molecules with precise control over concentration. Herein, resorbable polymer scaffolds were prepared from propargyl functionalized L-phenylalanine-based poly(ester urea)s (PEUs). Osteogenic growth peptide (OGP) or bone morphogenic protein-2 (BMP-2) peptides were immobilized on PEU scaffolds through surface available propargyl groups via copper-catalyzed azide alkyne cycloaddition (CuAAC) post 3D printing. The presence of either OGP or BMP-2 significantly enhanced hMSCs osteogenic differentiation compared to unfunctionalized scaffolds.

AbstractShape memory materials have emerged as an important class of materials in medicine due to their ability to change shape in response to a specific stimulus, enabling the simplification of medical procedures, use of minimally invasive techniques, and access to new treatment modalities. Shape memory polymers, in particular, are well suited for such applications given their excellent shape memory performance, tunable materials properties, minimal toxicity, and potential for biodegradation and resorption. This review provides an overview of biodegradable shape memory polymers that have been used in medical applications. The majority of biodegradable shape memory polymers are based on thermally responsive polyesters or polymers that contain hydrolyzable ester linkages. These materials have been targeted for use in applications pertaining to embolization, drug delivery, stents, tissue engineering, and wound closure. The development of biodegradable shape memory polymers with unique properties or responsiveness to novel stimuli has the potential to facilitate the optimization and development of new medical applications.

The thermal shape memory behavior of a series of α-amino acid-based poly(ester urea)s has been explored. We demonstrate that these materials exhibit excellent shape memory performance in dual- and triple-shape thermomechanical testing. Significant activation of chain mobility above the Tg as well as a hydrogen bonding network provide the basis for shape transformations and recovery. Additionally, we tuned the shape memory properties of these materials with polymer blending, enabling the demonstration of quadruple-shape memory cycles.

Bone and tissue adhesives are essential in surgeries for wound healing, hemostasis, tissue reconstruction, and drug delivery. However, there are very few degradable materials with high adhesion strengths that degrade into bioresorbable byproducts. Caddisfly adhesive silk is interesting due to the presence of phosphoserines, which are thought to afford adhesive properties. In this work, phosphoserine–valine poly(ester urea) copolymers with 2% and 5% phosphoserine content were synthesized to mimic caddisfly adhesive silk. Significantly, the materials are ethanol soluble and water insoluble, making them clinically relevant. Their physical properties were quantified, and the adhesion properties were studied on aluminum and bovine bone substrates before and after cross-linking with Ca2+ ions. The adhesive strength of the phosphorylated copolymer on a bone substrate after cross-linking with Ca2+ was 439 ± 203 kPa, comparable to commercially available PMMA bone cement (530 ± 133 kPa).

Thermoplastic polyurethane (TPU) sulfonate ionomers with quaternary ammonium cations were synthesized to achieve soft TPUs without using conventional low molecular weight plasticizers. The sulfonated monomer N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES) neutralized with bulky ammonium counterions was incorporated as a chain extender to internally plasticize the TPU. Increasing the steric bulk of the counterion and the concentration of the ionic species produced softer TPUs with improved melt processability. The incorporation of the sulfonate species suppressed crystallinity of the TPU hard block, which was mainly responsible for the softening of the polymer. The synthetic procedure developed allows for facile tuning of the mechanical properties of the TPU by simply switching the counterion and/or increasing the feed ratio of ionic monomer. The precursors in this study were synthesized and analyzed via 1H NMR, and the thermomechanical properties of the resulting TPU ionomers were characterized by differential scanning calorimetry, dynamic mechanical analysis, Shore A hardness, and static mechanical testing.

Degradable poly(ester urea)s (PEU)s were electrospun into nanofiber sheets and assessed for their potential to be used in soft tissue repair. The level of residual solvent was measured and the effects of ethylene oxide and electron beam sterilization techniques on molecular mass, mass distribution, and morphology were quantified. Two PEU compositions that formed stable nanofiber sheets were advanced into a pilot study in vitro and in vivo as candidate materials for hernia repair. Cell viability, spreading, proliferation, and migration were examined in vitro. Nanofiber sheets were implanted subcutaneously into mice and analyzed via microangiography and histology for tissue incorporation. Nanofiber sheets performed similarly to decellularized extracellular matrix (ECM) in vitro, but the lack of sufficient pore structure inhibited cellular infiltration after 14 days of culture. The lack of microporous features in nanofiber sheets also contributed to low levels of cellular infiltration, angiogenesis, and matrix deposition in vivo. A preliminary study to increase pore size in nanofibers was performed using coaxial electrospinning resulting in significant improvement in tissue infiltration in vivo.

The competitive absorption of blood plasma components including fibrinogen (FG), bovine serum albumin (BSA), and platelet-rich plasma (PRP) on l-valine-based poly(ester urea) (PEU) surfaces were investigated. Using four different PEU polymers, possessing compositionally dependent trends in thermal, mechanical, and critical surface tension measurements, water uptake studies were carried out to determine in vitro behavior of the materials. Quartz crystal microbalance (QCM) measurements were used to quantify the adsorption characteristics of PRP onto PEU thin films by coating the surfaces initially with FG or BSA. Pretreatment of the PEU surfaces with FG inhibited the adsorption of PRP and BSA decreased the absorption 4-fold. In vitro studies demonstrated that cells cultured on l-valine-based PEU thin films allowed attachment and spreading of rat aortic cells. These measurements will be critical toward efforts to use this new class of materials in blood-contacting biomaterials applications.

A ring opening polymerization method for synthesizing oligomeric poly(propylene fumarate) (PPF) provides a rapid, and scalable method of synthesizing PPF with well-defined molecular mass, molecular mass distribution (Đm), and viscosity properties suitable for 3D printing. These properties will also reduce the amount of solvent necessary to ensure sufficient flow of material during 3D printing. MALDI mass spectrometry precisely shows the end group fidelity, and size exclusion chromatography (SEC) demonstrates narrow mass distributions (<1.6) of a series of low molecular mass oligomers (700–3000 Da). The corresponding intrinsic viscosities range from 0.0288 ± 0.0009 dL/g to 0.0780 ± 0.0022 dL/g. The oligomers were printed into scaffolds via established photochemical methods and standardized ISO 10993-5 testing shows that the 3D printed materials are nontoxic to both L929 mouse fibroblasts and human mesenchymal stem cells.

Self-assembled monolayer substrates containing tethered orthogonal concentration profiles of GRGDS (glycine/arginine/glycine/aspartic acid/serine) and BMP-2 (bone morphogenetic protein) peptides are shown to accelerate or decelerate, depending on the concentrations, the proliferation and osteoblastic differentiation of human mesenchymal stem cell (hMSC) populations in vitro without the use of osteogenic additives in culture medium. Concurrently, the single peptide gradient controls (GRGDS or BMP-2 only) induce significantly different proliferation and differentiation behavior from the orthogonal substrates. Bone sialoprotein (BSP) and Runt-related transcription factor 2 (Runx2) PCR data acquired from hMSC populations isolated by laser capture microdissection correspond spatially and temporally to protein marker data obtained from immunofluorescent imaging tracking of the differentiation process. Although genomic and protein data at high concentrations area GRGDS (71–83 pmol/cm2):BMP-2 (25 pmol/cm2) reveal an implicit acceleration on the hMSC differentiation timeline relative to the individual peptide concentrations, most of the GRGDS and BMP-2 combinations displayed significant antagonistic behavior during the hMSC differentiation. These data highlight the utility of the orthogonal gradient approach to aid in identifying optimal concentration ranges of translationally relevant peptides and growth factors for targeting cell lineage commitment.

4-Dibenzocyclooctynol (DIBO) was used as an initiator for the ring-opening copolymerization of ε-caprolactone and 1,4,8-trioxaspiro[4.6]-9-undecanone (TOSUO) resulting in a series of DIBO end-functionalized copolymers. Following deprotection of the ketone group, the polymers were derivatized with aminooxyl-containing compounds by oxime ligation. Mixtures of keto- and alkyne-derivatized polymers were co-electrospun into well-defined nanofibers containing three separate chemical handles. Strain-promoted azide alkyne cycloaddition (SPAAC), oxime ligation, and copper-catalyzed azide alkyne cycloaddition (CuAAC) were used to sequentially functionalize the nanofibers first with fluorescent reporters and then separately with bioactive Gly-Arg-Gly-Asp-Ser (GRGDS), BMP-2 peptide, and dopamine. This translationally relevant approach facilitates the straightforward derivatization of diverse bioactive molecules that can be controllably tethered to the surface of nanofibers.

Dual end-functionalized telechelic poly(N-isopropylacrylamide) (PNIPAM) was synthesized using reversible addition–fragmentation chain-transfer (RAFT) polymerization. One end was coupled to a fluorescent dye and the other end was covalently coupled to CdSe/ZnS quantum dots (QDs) through carbodiimide chemistry. The hybrid nanoparticle shows ratiometric changes in fluorescence emission upon temperature cycling between 25 °C and 45 °C.

Poly(ester urea)s (PEUs) derived from α-amino acids are promising for vascular tissue engineering applications. The objective of this work was to synthesize and characterize l-leucine-based PEUs and evaluate their suitability for vascular tissue engineering. Four different PEUs were prepared from di-p-toluenesulfonic acid salts of bis-l-leucine esters and triphosgene using interfacial condensation polymerizations. Mechanical testing indicated that the elastic moduli of the respective polymers were strongly dependent on the chain length of diols in the monomers. Three of the resulting PEUs showed elastic moduli that fall within the range of native blood vessels (0.16 to 12 MPa). The in vitro degradation assays over 6 months indicated that the polymers are surface eroding and no significant pH drop was observed during the degradation process. Human umbilical vein endothelial cells (HUVECs) and A-10 smooth muscle cells (A-10 SMCs) were cultured on PEU thin films. Protein adsorption studies showed the PEUs did not led to significant platelet adsorption in platelet rich plasma (PRP) after pretreatment with fibrinogen. Taken together, our data suggest that the l-leucine-based PEUs are viable candidate materials for use in vascular tissue engineering applications.Keywords: biomaterial; poly(ester urea)s; protein absorption; resorbable; vascular tissue engineering

Amino acid-based poly(ester urea) (PEU) copolymers functionalized with pendant catechol groups that address the need for strongly adhesive yet degradable biomaterials have been developed. Lap-shear tests with aluminum adherends demonstrated that these polymers have lap-shear adhesion strengths near 1 MPa. An increase in lap-shear adhesive strength to 2.4 MPa was achieved upon the addition of an oxidative cross-linker. The adhesive strength on porcine skin adherends was comparable with commercial fibrin glue. Interfacial energies of the polymeric materials were investigated via contact angle measurements and Johnson–Kendall–Roberts (JKR) technique. The JKR work of adhesion was consistent with contact angle measurements. The chemical and physical properties of PEUs can be controlled using different diols and amino acids, making the polymers candidates for the development of biological glues for use in clinical applications.

Amino acid-based poly(ester urea)s (PEU) are high modulus, resorbable polymers with many potential uses, including the surgical repair of bone defects. In vitro and in vivo studies have previously shown that phenylalanine-based PEUs have nontoxic hydrolytic byproducts and tunable degradation times. Phenylalanine PEUs (poly(1-PHE-6)) have been further modified by tethering osteogenic growth peptide (OGP) to tyrosine-based monomer subunits. These OGP-tethered PEUs have been fabricated into porous scaffolds and cultured in vitro to examine their effect on differentiation of human mesenchymal stem cells (hMSCs) toward the osteogenic lineage. The influence of tethered OGP on the hMSC proliferation and differentiation profile was measured using immunohistochemistry, biochemistry, and quantitative real time polymerase chain reaction (qRT-PCR). In vitro data indicated an enhanced expression of BSP by 130–160% for hMSCs on OGP-tethered scaffolds compared to controls. By 4 weeks, there was a significant drop (60–85% decrease) in BSP expression on OGP-functionalized scaffolds, which is characteristic of osteogenic differentiation. ALP and OSC expression was significantly enhanced for OGP-functionalized scaffolds by week 4, with values reaching 145% and 300% greater, respectively, compared to nonfunctionalized controls. In vivo subcutaneous implantation of poly(1-PHE-6) scaffolds revealed significant tissue-scaffold integration, as well as the promotion of both osteogenesis and angiogenesis.

A series of amino-acid based poly(ester urea)s (PEU) with controlled amounts of branching was synthesized and characterized. The mechanical properties, thermal characteristics and water absorptions varied widely with the extent of branch unit incorporation. Herein, the details of the synthesis of a linear bis(l-phenylalanine)-hexane 1,6-diester monomer, a branch tri-O-benzyl-l-tyrosine-1,1,1-trimethylethane triester monomer and a series of copolymers are described. The extent of branching was varied by adjusting the molar ratio of linear to branched monomer during the interfacial polymerization. The elastic moduli span a range of values (1.0–3.1 GPa) that overlaps with several clinically available degradable polymers. Increasing the amount of branching monomers reduces the molecular entanglement, which results in a decrease in elastic modulus values and an increase in values of elongation at break. The l-phenylalanine-based poly(ester urea)s also exhibited a branch density dependent water uptake ability that varied between 2 and 3% after 24 h of immersion in water. Nanofibers incorporating 8% branching were able to maintain their morphology at elevated temperature, in hydrated conditions, and during ethylene oxide sterilization which are critical to efforts to translate these materials to clinical soft tissue applications.

Peptides that comprise the functional subunits of proteins have been conjugated to versatile materials (biomolecules, polymers, surfaces and nanoparticles) in an effort to modulate cell responses, specific binding affinity and/or self-assembly behavior. However, the efficient and convenient synthesis of peptide-conjugates, especially the constructs with multiple types of peptide functionality remains challenging. In this critical review, we focus on “click” reactions that have been used to synthesis peptide-functionalized conjugates, introducing their reaction conditions, specifically elucidating parameters that influence reaction kinetics and total conversion, and highlighting examples that have been completed recently. Moreover, orthogonal “click” reactions that synthesize multi-functional biomaterials in a one-pot or sequential manner are noted. Through this review, a comprehensive understanding of “click” reactions aims to provide insight on how one might choose suitable “click” reactions to constitute peptide-functionalized molecules/surfaces/matrices for the development of advanced biomaterials.

A series of multivalent dendrons containing a bioactive osteogenic growth peptide (OGP) domain and surface-binding catechol domains were obtained through solid phase synthesis, and their binding affinity to hydroxyapatite, TiO2, ZrO2, CeO2, Fe3O4 and gold was characterized using a quartz crystal microbalance with dissipation (QCM-d). Using the distinct difference in binding affinity of the bioconjugate to the metal oxides, TiO2-coated glass slides were selectively patterned with bioactive peptides. Cell culture studies demonstrated the bioavailability of the OGP and that OGP remained on the surface for at least 2 weeks under in vitro cell culture conditions. Bone sialoprotein (BSP) and osteocalcein (OCN) markers were upregulated 3-fold and 60-fold, respectively, relative to controls at 21 days. Similarly, 3-fold more calcium was deposited using the OGP tethered dendron compared to TiO2. These catechol-bearing dendrons provide a fast and efficient method to functionalize a wide range of inorganic materials with bioactive peptides and have the potential to be used in coating orthopaedic implants and fixation devices.

Well-defined poly(caprolactone)s were generated using alkyne-based initiating systems catalyzed by Candida antarctica Lipase B (CALB). Propargyl alcohol and 4-dibenzocyclooctynol (DIBO) were shown to efficiently initiate the polymerizations under metal free conditions and yielded 4 to 24 KDa polymers with relatively narrow molecular mass distribution. The alkyne bonds in the strained and unstrained initiating systems survived the polymerization conditions and the purification process. The resulting bonds can be sequentially functionalized post-polymerization using metal free and copper catalyzed click chemistry conditions.

Strain-promoted azide–alkyne cycloaddition reactions are combined with a dopamine functional species to generate a highly efficient method for surface modification. The resulting conjugate containing 4-dibenzocyclooctynol (DIBO) and dopamine results in a versatile surface labeling technology that can replicate patterns generated from photolithography and microcontact printing techniques.

A new class of l-phenylalanine-based poly(ester urea)s (PEU) was developed that possess tunable mechanical properties and degradation rates. Our preliminary data have shown that 1,6-hexanediol-l-phenylalanine-based poly(ester urea)s possess an elastic modulus nearly double that of poly(lactic acid). The data in this article detail the synthesis of a series of l-phenylalanine-based poly(ester urea)s possessing a variation in diol chain length and how these subtle structural differences influence the mechanical properties and in vitro biodegradation rates. The mechanical data span a range of values that overlaps with several currently clinically available degradable polymers. Increasing the diol chain lengths increases the amount of flexible segment in the chemical structure, which results in reduced elastic modulus values and increased values of elongation at break. The l-phenylalanine-based poly(ester urea)s also exhibited a diol length dependent degradation process that varied between 1 and 5% over 16 weeks. Compared with PLLA, PEUs degrade more quickly, and the rate can be tuned by changing the diol chain length.

Water-soluble cadmium telluride (CdTe) quantum dots (QDs) used as an anode interlayer in solution-processed near infrared (NIR) polymer photodetectors (PDs) were demonstrated. Polymer PDs incorporated with CdTe QDs as an anode interlayer exhibited 10-fold suppressed dark current density and analogous photocurrent density relative to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which resulted in enhanced detectivities over 1011 Jones in the spectral range from 350 nm to 900 nm. Moreover, with the substitution of PEDOT:PSS by CdTe QDs, the stability of unencapsulated NIR polymer PDs was extended up to 650 hours, which is more than 3 times longer than those with PEDOT:PSS as an anode interlayer. These results indicated that CdTe QDs can be utilized as a solution-processable alternative to PEDOT:PSS as an anode interlayer for high performance NIR polymer PDs.

Utilization of 4-dibenzocyclooctynol (DIBO) as an initiator for the ring-opening polymerization of ε-caprolactone yields well-defined, high molecular weight poly(ε-caprolactone) end functionalized with DIBO (DIBO–PCL). Nanofibers bearing reactive DIBO groups were generated via electrospinning and functionalized post-fabrication with azide-tethered molecules.

Cell sourcing continues to be a significant limitation to regenerative medicine especially in neural lineages where population heterogeneity during in vitro culture prevents definitive phenotype assessment. For nearly 40 years, the biological community has worked with amine-derivated surfaces and hydrogels, especially alginate, with little quantitative assessment of how local amine concentration influences the extent of neural differentiation and neurite extension. In this manuscript we show that the local concentration of amines distinctly influences mouse embryonic stem cell (ESC) lineage commitment and the length of neurite extensions both of which are early indicators of differentiation. The well-defined amine gradients are a highly relevant tool for identifying these critical concentrations and thresholds. We feel these results will be of critical importance to researchers developing new ex vivo culture materials for neural applications as well as the community exploring nerve regeneration in vivo.

Collective surface plasmons (SPs) generated by two-dimensional (2-D) Au nanostructures are attractive for developing miniaturized photonic devices with sub-wavelength plasmonic structures and bimolecular sensors with single molecule sensitivity. However, high-fidelity methods for fabricating ordered arrays of AuNPs with nanoscale precision are limited. Herein, the fabrication of AuNP hierarchical structures with tunable domain spacing controlled precisely by Au phage peptide (A3) end-functionalized poly(methyl methacrylate) (PMMA) domains in a hexagonally packed polystyrene-PMMA block copolymer (BCP-A3) thin films is demonstrated. Dynamic thermal field processing techniques control the BCP-A3 orientation and ordered-assembly, thus controlling the arrangement of AuNP superstructures from hexagonal clusters to parallel wires.

Amino acid-based poly(ester urea)s (PEU) are emerging as a new class of degradable polymers that have shown promise in regenerative medicine applications. Herein, we report the synthesis of PEUs carrying pendent “clickable” groups on modified tyrosine amino acids. The pendent species include alkyne, azide, alkene, tyrosine–phenol, and ketone groups. PEUs with Mw exceeding to 100K Da were obtained via interfacial polycondensation methods, and the concentration of pendent groups was varied using a copolymerization strategy. The incorporation of derivatizable functionalities is demonstrated using 1H NMR and UV–vis spectroscopy methods. Electrospinning was used to fabricate PEU nanofibers with a diameters ranging from 350 to 500 nm. The nanofiber matricies possess mechanical strengths suitable for tissue engineering (Young’s modulus: 300 ± 45 MPa; tensile stress: 8.5 ± 1.2 MPa). A series of bioactive peptides and fluorescent molecules were conjugated to the surface of the nanofibers following electrospinning using bio-orthogonal reactions in aqueous media. The ability to derivatize PEUs with biological molecules using translationally relevant chemical methods will significantly expand their use in vitro and in vivo.

A series of low bandgap semi-random copolymers incorporating various ratios of two acceptor units—thienothiadiazole and benzothiadiazole—were synthesized by Pd-catalyzed Stille coupling. The polymer films exhibited broad and intense absorption spectra, covering the spectral range from 350 nm up to 1240 nm. The optical bandgaps and HOMO levels of the polymers were calculated from ultraviolet–visible spectroscopy and cyclic voltammetry measurements, respectively. By changing the ratio of the two acceptor monomers, the HOMO levels of the polymers were tuned from −4.42 to −5.28 eV and the optical bandgaps were varied from 1.00 to 1.14 eV. The results indicate our approach could be applied to the design and preparation of conjugated polymers with specifically desired energy levels and bandgaps for photovoltaic applications.

We demonstrate the formation of polyethylene glycol (PEG) based hydrogels via oxime ligation and the photoinitiated thiol–ene 3D patterning of peptides within the hydrogel matrix postgelation. The gelation process and final mechanical strength of the hydrogels can be tuned using pH and the catalyst concentration. The time scale to reach the gel point and complete gelation can be shortened from hours to seconds using both pH and aniline catalyst, which facilitates the tuning of the storage modulus from 0.3 to over 15 kPa. Azide- and alkene-functionalized hydrogels were also synthesized, and we have shown the post gelation “click”-type Huisgen 1,3 cycloaddition and thiolene-based radical reactions for spatially defined peptide incorporation. These materials are the initial demonstration for translationally relevant hydrogel materials that possess tunable mechanical regimes attractive to soft tissue engineering and possess atom neutral chemistries attractive for post gelation patterning in the presence or absence of cells.

Stem cells have shown lineage-specific differentiation when cultured on substrates possessing signaling groups derived from the native tissue. A distinct determinant in this process is the concentration of the signaling motif. While several groups have been working actively to determine the specific factors, concentrations, and mechanisms governing the differentiation process, many have been turning to combinatorial and gradient approaches in attempts to optimize the multiple chemical and physical parameters needed for the next advance. However, there has not been a direct comparison between the cellular behavior and differentiation of human mesenchymal stem cells cultured in gradient and discrete substrates, which quantitates the effect of differences caused by cell-produced, soluble factors due to design differences between the culture systems. In this study, the differentiation of human mesenchymal stem cells in continuous and discrete polyethylene glycol dimethacrylate (PEGDM) hydrogels containing an RGD concentration gradient from 0 to 14 mM were examined to study the effects of the different culture conditions on stem-cell behavior. Culture condition was found to affect every osteogenic (alkaline phosphatase, Runx 2, type 1 collagen, bone sailoprotein, and calcium content) and adipogenic marker (oil red and peroxisome proliferator-activated receptor gamma) examined regardless of RGD concentration. Only in the continuous gradient culture did RGD concentration affect human mesenchymal stem-cell lineage commitment with low RGD concentrations expressing higher osteogenic differentiation than high RGD concentrations. Conversely, high RGD concentrations expressed higher adipogenic differentiation than low RGD concentrations. Cytoskeletal actin organization was only affected by culture condition at low RGD concentrations, indicating that it played a limited role in the differences in lineage commitment observed. Therefore, the role of discrete versus gradient strategies in high-throughput experimentation needs to be considered when designing experiments as we show that the respective strategies alter cellular outcomes even though base scaffolds have similar material and chemical properties.

Peptides, proteins, and extracellular matrix act synergistically to influence cellular function at the biotic–synthetic interface. However, identifying the individual and cooperative contributions of the various combinations and concentration regimes is extremely difficult. The confined channel deposition method we describe affords highly tunable orthogonal reactive concentration gradients that greatly expand the dynamic range, spatial control, and chemical versatility of the reactive silanes that can be controllably deposited. Using metal-free “dual click” immobilization chemistries, multiple peptides with a variety of functionality can be immobilized efficiently and reproducibly enabling optimal concentration profiling and the assessment of synergistic interactions.

A series of mono- and multifunctionalized degradable polyesters bearing various “clickable” groups, including ketone, alkyne, azide, and methyl acrylate (MA) are reported. Using this approach, we demonstrate a cascade approach to immobilize and quantitate three separate bioactive groups onto poly(caprolactone) (PCL) thin films. The materials are based on tunable copolymer compositions of ε-caprolactone and 2-oxepane-1,5-dione. A quartz crystal microbalance (QCM) was used to quantify the rate and extent of surface conjugation between RGD peptide and polymer thin films using “click” chemistry methods. The results show that alkyne-functionalized polymers have the highest conversion efficiency, followed by MA and azide polymers, while polymer films possessing keto groups are less amenable to surface functionalization. The successful conjugation was further confirmed by static contact angle measurements, with a smaller contact angle correlating directly with lower levels of surface peptide conjugation. QCM results quantify the sequential immobilization of peptides on the PCL thin films and indicate that Michael addition must occur first, followed by azide–alkyne Huisgen cycloadditions.

Janus particles have attracted significant attention in recent years, due to their potential in nanomanufacturing. Their asymmetric features impart unique physical and chemical properties, which can be tuned and utilized to control their solution-state assembly. While several examples of Janus nanostructures have been reported in the literature, the scientific community continues to pursue synthetic routes which are less time and resource intensive. Herein, we describe a facile method to synthesize Janus nanoparticles in which colloidal micelles template the in situ formation of Au nanoparticles in the shell layer. The resulting morphologies of the hybrid Au–polymer nanoparticles can be adjusted widely by controlling the polymer and reducing agent (HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid)) concentrations. High-fidelity populations of Au–micelle Janus nanoparticles are obtained when the polymer concentration is high (≥1.0 × 10−6 mmol L−1) and the HEPES concentration is low (≤0.3 mol L−1). Conversely, when the HEPES concentration is high (≥0.5 mol L−1) and the polymer concentration is low (≤2.4 × 10−7 mmol L−1), raspberry-like clusters are formed, where each micelle encumbers several Au nanocrystals. Our method is attractive in that the number of Au nanoparticles on each Au–micelle entity can be controlled using a scalable, aqueous solution process. It also has significant potential for the directed assembly of Au nanoparticle superstructures, as the nature and geometry of the polymer precursors can be varied.

We present a molecular dynamics study of the binding process of peptide A3 (AYSSGAPPMPPF) and other similar peptides onto gold surfaces, and identify the functions of many amino acids. Our results provide a clear picture of the separate regimes present in the binding process: diffusion, anchoring, crawling and binding. Moreover, we explored the roles of individual residues. We found that tyrosine, methionine, and phenylalanine are strong binding residues; serine serves as an effective anchoring residue; proline acts as a dynamic anchoring point, while glycine and alanine give flexibility to the peptide backbone. We then show that our findings apply to unrelated phage-derived sequences that have been reported recently to facilitate AuNP synthesis. This new knowledge may aid in the design of new peptides for the synthesis of gold nanostructures with novel morphologies.

The development of advanced materials that facilitate hyaline cartilage formation and regeneration in aging populations is imperative. Critical to the success of this endeavor is the optimization of ECM production from clinically relevant cells. However, much of the current literature focuses on the investigation of primary bovine chondrocytes from young calves, which differ significantly than osteoarthritic cells from human sources. This study examines the levels of extracellular matrix (ECM) production using various levels of type I collagen and hyaluronic acid in poly(ethylene glycol) dimethacrylate (PEGDM) hydrogels in total knee arthroplasties, compared with the results from bovine chondrocytes. The addition of type 1 collagen in both the presence and absence of low levels of hyaluronic acid increased ECM production and/or retention in scaffolds containing either bovine or human chondrocytes. These findings are supported consistently with colorimetric quantification, whole mount extracellular matrix staining for both cell types, and histological staining for glycoaminoglycans and collagen of human chondrocyte containing samples. While exhibiting similar trends, the relative ECM productions levels for the primary human chondrocytes are significantly less than the bovine chondrocytes which reinforces the need for additional optimization.

Imaging of polymer implants during surgical implantations is challenging in that most materials lack sufficient X-ray contrast. Synthetic derivatization with iodine serves to increase the scattering contrast but results in distinct physicochemical properties in the material which influence subsequent protein adsorption and cell morphology behavior. Herein we report the impact of increasing iodine inclusion on the cell morphology (cell area and shape) of MC3T3-E1 osteoblasts on a series of homopolymers and discrete blend thin films of poly(desaminotyrosyl tyrosine ethyl ester carbonate), poly(DTE carbonate), and an iodinated analogue poly(I2-DTE carbonate). Cell morphology is correlated to film chemical composition via measuring fibronectin (FN) adhesion protein adsorption profile on these films. FN exhibits up to 2-fold greater adsorption affinity for poly(I2-DTE carbonate) than (poly(DTE carbonate)). A correlation was established between cell area, roundness, and the measured FN adsorption profile on the blend films up to 75% by mass poly(I2-DTE carbonate). Data suggest that incorporation of iodine within the polymer backbone has a distinct impact on the way FN proteins adsorb to the surface and within the studied blend systems; the effect is composition dependent.

Peptides have been shown to mediate the reduction and clustering of inorganic ions during biomineralization processes to build nanomaterials with well-defined shape, size, and composition. This precise control has been linked to specific amino acid sequence; however, there is a lack of information about the role of peptides during mineralization. Here, we investigate the nucleation and growth behavior of Au nanocrystals that are mediated by the engineered peptide AYSSGAPPMPPF. Unlike other nanocrystal synthesis schemes, this peptide produces Au nanocrystals from Au(III) ions at very low relative peptide concentrations, at ambient temperature, and in water at neutral pH. Our data show that (i) the peptide AYSSGAPPMPPF actually inhibits nucleation and growth of nanocrystals, (ii) HEPES plays an active chemical role as the reducing agent, and (iii) HAuCl4 accelerates the kinetics of nanoparticle nucleation and growth. Herein, we propose empirical rate laws for nucleation and growth of Au nanocrystals and compare kinetic rate laws for this peptide, citrate, and various other polymer ligands. We find that the peptide belongs to a unique class of nonreducing inhibitor ligands regulating the surface-reaction-limited growth of nanocrystals.

Cell sourcing continues to be a significant limitation to regenerative medicine especially in neural lineages where population heterogeneity during in vitro culture prevents definitive phenotype assessment. For nearly 40 years, the biological community has worked with amine-derivated surfaces and hydrogels, especially alginate, with little quantitative assessment of how local amine concentration influences the extent of neural differentiation and neurite extension. In this manuscript we show that the local concentration of amines distinctly influences mouse embryonic stem cell (ESC) lineage commitment and the length of neurite extensions both of which are early indicators of differentiation. The well-defined amine gradients are a highly relevant tool for identifying these critical concentrations and thresholds. We feel these results will be of critical importance to researchers developing new ex vivo culture materials for neural applications as well as the community exploring nerve regeneration in vivo.