Collagen-cellulose composite thin films that mimic soft-tissue and allow stem-cell orientation

Synthesis and Evaluation of Metal Lipoate Adhesives

The development of new bioadhesives with integrated properties remains an unmet clinical need to replace staples or sutures. Current bioadhesives do not allow electronic activation, which would allow expansion into laparoscopic and robotic surgeries. To address this deficiency, voltage-activated adhesives have been developed on both carbene- and catechol-based chemical precursors. Herein, a third platform of voltage-activated adhesive is evaluated based on lipoic acid, a non-toxic dithiolane found in aerobic metabolism and capable of ring-opening polymerization. The electro-rheological and adhesive properties of lithium, sodium, and potassium salts of lipoic acid are applied for wet tissue adhesion. At ambient conditions, potassium lipoate displays higher storage modulus than lithium or sodium salt under similar conditions. Voltage stimulation significantly improves gelation kinetics to Na- and K-lipoates, while Li-lipoate is found to not require voltage stimulation for gelation. Lap shear adhesion strength on wetted collagen substrates reveals that the synthetic metal lipoates have comparable adhesion strength to fibrin sealants without viral or ethical risks.

Mechanical characterization of rose bengal and green light crosslinked collagen scaffolds for regenerative medicine

Collagen is one of the most important biomaterials for tissue engineering approaches. Despite its excellent biocompatibility, it shows the non-negligible disadvantage of poor mechanical stability. Photochemical crosslinking with rose bengal and green light (RGX) is an appropriate method to improve this property. The development of collagen laminates is helpful for further adjustment of the mechanical properties as well as the controlled release of incorporated substances. In this study, we investigate the impact of crosslinking and layering of two different collagen scaffolds on the swelling behavior and mechanical behavior in micro tensile tests to obtain information on its wearing comfort (stiffness, strength and ductility). The mechanical stability of the collagen material after degradation due to cell contact is examined using thickness measurements. There is no linear increase or decrease due to layering homologous laminates. Unexpectedly, a decrease in elongation at break, Young's modulus and ultimate tensile strength are measured when the untreated monolayer is compared to the crosslinked one. Furthermore, we can detect a connection between stability and cell proliferation. The results show that with variation in number and type of layers, collagen scaffolds with tailored mechanical properties can be produced. Such a multi-layered structure enables the release of biomolecules into inner or outer layers for biomedical applications.

Rapid Activation of Diazirine Biomaterials with the Blue Light Photocatalyst

Carbene-based macromolecules are an emerging new stimuli-sensitive class of biomaterials that avoid the impediments of free radical polymerization but maintain a rapid liquid-to-biorubber transition. Activation of diazirine-grafted polycaprolactone polyol (CaproGlu) is limited to UVA wavelengths that have tissue exposure constraints and limited light intensities. For the first time, UVA is circumvented with visible light-emitting diodes at 445 nm (blue) to rapidly activate diazirine-to-carbene covalent cross-linking. Iridium photocatalysts serve to initiate diazirine, despite having little to no absorption at 445 nm. CaproGlu's liquid organic matrix dissolves the photocatalyst with no solvents required, creating a light transparent matrix. Considerable differences in cross-linking chemistry are observed in UVA vs visible/photocatalyst formulations. Empirical analysis and theoretical calculations reveal a more efficient conversion of diazirine directly to carbene with no diazoalkane intermediate detected. Photorheometry results demonstrate a correlation between shear moduli, joules light dose, and the lower limits of photocatalyst concentration required for the liquid-to-biorubber transition. Adhesion strength on ex vivo hydrated tissues exceeds that of cyanoacrylates, with a fixation strength of up to 20 kg·f·cm². Preliminary toxicity assessment on leachates and materials directly in contact with mammalian fibroblast cells displays no signs of fibroblast cytotoxicity.

Dry vs. wet: Properties and performance of collagen films. Part I. Mechanical behaviour and strain-rate effect

Collagen forms one-third of the body proteome and has emerged as an important biomaterial for tissue engineering and wound healing. Collagen films are used in tissue regeneration, wound treatment, dural substitute etc. as well as in flexible electronics. Thus, the mechanical behaviour of collagen should be studied under different environmental conditions and strain rates relevant for potential applications. This study's aim is to assess the mechanical behaviour of collagen films under different environmental conditions (hydration, submersion and physiological temperature (37 °C)) and strain rates. The combination of all three environment factors (hydration, submersion and physiological temperature (37 °C)) resulted in a drop of tensile strength of the collagen film by some 90% compared to that of dry samples, while the strain at failure increased to about 145%. For the first time, collagen films were subjected to different strain rates ranging from quasi-static (0.0001 s^-1) to intermediate (0.001 s^-1, 0.01 s^-1) to dynamic (0.1 s^-1, 1 s^-1) conditions, with the strain-rate-sensitivity exponent (m) reported. It was found that collagen exhibited a strain-rate-sensitive hardening behaviour with increasing strain rate. The exponent m ranged from 0.02-0.2, with a tendency to approach zero at intermediate strain rate (0.01 s^-1), indicating that collagen may be strain-rate insensitive in this regime. From the identification of hyperelastic parameter of collagen film, it was found that the Ogden Model provides realistic results for future simulations.

Production of Biocompatible Protein Functionalized Cellulose Membranes by a Top-Down Approach

Protein functionalized cellulose fibrils were isolated from the tunic of Pyura chilensis and subsequently used to produce protein functionalized cellulose membranes. Bleached cellulose membranes were also obtained and used as reference material. FTIR and Raman spectroscopy demonstrated that the membranes are mostly constituted of cellulose along with the presence of residual proteins and pigments. Protein functionalized cellulose membranes were found to possess ∼3.1% of protein at their surface as measured by X-ray photoelectron spectroscopy. Powder X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis were used to identify and semiquantify the amount of residual sand grains present within the structure of the membranes. The presence of residual proteins was found not to significantly affect the tensile mechanical properties of the membranes. Streaming ζ-potential was used to assess surface charges of the membranes. Below pH 4, nonbleached cellulose membranes possessed highly negative surfaces charges and also significantly less negative surface charges at physiological pH when compared to bleached cellulose membranes. No significant difference was found with respect to growth kinetics of myoblasts at the surface of the membranes for cell culturing times of 48 and 72 h. After 48 h of culture, protein functionalized cellulose-based membranes that possess ∼3.1% of proteins at their surface (H1) were, however, found to promote higher cell density, cell spreading, and more orientated shape cell morphology when compared to the other cellulose-based membranes (H3 and B) evaluated in the present study.

Non-aqueous, tissue compliant carbene-crosslinking bioadhesives

Surgical adhesives are an attractive alternative to traditional mechanical tissue fixation methods of sutures and staples. Ease of application, biocompatibility, enhanced functionality (drug delivery) are known advantages but weak adhesion strength in the wet environment and lack of tissue compliant behavior still pose a challenge. In order to address these issues, non-aqueous bioadhesive based on blends of polyamidoamine (PAMAM) dendrimer, conjugated with 4-[3-(trifluoromethyl)-3H-diazirin-3-yl] benzyl bromide (PAMAM-g-diazirine) and liquid polyethylene glycol (PEG 400) has been developed. PEG 400 biocompatible solvent reduces the viscosity of PAMAM-g-diazirine dendrimer without incorporating aqueous solvents or plasticizers, allowing application by syringe or spray. Upon UV activation, diazirine-generated reactive intermediates lead to intermolecular dendrimer crosslinking. The properties of the crosslinked matrix are tissue compliant, with anisotropic material properties dependent on the PEG 400 wt%, UV dose, pressure and uncured adhesive thickness. The hygroscopic PAMAM-g-diazirine/PEG 400 blend was hypothesized to absorb water at the tissue interface, leading to high interfacial adhesion, however porous matrices led to cohesive failure. The hydrophilic nature of the polyether backbone (PEG 400) shielded cationic PAMAM dendrimers with cured bioadhesive film displaying significantly less platelet activation than neat PAMAM-g-diazirine or PLGA thin films.

On-Demand Bioadhesive Dendrimers with Reduced Cytotoxicity

Tissue adhesives based on polyamidoamine (PAMAM) dendrimer, grafted with UV-sensitive aryldiazirine (PAMAM-g-diazirine) are promising new candidates for light active adhesion on soft tissues. Diazirine carbene precursors form interfacial and intermolecular covalent crosslinks with tissues after UV light activation that requires no premixing or inclusion of free radical initiators. However, primary amines on the PAMAM dendrimer surface present a potential risk due to their cytotoxic and immunological effects. PAMAM-g-diazirine formulations with cationic pendant amines converted into neutral amide groups were evaluated. In vitro toxicity is reduced by an order of magnitude upon amine capping while retaining bioadhesive properties. The in vivo immunological response to PAMAM-g-diazirine formulations was found to be optimal in comparison to standard poly(lactic-co-glycolic acid) (PLGA) thin films.

Self-assembled photoadditives in polyester films allow stop and go chemical release

Near-infrared (NIR) triggered chemical delivery allows on-demand release with the advantage of external tissue stimulation. Bioresorbable polyester poly-l-lactic acid (PLLA) was compounded with photoadditives of neat zinc oxide (ZnO) nanoparticles and 980→365nm LiYF₄:Tm³⁺, Yb³⁺ upconverting nanoparticles (UCNP). Subsequently, neat ZnO and UCNP blended PLLA films of sub-50μm thickness were knife casted with a hydrophobic small molecule drug mimic, fluorescein diacetate. The PLLA films displayed a 500 times increase in fluorescein diacetate release from the 50mW NIR irradiated PLLA/photoadditive film compared to non-irradiated PLLA control films. Larger ratios of UCNP/neat ZnO increased photocatalysis efficiency at low NIR duty cycles. The synergistic increase results from the self-assembled photoadditives of neat zinc oxide and upconverting nanoparticles (UCNPs), as seen in transmission electron microscopy. Colloidal ZnO, which does not self-assemble with UCNPs, had less than half the release kinetics of the self-assembled PLLA films under similar conditions, advocating Förster resonance energy transfer as the mechanism responsible for the synergistic increase. Alternative to intensity modulation, pulse width modulation (duty cycles from 0.1 to 1) of the low intensity 50mW NIR laser diode allowed tailorable release rates from 0.01 to 1.4% per day. With the low intensity NIR activation, tailorable release rates, and favorable biocompatibility of the constituents, implanted PLLA photoadditive thin films could allow feedback mediated chemical delivery.

STATEMENT OF SIGNIFICANCE

Upconverting nanoparticles and zinc oxide nanorods were found to spontaneously self-assemble into submicron particles in organic solvents. Exposure of the submicron particles to near-infrared light allows stop and go chemical release from biocompatible polymers. Sample preparation of thin films is done with ease through physical mixing of the photoadditives followed by air-dried knife casting. A colloidal ZnO variant that does not self-assemble with upconverting nanoparticles had slower chemical release, suggesting that synergistic chemical release is brought upon by highly efficient energy transfer mechanisms when the nanoparticles are less than 10nm apart. Never before seen composite particles of UCNP/ZnO are displayed, which shows the close interaction of the photoadditives within the polymer matrix.

Aligned collagen-GAG matrix as a 3D substrate for Schwann cell migration and dendrimer-based gene delivery

The development of artificial off-the-shelf conduits that facilitate effective nerve regeneration and recovery after repair of traumatic nerve injury gaps is of fundamental importance. Collagen-glycosaminoglycan (GAG) matrix mimicking Schwann cell (SC) basal lamina has been proposed as a suitable and biologically rational substrate for nerve regeneration. In the present study, we have focused on the permissiveness of this matrix type for SC migration and repopulation, as these events play an essential role in nerve remodeling. We have also demonstrated that SCs cultured within collagen-GAG matrix are compatible with non-viral dendrimer-based gene delivery, that may allow conditioning of matrix-embedded cells for future gene therapy applications.