Colorimetric Properties of Bombyx mori Silk Fibroin.

Recent reports highlighted several novel applications for the Bombyx mori silk fibroin (SF), as edible coatings for the preservation of food freshness, smart labels, or packaging materials. This study complements these reports and additionally describes the colorimetric sensing properties of the natural protein that could be explored to enhance the practical value of such applications. Our data show that in response to pH changes, reconstituted SF is able to undergo visible color changes that correlate with the intensity of the stimuli, regardless of its physical format or physical cross-linking state. The intensity of the developed color was proportional to the extent of the protein's hydrolytic degradation. We also found that these pH-driven color changes were reversible and interchangeable, with colorless samples at neutral pH, purple in acidic environments, and yellow under basic conditions. Our mechanistic studies identified tryptophan as being responsible for these colorimetric responses, which could be further intensified by the presence of ionized tyrosine functionalities. In addition, we determined that SF's sensing properties also applied to ultraviolet light exposure. Finally, we showed that the innate sensing capabilities of activated SF can be enhanced via the covalent incorporation of additional tryptophan into the protein. Overall, our results further support the utility of SF for sensing applications.

Manufacturability of a Tetraethyl Orthosilicate-Based Hydrogel for Use as a Single Application Otitis Externa Therapeutic.

Otitis externa, also known as outer ear infection, is a frequent affliction in both humans and animals. The most prevalent treatment for otitis externa is ear drops, but it is difficult to adhere properly to this treatment, causing poor patient compliance and the potential for complications. As a result, we have developed a tetraethyl orthosilicate-based hydrogel for use as single application treatment for otitis externa to increase ease of use and improve patient outcomes. Herein, we investigated the manufacturability of the hydrogel, focusing on several key aspects: formulation repeatability and reproducibility, material source and tolerances, release of a variety of model drugs, and impact of application-specific physiological factors, specifically local pH and enzymatic activity on drug release. Overall, our results indicate that these hydrogels are well suited for production and scalability, as they have a robust manufacturing process, have a wide tolerance for pH level, release a variety of model drugs, and are not impacted by outer ear canal-specific physiological factors.

Single Application Cold-Chain Independent Drug Delivery System for Outer Ear Infections.

Outer ear infections (OE) affect millions of people annually with significant associated healthcare costs. Incorrect administration or non-compliance with the treatment regimen can lead to infection persistence, recurrence, antibiotic resistance, and in severe cases aggravation to malignant otitis externa. Such issues are particularly pertinent for military personnel, patients in nursing homes, the geriatric population, for patients with head or hand tremors and for those with limited or no access to proper healthcare. With the intent of using traditional material science principles to deconvolute material design while increasing relevance and efficacy, we developed a single application, cold-chain independent thixotropic drug delivery system. This can be easily applied into the ear as a liquid, then gels to deliver effective concentrations of antibiotics against bacterial strains commonly associated with OE. The system maintains thixotropic properties over several stress/no stress cycles, shows negligible swelling and temperature dependence, and does not impact the minimum inhibitory concentration or bactericidal effects of relevant antibiotics. Moreover, the thixogels are biocompatible and are well tolerated in the ear. This drug delivery system can readily translate into a user-friendly product, could improve compliance via a single application by the diagnosing health care provider, is expected to effectively treat OE and minimize the development of antibiotic resistance, infection recurrence or exacerbation.

Recombinant Silk Fibroin Crystalline Regions as Biomaterial Alternatives to the Full-Length Protein.

Silk fibroin is a natural polymer with a unique repetitive structure that translates to extraordinary properties in terms of processability and mechanical properties. The Bombyx mori silk has a molecular weight of ∼415 kDa and consists of a light chain and a heavy chain. Its heavy chain is organized into 12 crystalline domains. Each of these crystalline domains contains subdomains of ∼70 amino acid containing blocks. It is well understood that the heavy chain of the protein is responsible for its processing versatility and excellent mechanical properties; however, the need for the high number of monomeric repeating units is unclear, and the individual properties of crystalline regions compared to those of the full-length protein are not understood. The work described herein assessed the possibility of using recombinant crystalline regions as alternative biomaterials for applications such as tissue adhesives. Our results indicate that while the two tested substructures do not fully recapitulate the native silk fibroin's properties, they appear to be a suitable alternative for the production of silk-based medical adhesives.

Tetraethyl Orthosilicate-Based Hydrogels for Drug Delivery-Effects of Their Nanoparticulate Structure on Release Properties.

Tetraethyl orthosilicate (TEOS)-based hydrogels, with shear stress response and drug releasing properties, can be formulated simply by TEOS hydrolysis followed by volume corrections with aqueous solvents and pH adjustments. Such basic thixotropic hydrogels (thixogels) form via the colloidal aggregation of nanoparticulate silica. Herein, we investigated the effects of the nanoparticulate building blocks on the drug release properties of these materials. Our data indicate that the age of the hydrolyzed TEOS used for the formulation impacts the nanoparticulate structure and stiffness of thixogels. Moreover, the mechanism of formation or the disturbance of the nanoparticulate network significantly affects the release profiles of the incorporated drug. Collectively, our results underline the versatility of these basic, TEOS-only hydrogels for drug delivery applications.

Mechanistic insights into silk fibroin's adhesive properties via chemical functionalization of serine side chains.

Bombyx mori-derived silk fibroin (SF) has recently gained interest for its intrinsic or engineered adhesive properties. In a previous study by our group, the mechanism of the protein's intrinsic adhesiveness to biological substrates such as leather has been hypothesized to rely on hydrogen bond formation between amino acid side chains of SF and the substrate. In this study, the serine side chains of SF were chemically functionalized with substituents with different hydrogen bonding abilities. The effect of these changes on adhesion to leather was investigated along with protein structural assessments. The results confirm our hypothesis that adhesive interactions are mediated by hydrogen bonds and indicate that the length and nature of the side chains are important for both adhesion and secondary structure formation.

A Controlled Antibiotic Release System for the Development of Single-Application Otitis Externa Therapeutics.

Ear infections are a commonly-occurring problem that can affect people of all ages. Treatment of these pathologies usually includes the administration of topical or systemic antibiotics, depending on the location of the infection. In this context, we sought to address the feasibility of a single-application slow-releasing therapeutic formulation of an antibiotic for the treatment of otitis externa. Thixotropic hydrogels, which are gels under static conditions but liquefy when shaken, were tested for their ability to act as drug controlled release systems and inhibit Pseudomonas aeruginosa and Staphylococcus aureus, the predominant bacterial strains associated with outer ear infections. Our overall proof of concept, including in vitro evaluations reflective of therapeutic ease of administration, formulation stability, cytocompatibility assessment, antibacterial efficacy, and formulation lifespan, indicate that these thixotropic materials have strong potential for development as otic treatment products.