Donna M. Ebenstein

Nanoindentation of biological materials

Nanoindentation has recently emerged as a powerful tool for measuring nano- and microscale mechanical properties in tissues and other biomaterials. This technique has been used to measure the mechanical properties of microstructural features in bone and teeth, investigate variations in mechanical properties with changes in tissue organization or composition in mineralized and soft tissues, and map mechanical properties spatially in complex biomaterials. Continuing advancements in indentation data analysis will increase the methods utility in the characterization of biomaterials.

Eliminating Adhesion Errors in Nanoindentation of Compliant Polymers and Hydrogels

Nanoindentation is a valuable tool for characterization of biomaterials due to its ability to measure local properties in heterogeneous, small or irregularly shaped samples. However, applying nanoindentation to compliant, hydrated biomaterials leads to many challenges including adhesion between the nanoindenter tip and the sample. Although adhesion leads to overestimation of the modulus of compliant samples when analyzing nanoindentation data using traditional analysis techniques, most studies of biomaterials have ignored its effects. This paper demonstrates two methods for managing adhesion in nanoindentation analysis, the nano-JKR force curve method and the surfactant method, through application to two biomedically-relevant compliant materials, poly(dimethyl siloxane) (PDMS) elastomers and poly(ethylene glycol) (PEG) hydrogels. The nano-JKR force curve method accounts for adhesion during data analysis using equations based on the Johnson-Kendall-Roberts (JKR) adhesion model, while the surfactant method eliminates adhesion during data collection, allowing data analysis using traditional techniques. In this study, indents performed in air or water resulted in adhesion between the tip and the sample, while testing the same materials submerged in Optifree Express(®) contact lens solution eliminated tip-sample adhesion in most samples. Modulus values from the two methods were within 7% of each other, despite different hydration conditions and evidence of adhesion. Using surfactant also did not significantly alter the properties of the tested material, allowed accurate modulus measurements using commercial software, and facilitated nanoindentation testing in fluids. This technique shows promise for more accurate and faster determination of modulus values from nanoindentation of compliant, hydrated biological samples.

Multiscale characterization of acrylic bone cement modified with functionalized mesoporous silica nanoparticles.

Acrylic bone cement is widely used to anchor orthopedic implants to bone and mechanical failure of the cement mantle surrounding an implant can contribute to aseptic loosening. In an effort to enhance the mechanical properties of bone cement, a variety of nanoparticles and fibers can be incorporated into the cement matrix. Mesoporous silica nanoparticles (MSNs) are a class of particles that display high potential for use as reinforcement within bone cement. Therefore, the purpose of this study was to quantify the impact of modifying an acrylic cement with various low-loadings of mesoporous silica. Three types of MSNs (one plain variety and two modified with functional groups) at two loading ratios (0.1 and 0.2wt/wt) were incorporated into a commercially available bone cement. The mechanical properties were characterized using four-point bending, microindentation and nanoindentation (static, stress relaxation, and creep) while material properties were assessed through dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, FTIR spectroscopy, and scanning electron microscopy. Four-point flexural testing and nanoindentation revealed minimal impact on the properties of the cements, except for several changes in the nano-level static mechanical properties. Conversely, microindentation testing demonstrated that the addition of MSNs significantly increased the microhardness. The stress relaxation and creep properties of the cements measured with nanoindentation displayed no effect resulting from the addition of MSNs. The measured material properties were consistent among all cements. Analysis of scanning electron micrographs images revealed that surface functionalization enhanced particle dispersion within the cement matrix and resulted in fewer particle agglomerates. These results suggest that the loading ratios of mesoporous silica used in this study were not an effective reinforcement material. Future work should be conducted to determine the impact of higher MSN loading ratios and alternative functional groups.

Mechanical and Chemical Characteristics of Mineral Produced by Basic Fibroblast Growth Factor-Treated Bone Marrow Stromal Cells in Vitro

It has been shown that various organ and cell cultures exhibit increased mineral formation with the addition of basic fibroblast growth factor (bFGF) and phosphate ions in the medium. However, to date there has been no attempt to relate the chemical composition of mineral formed in vitro to a measure of its mechanical properties. This information is important for understanding the in vivo mineralization process, the development of in vitro models, and the design of tissue-engineered bone substitutes. In this study we examined the reduced modulus; hardness; and mineral-to-matrix, crystallinity, carbonate-to-mineral, and calcium-to-phosphorus ratios of mineral formed by bFGF-treated rat-derived bone marrow stromal cells in vitro. The cells were treated with 1 or 3 mM beta-glycerophosphate for 3 and 4 weeks. Both mechanical parameters, reduced modulus and hardness, increased with increasing beta-glycerophosphate concentration. The only chemical measure of the mineral composition that exhibited the same dependency was the mineral-to-matrix ratio. The values of crystallinity and carbonate fraction were similar to those for intact cortical bone, but the calcium-to-phosphorus ratio was substantially lower than that of normal bone. These data indicate that the mineral formed by bFGF-treated bone cells is mechanically and chemically different from naturally formed lamellar bone tissue after 4 weeks in culture. These results can be used to improve in vitro models of mineral formation as well as enhance the design of tissue-engineered bone substitutes.

Characterization of dermal plates from armored catfish Pterygoplichthys pardalis reveals sandwich-like nanocomposite structure

Dermal plates from armored catfish are bony structures that cover their body. In this paper we characterized structural, chemical, and nanomechanical properties of the dermal plates from the Amazonian fish Pterygoplichthys pardalis. Analysis of the morphology of the plates using scanning electron microscopy (SEM) revealed that the dermal plates have a sandwich-like structure composed of an inner porous matrix surrounded by two external dense layers. This is different from the plywood-like laminated structure of elasmoid fish scales but similar to the structure of osteoderms found in the dermal armour of some reptiles and mammals. Chemical analysis performed using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) results revealed similarities between the composition of P. pardalis plates and the elasmoid fish scales of Arapaima gigas. Reduced moduli of P. pardalis plates measured using nanoindentation were also consistent with reported values for A. gigas scales, but further revealed that the dermal plate is an anisotropic and heterogeneous material, similar to many other fish scales and osteoderms. It is postulated that the sandwich-like structure of the dermal plates provides a lightweight and tough protective layer.

Nanomechanical and Microstructural Properties of Bombyx mori Silk Films

Instrumented indentation and micro-Raman spectroscopy were used to investigate the mechanical and structural properties of Bombyx mori silk films. Twelve different films were prepared from B. mori silk fibroin protein using a variety of post-deposition processing treatments (e.g. soaking in methanol, soaking in water, stretching, and/or enzymatic etching). The results show that different treatments lead to changes in both the conformation of the silk fibroin protein and the mechanical properties of the films.

Comparison of Artificial Saliva vs Saline Solution on Rate of Suture Degradation in Oropharyngeal Surgery

Importance Absorbable sutures are designed to degrade and lose strength over time. Manufacturers warn that exposure to various body fluids can change the estimated degradation rate of these sutures, but few studies have been conducted to quantify the degree of change associated with saliva. Objective To quantify the association of increased loss of strength of sutures over time after exposure to artificial saliva (hereinafter referred to as “saliva”). Design, Setting, and Participants This experimental in vitro study was conducted at Bucknell University (Lewisburg, Pennsylvania) from June 19, 2015, to July 4, 2015. No participants were involved. The loss of strength over time of sutures submerged in physiological saline and artificial saliva solutions was compared. Three types of absorbable sutures commonly used in oral surgery were tested: chromic, poliglecaprone 25, and polyglactin 910. Data analysis was conducted from July 15, 2016, to August 16, 2016. Main Outcomes and Measures The primary outcome measure was 50% strength reduction. To measure breaking strength, 6 knotted sutures of each type were pulled to failure at regular time intervals after immersion in either saline or synthetic saliva at 37°C. Regression analysis was used to interpret strength degradation profiles and to estimate the time to reach 50% of the original breaking strength. Results Of the 3 suture types submerged in the 2 solutions, all 3 degraded to 50% strength faster (by 2 to 13 days) in saliva than in saline. The differences in the degradation profiles varied by suture type. Poliglecaprone 25 sutures demonstrated a sudden decrease in failure strength between day 5 and day 8 in both solutions, but the decrease was greater in saliva (–10.2 N; 95% CI, –15.5 to –4.9 N) than in saline (–6.1 N; 95% CI, –11.2 to –0.9 N). The polyglactin 910 and chromic sutures share a similar degradation profile when implanted in tissue, but saliva was associated with more degradation of chromic sutures. Differences in degradation rate were seen in polyglactin 910 sutures after day 6 (saline: –0.9 N/d; 95% CI, –1.0 to –0.7 vs saliva: –1.2 N/d; 95% CI, –1.4 to –1.1). After day 2, chromic sutures had a degradation rate of –0.3 N/d (95% CI, –0.5 to –0.2) in saline and –0.5 N/d (95% CI, –0.6 to –0.3) in saliva. Conclusions and Relevance Knowing the association of saliva with suture degradation rates of various suture types may enable oropharyngeal surgeons to select sutures that retain their strength and degrade at an appropriate rate to allow for the effective healing of the wound.

A Nanoindentation Method for Hydrated Compliant Materials

Nanoindentation is becoming an increasingly popular tool in the biomaterials field due to its ability to measure local mechanical properties in small, irregularly-shaped or heterogeneous samples.1 Although this technique was readily adapted to the study of mineralized tissues, the application of nanoindentation to compliant, hydrated biomaterials such as soft tissues and hydrogels has led to many challenges.1 Three key concerns associated with nanoindentation of compliant, hydrated materials are inaccurate surface detection, errors due to adhesion forces, and fluid interactions with the tip.1–4Copyright