Polyvinyl alcohol (PVA) is a hydrophilic, biodegradable, semicrystalline polymer with a wide array of commercial uses ranging from textiles and packaging to medicine. Film samples of PVA were ...investigated to assess crystallization and melting behavior during self-nucleation experiments and thermal degradation, using differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis, respectively. TG results show that degradation occurred at temperatures in excess of 200 °C which is close to the observed peak melting temperature of 223 °C. PVA was heated to various self-nucleation temperatures,
T
s
, within its melting range, and then cooled and reheated. Three distinct crystallization regimes were observed upon cooling, depending upon the self-nucleation temperature (
T
s
) selected. At low values of
T
s
,
T
s
< 227 °C, PVA only partially melts, and upon cooling the residual crystals anneal and become more stable. At intermediate values of
T
s
, 228 °C <
T
s
< 234 °C, PVA was found to crystallize exclusively from self-nucleation. For
T
s
> 235 °C, the PVA melts completely and absence of self-nucleation sites causes crystallization to occur at lower temperatures.
A porous material that is both hydrophobic and fouling-resistant is needed in many applications, such as water purification by membrane distillation. In this work, we take a novel approach to ...fabricating such membranes. Using the zwitterionic amphiphilic copolymer poly(trifluoroethyl methacrylate-random-sulfobetaine methacrylate), we electrospin nonwoven, porous membranes that combine high hydrophobicity with resistance to protein adsorption. By changing the electrospinning parameters and the solution composition, membranes can be prepared with a wide range of fiber morphologies including beaded, bead-free, wrinkly, and ribbonlike fibers, with diameters ranging between ∼150 nm and 1.5 μm. The addition of LiCl to the spinning solution not only helps control the fiber morphology but also increases the segregation of zwitterionic groups on the membrane surface. The resultant electrospun membranes are highly porous and very hydrophobic, yet resist the adsorption of proteins and retain a high contact angle (∼140°) even after exposure to a protein solution. This makes these materials promising candidates for the membrane distillation of contaminated wastewater streams and as self-cleaning materials.
Abstract The anti-parallel beta pleated sheet is a fundamental secondary structure in proteins and a major component in silk fibers generated by silkworms and spiders, with a key role to stabilize ...these proteins via physical cross-links. Importantly, these beta-sheets are fully degradable and nontoxic structures in biology, in contrast for example to beta-amyloid structures formed in disease states. Thus, insight into mechanism of enzymatic degradation would be instructive as a route to elucidating differences among these stable yet different structural features in biological systems. We report on the mechanism of enzymatic degradation of anti-parallel beta pleated sheets with Bombyx mori silk structures, leading to fibrils and subsequently to nanofilaments (2 nm thickness and 160 nm length). These nanofilaments play a role as nucleators of the crystalline regions, an important feature of the system that can be exploited to design silk-based biomaterials with predictable biodegradability and mechanical properties. The potential toxicity of degradation products from these proteolytic enzymes was also assessed in vitro and no cell toxicity found in vitro for the protease found in vivo in the human body. The degradation mechanism of beta-sheet silk crystals provides additional insight into the significant differences in biological impact between the anti-parallel beta-sheet silk biomaterials reported in this work vs. amyloid structures in disease states, adding to prior descriptions of chemical and structural differences that are more extensively documented.
Silk fibroin in material formats provides robust mechanical properties, and thus is a promising protein for 3D printing inks for a range of applications, including tissue engineering, bioelectronics, ...and bio-optics. Among the various crosslinking mechanisms, photo-crosslinking is particularly useful for 3D printing with silk fibroin inks due to the rapid kinetics, tunable crosslinking dynamics, light-assisted shape control, and the option to use visible light as a biocompatible processing condition. Multiple photo-crosslinking approaches have been applied to native or chemically modified silk fibroin, including photo-oxidation and free radical methacrylate polymerization. The molecular characteristics of silk fibroin, i.e., conformational polymorphism, provide a unique method for crosslinking and microfabrication via light. The molecular design features of silk fibroin inks and the exploitation of photo-crosslinking mechanisms suggest the exciting potential for meeting many biomedical needs in the future.
We present a simple and effective method to obtain refined control of the molecular structure of silk biomaterials through physical temperature-controlled water vapor annealing (TCWVA). The silk ...materials can be prepared with control of crystallinity, from a low content using conditions at 4 °C (α helix dominated silk I structure), to highest content of ∼60% crystallinity at 100 °C (β-sheet dominated silk II structure). This new physical approach covers the range of structures previously reported to govern crystallization during the fabrication of silk materials, yet offers a simpler, green chemistry, approach with tight control of reproducibility. The transition kinetics, thermal, mechanical, and biodegradation properties of the silk films prepared at different temperatures were investigated and compared by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), uniaxial tensile studies, and enzymatic degradation studies. The results revealed that this new physical processing method accurately controls structure, in turn providing control of mechanical properties, thermal stability, enzyme degradation rate, and human mesenchymal stem cell interactions. The mechanistic basis for the control is through the temperature-controlled regulation of water vapor to control crystallization. Control of silk structure via TCWVA represents a significant improvement in the fabrication of silk-based biomaterials, where control of structure−property relationships is key to regulating material properties. This new approach to control crystallization also provides an entirely new green approach, avoiding common methods that use organic solvents (methanol, ethanol) or organic acids. The method described here for silk proteins would also be universal for many other structural proteins (and likely other biopolymers), where water controls chain interactions related to material properties.
Structural Origins of Silk Piezoelectricity Yucel, Tuna; Cebe, Peggy; Kaplan, David L.
Advanced functional materials,
February 22, 2011, Letnik:
21, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Uniaxially oriented, piezoelectric silk films are prepared by a two‐step method that involves first air drying aqueous, regenerated silk fibroin solutions into films, and then drawing the silk films ...to a desired draw ratio. The utility of two different drawing techniques—zone drawing and water‐immersion drawing—is investigated for processing the silk for piezoelectric studies. Silk films zone drawn to a ratio of λ 5 2.7 display relatively high dynamic shear piezoelectric coefficients of d14 5 –1.5 pC N21, corresponding to an increase in d14 of over two orders of magnitude due to film drawing. A strong correlation is observed between the increase in silk II, β‐sheet content with increasing draw ratio as measured by FTIR spectroscopy (Cb $ \propto $ e2.5λ), the concomitant increasing degree of orientation of β‐sheet crystals detected via wide‐angle X‐ray diffraction (full width half maximum (FWHM) = 0.22° for λ = 2.7), and the improvement in silk piezoelectricity (d14 $ \propto $ e2.4λ). Water‐immersion drawing leads to a predominantly silk I structure with a low degree of orientation (FWHM 5 75°) and a much weaker piezoelectric response compared to zone drawing. Similarly, increasing the β‐sheet crystallinity without inducing crystal alignment, e.g., by methanol treatment, does not result in a significant enhancement of silk piezoelectricity. Overall, a combination of a high degree of silk II, β‐sheet crystallinity and crystalline orientation are prerequisites for a strong piezoelectric effect in silk. Further understanding of the structural origins of silk piezoelectricity provides important options for future biotechnological and biomedical applications of this protein.
Uniaxially oriented silk films exhibit high shear piezoelectric coefficient values. A combination of a high degree of silk II, β‐sheet crystallinity and crystallite orientation are prerequisites for a strong piezoelectric effect in silk. Further understanding of the structural origins of silk piezoelectricity provides important options for future biotechnological and biomedical applications of this protein.
•We studied 19 silk protein samples by fast scanning calorimetry using a Flash DSC1.•Samples were non-crystalline as-cast, or made non-crystalline after first melting.•Mass was evaluated one way, by ...using the known glassy state heat capacity.•Mass was evaluated another way, by using the known heat capacity increment at Tg.•We find (for T in K): Cpliquid(T)=(1.98±0.06)J/gK+T·(6.82±1.4)×10−4J/gK2.
In this technical note, we report a study of the heat capacity of amorphous silk fibroin protein evaluated using the Flash DSC1 for fast scanning calorimetry. Nineteen amorphous thin films were obtained either after casting directly from water solutions, or after melting of previously crystalline films. Fibroin films were mounted onto Flash DSC1 sensors, dried free of bound water, relaxed just above the glass transition, Tg, then scanned in heating and cooling at ±2000K/s. The heat flow rate data were analyzed by finding the symmetry line between the heating and cooling scans, and by subtraction of the empty sensor heat flow rate, similarly corrected for its symmetry line. To evaluate the sample mass, two approaches were compared. First, the mass was obtained from known solid state heat capacity, Cpsolid(T), at a temperature below Tg. Second, the mass was obtained from the known heat capacity increment at the glass transition, ΔCp(Tg). The Cpsolid(T) and ΔCp(Tg) had been obtained previously from slow scanning differential scanning calorimetry. The use of either method for mass determination necessitated additional corrections to the heat capacity data to bring them into agreement with the literature values. After these corrections, the heat capacity of silk fibroin in the liquid state was evaluated over a wide temperature range above Tg. We find Cpliquid(T)=(1.98±0.06)J/gK+T·(6.82±1.4)×10−4J/gK2 in the temperature interval from 510 to 570K with an uncertainty of about ±5%.
Beta-pleated-sheet crystals are among the most stable of protein secondary structures, and are responsible for the remarkable physical properties of many fibrous proteins, such as silk, or proteins ...forming plaques as in Alzheimer's disease. Previous thinking, and the accepted paradigm, was that beta-pleated-sheet crystals in the dry solid state were so stable they would not melt upon input of heat energy alone. Here we overturn that assumption and demonstrate that beta-pleated-sheet crystals melt directly from the solid state to become random coils, helices, and turns. We use fast scanning chip calorimetry at 2,000 K/s and report the first reversible thermal melting of protein beta-pleated-sheet crystals, exemplified by silk fibroin. The similarity between thermal melting behavior of lamellar crystals of synthetic polymers and beta-pleated-sheet crystals is confirmed. Significance for controlling beta-pleated-sheet content during thermal processing of biomaterials, as well as towards disease therapies, is envisioned based on these new findings.
Silk fibroin films cast from water solution, and containing bound water, are quantitatively studied in this work. First, to obtain the solid and liquid heat capacities of the pure dry silk fibroin, ...cyclic heat treatment was used to monitor the process of removing the bound water. After water removal, the glass transition of pure non-crystalline silk was observed at 451
K (178
°C). The solid and liquid heat capacities of the pure silk fibroin were then measured using differential scanning calorimetry (DSC), temperature-modulated DSC (TMDSC), and quasi-isothermal TMDSC, and found to be:
C
p
(
T)
solid
=
0.134
+
3.696
×
10
−3
T
J/g
K and
C
p
(
T)
liquid
=
0.710
+
3.47
×
10
−3
T
J/g
K over the temperature region from 200 to 450
K. These heat capacities were used to construct the underlying baseline heat capacity for the combined silk–water system.
When the combined silk–water system is studied, bound water is lost from the film during heating, and the loss of mass is quantified using thermogravimetric analysis (TGA). Bound water in the silk film acts as a plasticizer, and a lower glass transition of the silk–water system is observed. Comparison of the measured heat capacity of the silk–water system to the calculated total baselines was made in the vicinity of the water-induced glass transition. Results show that the total solid specific heat capacity is in good agreement with the calculated solid baseline in the low-temperature region below about 240
K. As temperature increases above the lower glass transition, all bound water eventually leaves the silk, and the free volume and the silk mobility are reduced. This allows the upper glass transition of the dried silk to be observed.
We report formation of biocompatible hydrogels using physically cross-linked biopolymers. Gelation of silk fibroin (from B. mori silkworm) aqueous solution was effected by ultrasonication and used to ...entrap blended, un-cross-linked, hyaluronic acid (HA) without chemical cross-linking. HA was formed into silk/HA blended hydrogels with different mixing ratios, forming homogeneous materials with stable swelling behavior when the HA content was less than 40 wt %. This is a novel approach to HA hydrogel systems, which otherwise require chemical cross-linking. Further, these systems exploit the beneficial material and biological properties of both polymers. Differential scanning calorimetry (DSC), temperature modulated DSC, and thermal gravimetric analysis were used to show that well-blended silk/HA hydrogel systems formed without macrophase separation. Fourier transform infrared spectroscopy was used to determine secondary structures from the amide I region of silk protein by spectral subtraction and Fourier-self-deconvolution. The β-sheet crystal fraction of the silk protein increased with increase of HA content (26−35 wt %), which resulted in stable, crystalline features in the blend hydrogel materials, favorable features to support human mesenchymal stem cell attachment and proliferation. Scanning electron microscopy was used to characterize morphology. β-Sheet content controlled the stability of the silk/HA hydrogel systems, with a minimum crystalline content needed to maintain a stable hydrogel system of ∼26 wt %. This value is close to the β-sheet content in pure silk fibroin hydrogels. These novel nonchemically cross-linked blend hydrogels may be useful for biomedical applications due to biocompatibility and the widespread utility of hydrogel systems. The attributes of HA in combination with the features of silk, offer a useful suite of properties, combining the mechanical integrity and slow degradation of silk with the control of water interactions and biological signaling of HA.
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