Cross-plane thermal transport in micrometer-thick spider silk films

Abstract

This work reports on the first study of thermal transport capacity in the thickness direction (∼μm scale) for spider silk films. Fresh (minimally processed) and hexafluoroisopropanol (HFIP) films of Nephila clavipes and Latrodectus hesperus major ampullate silk are studied. Detailed Raman spectroscopy reveals that the fresh films have more crystalline secondary protein structures such as antiparallel β-sheets than the HFIP films for N. clavipes. For N. clavipes, the randomly distributed antiparallel β-sheets in fresh films have nearly no effect in improving thermal conductivity in comparison with HFIP films. For L. hesperus, the films mainly consist of α-helices and random coils while the fresh film has a higher concentration of α-helices. The higher concentration of α-helices in fresh films gives rise to a higher heat capacity than HFIP films, while the thermal conductivity shows little effect from the α-helices concentration. Thickened HFIP films are heated at different temperatures to study the effect of heat treatment on structure and thermal transport capacity. These experiments demonstrate that α-helices are formed by thermal treatment and that thermal effusivity increases with the appearance of α-helices in films.

Introduction

For over 50 years, spider silk has attracted significant attention due to its outstanding mechanical properties. For example, with tensile strengths as high as 1.75 GPa and elongations of 26%–35% [1], [2], [3], some spider silks surpass the toughness of steel. Silk also behaves like rubber on a weight to weight basis and can be two to three times as tough as Nylon or Kevlar [4]. In addition to these superb features, with its biocompatibility and biodegradability, spider silk offers further advantages over inorganic polymers. As early as 1901, spider silk was described to be absorbable in the human body and cause low inflammation. It has also been substituted for cat-gut sutures and has become a new biomaterial for other medical applications [5], [6].

Taking advantage of its excellent mechanical properties and biocompatibility, spider silk can be used in tissue engineering. Although many other artificial polymers were produced and developed a few decades ago, spider silk outperforms almost all synthetic materials [7] due to its combination of mechanical strength and elasticity [8]. Moreover, the biomedical functionality of this material could be deployed for applications in tissue replacement [9], [10], suture [6], [11], drug carrier [9], ligament/tendon tissue [12], biomaterial scaffold [13], [14], and artificial blood vessels [5].

Compared with other kinds of fibers, the preeminent properties of spider silk come from its unique internal structure. A spider produces more than one type of silk, however, dragline silk is the most widely studied and has more desirable mechanical properties than others. Dragline silk, synthesized in the major ampullate glands in the abdomen of a spider, is composed of many parallel fibrils [15], [16], [17]. Spidroins (spider fibroins) are the main component of a silk fibril, and dragline silk in particular is composed of two spidroins, major ampullate spidroin 1 (MaSp1) and major ampullate spidroin 2 (MaSp2). In major ampullate silk, antiparallel β-sheets and random coils are the main secondary structures. The synthesis of dragline silk happens at the tail of the gland within specialized cells and then the silk proteins are stored in high concentration inside of the glands lumens as a liquid crystalline solution [18]. The liquid silk forms antiparallel β-sheets during spinning.

One approach to study the structure and properties of spider silk is to dissolve the silk protein in a solution, make a coating material to understand how the structure determines the physical properties, and then manipulate the structure [19]. Transmission electron microscopy (TEM) [20] has a higher resolution than standard optical microscopy making it very useful to observe the internal structure of spider silk film at the nanometer-scale. Fourier transform infrared spectroscopy (FTIR) [20], [21] can characterize detailed chemical bonds in spider silk proteins. Circular dichroism (CD) spectroscopy [21], [22] can analyze the α-helices and the antiparallel β-sheets conformation of spider silk protein in a solvent.

In addition to the above mentioned techniques, Raman spectroscopy is a powerful method to characterize the internal structure of spider silks and it has been employed in many studies [19]. Most Raman spectra of different silk samples from various spiders show two major peaks about amide III (1220–1279 cm⁻¹) and amide I (1650–1680 cm⁻¹). These represent antiparallel β-sheets, which silks from silkworm also have. These two peaks have their own distinct locations when they are in the antiparallel β-sheets and their Raman wavenumbers shift when they are in random coils.

In spider silks, the intrinsic thermal transport capacity is strongly determined by molecular weight, structure, crystallinity and alignment. For example, defects are the main source of reduction in strength and thermal conductivity. Under the same measurement condition, better internal structures (e.g., less defect, higher crystallinity, and better alignment) will lead to higher thermal transport properties. Therefore, thermal diffusivity and conductivity can be used as signatures to reflect the protein structures of spider silks. These thermal transport properties can complement the structural information determined by other techniques (e.g., XRD, SEM, FTIR), and provide new perspectives and understanding of the structure regularity and energy coupling in spider silk, as well as other synthetic and natural polymers. Unfortunately, very little research has been done on the thermal transport capacity in spider silks, thus, there has been very little use of this property to characterize its structure variation. According to Huang's discovery, the observed exceptionally high thermal conductivity of spider silk, from 348.7 ± 33.4 to 415.9 ± 33.0 W/m·K, is largely attributed to its extraordinary well-organized and less defective structures formed from strong self-assembly [23].

This work is focused on films made from native spider silk protein (major ampullate) that have either been cast directly from freshly dissected glands or from glands dissolved in hexafluoroisopropanol, HFIP. Two spider species are studied: Nephila clavipes (golden orb-weaver) and Latrodectus hesperus (Western black widow). The structure of original samples and heat treated HFIP films are studied and correlated with the thermophysical property change, in anticipation of revealing the unique structure of spider silk films and how energy transport is achieved. Additionally, the photothermal (PT) technique is used to characterize the thermophysical properties along the thickness direction of the films of interest.