The horn of the rhino consists of hairs tightly packed in the bulk of the protuberance and more loosely arranged at the outer shell (Fig. 1). The matrix material filling between the hairs is a very dense packing of cornified dead skin keratocyte cells that can be heavily pigmented with melanin3. Melanin is an interesting pigment that not only provides black colour but may also add to a material’s structural integrity10. Thus the native rhinoceros horn in essence is a composite material, structured by its growth, with the tubules of keratin hair forming ‘fibres’ that are embedded in a matrix material that may change in composition along and/or across the horn11. Throughout the rhinoceros horn each hair filament retains much of its natural hair structure including the medullary cavity although it is lacking the outermost layers of scaly cuticle so typical for external hairs3 (Figs 1B,C and 2A,C).

Figure 1 Schematic of Black Rhinoceros (Diceros bicornis) horn showing a section of horn with the hair tubules. The rhino head drawing is by Jonathan Kingdon (reproduced with permission). A single hair is circa 200 µm in diameter (length-section B, cross-section C). Full size image

Figure 2 Images of cross-section of a real rhino horn (A,C) and an artificial horn (B,D). We note that the hair filament density of our artificial rhino horn is about 9 mm−2, which is close to that (7 mm−2) of real horns11. Full size image

As the key structural material for the manufacture of our artificial rhino horns we used horsetail hair because of its phylogenetic origin (which suggests comparable chemical keratin composition) and its homologous morphological structure (which suggests comparable mechanical properties). Importantly, horsetail hairs also share with rhino horn hairs comparable dimensions, circular symmetry and spongy core structure (Figs 2 and 3). In order to copy the key feature separating the two, the lack of the outermost scaly layers in the rhino horn hairs, we used a Lithium Bromide (LiBr) wash to etch and remove the outer layer of the horsehair. We note that this treatment also facilitated the adhesion between the hair fibres and the matrix material that we used.

Figure 3 SEM Images of both natural and faux Rhino Hair Horn. The natural horn (upper row) and our faux horn (lower row) show a length section (a,d) and two cross sections in two different magnifications (b,e,c,f). Note that not all hairs are perfectly circular, while their partial disintegration is probably due to vacuum induced dehydration. Full size image

As there is no detailed information on the composition of the rhino’s nose-tip exudate and horn matrix material other than that it seems to be a sebatious gland exudate full of deceased highly melanised cells10. Such cells would contain high levels of intra-cellular proteins as well as carrying along the rather adhesive extra-cellular fibronectin glycoprotein. Thus the matrix of the native rhino horn would in essence be a largely proteinaceous glue with inclusions of soil and plant sap where the animal has rubbed the growing horn. Assuming such a highly proteinaceous and sticky horn matrix we used for this function in our faux horns the RSF silk fibroin, which we know how to prepare and deploy12. Importantly, the RSF material we used can also easily be moulded and cured into a tough matrix to fill-in between the horse-tail hairs.

By bundling the LiBr washed hairs as tightly as possible while infusing them with the RSF solution we were able to create solid composite cylinders of hair-horn. The smaller horn (around 4 cm diameter and 10 cm length) cured within a few days while the largest one (around 12 cm diameter and 35 cm length) took weeks in the vacuum oven to dry. The smaller ones, which were our focus for analysis, filed and polished very nicely into surfaces rather similar, indeed confusingly similar, to surfaces of native rhino horn naturally polished by rubbing. If carefully polished a faux horn could thus be easily modified to resemble the outside of a rhino horn. On the microscopic level our Light and Scanning-Electron Microscopy confirmed that not only the gross morphology and anatomy of the faux horn but also the more detailed fine structure was similar to those of real rhino horn.

Importantly for our more fundamental interests in the novel material, rather than the more superficial copying of structures, was the analysis of its material qualities. To this effect we used DSC and TGA to investigate the similarity of the thermal properties between samples of our artificial horns and the real horn.

Differential Scanning Calorimetry (DSC) is a thermo-analytical technique comparing the heat required to increase the temperature of a sample and a reference allowing us to study physical transformations such as phase transitions and determine whether the process is exothermic or endothermic as well as indicating a glass transition. Thermal Gravimetric Analysis (TGA), on the other hand, measures the mass of a single sample as it changes with temperature over time. This data allows us to probe not only physical phenomena such as phase transitions between the solid, liquid and gaseous states of the various components of the material studied but also chemical phenomena such as thermal decomposition and reactions between surfaces.

As shown in Fig. 4(a) the DSC analysis demonstrated that both materials were surprisingly similar with that peaks at 100 °C indicating the insipient moisture of the samples while the broad endothermic peak from about 200 °C to 400 °C indicates the degradation of the protein. The data in Fig. 4(b) on the other hand, shows that both real and artificial horns started to decompose approximately at 200 °C with final residues of 1.5 wt% and 1.3 wt%, respectively.

Figure 4 Thermal, spectral and Mechanical testing of the real rhino horns and artificial horn copies. The TGA tests (a), the DSC tests (b), the DMTA frequency sweep (c) and strain sweep (d), the FTIR Spectroscopy result (e). Full size image

To further probe the chemical composition and properties of both materials we applied the non-destructive method of FT-IR spectroscopy and compared the absorption bands of key constituent molecules of the both artificial and natural horn material8. The absorption bands observed (Fig. 4) at 1650 cm−1 and 3050 cm−1 were assigned to C=O stretching and N-H stretching. 3270 cm−1 belongs to O-H stretching vibration; 1540 cm−1 belongs to C=C stretching vibration. 1116 cm−1 was the S=O asymmetric stretching, while 1040 cm−1 was the S=O symmetric vibrations. 1450 cm−1 and 1240 cm−1 attributed to C-H bending and P=O stretching. Importantly, samples from both real and artificial horns showed very similar infrared spectra.

Perhaps the most interesting, because mechanically most important, measure of material properties was provided by Dynamic Mechanical Thermal Analysis (DMTA). Here a sample is continuously stress-strained and relaxed by tiny amounts to probe the underlying elastic and plastic properties of a material or composite. In the frequency sweep, samples were tested from 100 Hz to 0.1 Hz, and under that range materials were still in the liner region (Fig. 4). Thus, in the strain sweep, we applied the force with the frequency of 1 Hz, and the elastic modulus was 1.3 GPa, which meets the mechanical properties of natural rhino horns11. This similarity between rhinoceros horn and high-performance composites is not surprising; both materials are made up of stiff, inflexible fibres embedded into a flexible resin. The fibres break before they bend while the matrix bends before it breaks. The result is a composite that is able to withstand greater loads than either of its parts. When a stress is applied to the material, the matrix inhibits crack propagation and redistributes stress in the direction of the filaments.