Titanium Fiber Plates with Suitable Elastic Modulus and Porous Structure Facilitate Bone Tissue Repair

Titanium (Ti) is one of the most commonly used biomedical materials in orthopedics and dentistry. The excellent biocompatibility of Ti with high bone affinity makes it a suitable material for biomedical applications. Nevertheless, the higher Young’s modulus of Ti (≈110 GPa) than that of the cortical bone (10-30 GPa) causes stress shielding, leading to bone embrittlement. Since stress shielding is unavoidable with the use of Ti plates, it is generally recommended to remove them from the bone after completion of bone repair. However, removal of the Ti plate involves many risks; bone formation around the plate poses difficulty in removal of the plate besides pain and infections due to surgery. Researchers at Department of Orthopaedic Surgery, Shinshu University School of Medicine, Mechanical Systems Engineering, Shinshu University, Faculty of Engineering, Institute for Biomedical Sciences, Interdisciplinary Cluster for Cutting Edge Research, Shinshu University and Department of Applied Physical Therapy, Shinshu University School of Health Sciences, Japan have suggested the use of Ti fiber plates in place of the conventional Ti plates. Since the Young’s modulus of Ti fiber plates are similar to that of the cortical bone, stress shielding effect can be minimized. In addition, the porous structure of Ti fiber plates could act as scaffolds and promotes cell attachment.

The Ti fiber plates were prepared using Ti fibers (ASTM Grade 1 with 99.52% purity) with a mean diameter of 20 μm and a mean length of 500 μm (Fig. 1(a)). The Ti fibers were molded into a plate by simultaneously applying a compression stress of 1000 MPa and a shearing load of 400 kN at room temperature (Fig. 1(b)) followed by sintering at 300 K. The resultant Ti fiber plates had a thickness of 0.2 mm, Young’s modulus of ≈30 GPa and uniform porous structure with 30–40% porosity and 60–80 μm pore diameter (Fig. 1(c). The utility of the plates for repair of bone fracture and bone tissue regeneration was evaluated under in vitro and in vivo conditions.

Fig. 1 (a) SEM image of Ti fibers; (b) schematic diagram of the process used for preparing Ti fiber plates; and (c) SEM image of the Ti fiber plate

In vitro test results indicate that the extent of osteoblast adhesion and cell proliferation on the Ti fiber plates are quite similar to that of the conventional Ti plates. However, the difference in expression levels of cell-adhesion-related genes between cells on the Ti fiber plates and the cells on conventional Ti plates, suggests the existence of a difference in the mode of cell adhesion at the gene level between these two plates. The unique 3D structure of the Ti fiber plate is considered responsible for the increased level of osteoblast adhesion than those observed for the conventional titanium plates with a simple planar structure. In vivo study in rabbits with comminuted fracture at the center of the ulnar stem indicates that placing the titanium fiber plate in close contact with the fractured bone helps to immobilize and repair of small bone fragments.

Fig. 2 (a) Fixing of titanium fiber plate to the ulna using miniature screws for the repair of comminuted fracture (arrow mark) in rabbits; (b) Scout radiograms and (c) μCT images taken at Week 4 post-operation indicate complete bone union in the titanium fiber plate group but not with the control group

Unlike the conventional Ti plate, the Ti fiber plates could be easily prepared by compressing Ti fibers at room temperature without changing the fiber shape, which makes the process cost-effective and commercially viable. By suitably altering the length and thickness of the Ti fibers as well as the extent of compression and shear stress, Ti fiber plates with varying thickness, surface properties, porosity, Young’s modulus and strength can be prepared. Due to its malleability, the Ti fiber plates can be manually reshaped into a curved 3D structure and customized to the required size and shape of the fixation site for bone regeneration. The porous structure of Ti fibers with 30–40% porosity and 60–80 μm pore diameter is considered to be suitable for bone regeneration. Since the Young’s modulus of Ti fiber plate is similar to that of cortical bone, the deleterious stress shielding effect can be minimized and hence, it can remain implanted even after the fracture is healed. The use of pure Ti fibers ensures a better biosafety.

The Ti fiber plate is easy to deform manually and hence it can be shaped optimally during surgery to prevent loss of bone fragments from comminuted fractures. Since the titanium fiber plate is thin and easily deformable, it is suitable for fractures at sites where the space around the plate is limited such as ulnar and phalangeal fractures. The titanium fiber plates also allow holes to be drilled for insertion of small screws at given sites during surgery. Hence, the Ti fiber plates can be used for a wide variety of fracture treatments including bone regeneration. Titanium fiber plates are not so tough to withstand high mechanical loads. In addition, rubbing of Ti fibers against one another could produce wear particles. These limitations still remain to be solved.

T.S.N. Sankara Narayanan

For more information, the reader may kind refer: Takashi Takizaw et al., Titanium Fiber Plates for Bone Tissue Repair, Adv. Mater. 2017, 1703608, DOI: 10.1002/adma.201703608

Converting Wood to Graphene by Laser Scribing

Graphene has proved its supremacy in a wide variety of applications due to its high electrical conductivity, and excellent chemical and mechanical properties. The conventional method of synthesis of graphene by chemical vapor deposition (CVD) has certain limitations such as need for high temperature, type of substrates, requirement for post-treatments such as etching and critical point drying, or aerogel formation. Laser-induced graphene (LIG) can be formed on commercial polyimide (PI) films, which is considered as an alternate method for CVD to prepare graphene. Researchers at Rice University, USA and Beihang University, China have demonstrated that LIG can be formed on the surface of wood with a high electrical conductivity. Moreover, the graphene layer can be further modified by electrodeposition of polyaniline (PANI) to make it as a supercapacitor or plate it with Co-P and Ni-Fe hydroxides to catalyze hydrogen evolution and oxygen evolution reactions (OERs).

The 3D porous LIG graphene was formed on the surface of pine wood by irradiating it with a 10.6 μm CO₂ laser under Ar or H₂ atmosphere (Fig. 1(a)). The LIG graphene is formed only on the area of the pine wood, which is scribed by the laser whereas other areas remain unchanged (inset of Fig. 1(a)). This attribute helps patterning of LIG with different shapes (Fig. 1(b)). The inert atmosphere helps in the development of graphene with a stable structure while ablation of wood in air has lead to decomposition of the lignocellulose structure. The LIGs were prepared from pine wood using 10% (1.6 W), 30% (4.1 W), 50% (6.3 W), 70% (7.8 W), and 90% (8.6 W). The LIGs prepared using pine, oak and birch woods are designated as P-LIG-x, O-LIG-x and B-LIG-x, respectively, where x signifies the power percentage of the laser.

Fig. 1 (a) Schematic illustration of the formation of LIG using CO₂ laser under Ar or H₂ atmosphere; (b) Photograph of LIG patterned with letter ‘R’ on wood.

The morphology, chemical composition, structure, porosity, crystallite size, electrical properties and I_D/I_G ratio of P-LIG show a strong dependence on the laser power. Morphological features of the pine wood, laser scribed at varying laser powers indicate the evolution of a porous structure due to the liberation of gas during laser irradiation. The pore size of the P-LIGs is decreased with an increase in laser power from 10% to 70% (Figs. 2). The thermal stability and electrical conductivity of P-LIGs are increased with an increase in laser power.

Fig. 2 Morphological features of the laser-scribed pine wood at varying powers: (a) 30%; (b) 50%; and (f) 70%.

The chemical composition of the P-LIG is changed during laser irradiation; an increase in laser power leads to a decrease in C-O content and an increase an in C-C and carboxyl contents (Fig. 3(a)). The structural changes of P-LIG with laser power are confirmed by Raman spectra and TEM. At 10% laser power, no apparent Raman signal is detected while at 30% formation of amorphous carbon is evident by the broad D peak at ≈1350 cm⁻¹ and the weak 2D peak at ≈2700 cm⁻¹. As the laser power is increased from 50% to 90%, sharpening of the D and G peaks along with an enhancement in  the intensity of 2D peak indicate the formation of the graphene structure (Fig. 3(b)).

Fig. 3 (a) Change in chemical composition derived from XPS; and (b) Raman spectra of P-LIG as a function of laser power

The predominance of amorphous carbon without a clear graphene lattice at 30% laser power and graphene carbon over 50% laser power is confirmed by TEM (Fig. 4). The crystalline size of P-LIG reach a maximum at 70% power. The large I_2D/I_G ratio indicate that P-LIG-70 has a stacking of fewest-layered graphene. An increase in laser power, in general, has resulted in the formation of graphene with desired characteristics. Nevertheless, at 90% laser power, overheating of the pine wood has lead to an inferior LIG structure.

Fig. 4 TEM images of (a) P-LIG-30; (b) P-LIG-50; and (c) P-LIG-70

Similar to pine wood, 3D porous LIG is also formed using oak and birch woods. A comparison of the characteristics of LIGs formed using pine, oak and birch woods at a laser power of 70% indicates a lower I_D/I_G ratio of 0.48 for O-LIG-70, followed by at 0.73 and 0.85 for B-LIG-70 and P-LIG-70, respectively. Since aliphatic carbon moieties are more reactive than aromatic carbon, the hemicellulose and cellulose of oak and birch woods are easily decomposed during laser irradiation, thus resulting in O-LIG-70 and B-LIG-70 with more defects. In contrast, a higher aromatic lignin content favours the  generation of P-LIG-70 with a lower degree of defects. In spite of the lignin component, the formation of LIG also depends on the inherent composite structure of wood consisting of cellulose, hemicelluloses, and lignin.

The P-LIG is converted to a supercapacitor by electrodepositing polyaniline (P-LIG-PANI). The cyclic voltammetry curves of P-LIG-PANI in the potential window of −0.2 to 0.8 V indicate two characteristic pairs of redox peaks that correspond to leucoemeraldine/emeraldine and emeraldine/ pernigraniline transition of PANI, thus confirming the pseudocapacitive characteristics of PANI upon P-LIG (Fig. 5(a)). The galvanostatic charge-discharge curves of P-LIG-PANI indicate a specific areal capacitance of ≈780 and ≈320 mF/cm² at 1 and 10 mA/cm², respectively (Fig. 5(b)).

Fig. 5 (a) CV of P-LIG-PANI and P-LIG in 1 M H₂SO₄ at a scan rate of 20 mV/s; and (b) Galvanostatic charge–discharge curves of P-LIG-PANI at varying current densities.

Electrodeposition of Co-P on P-LIG-70 (P-LIG-Co-P) shows that if can be tuned to catalyze the hydrogen and oxygen evolution reaction (HER and OER). The polarization curves of P-LIG-Co-P in 1 M KOH delivers a HER current density of ≈62 mA/cm² at 200 mV overpotential and an OER current density of ≈20 mA/cm² at 400 mV overpotential over an area of ≈0.5 cm² (Fig. 6(a)). The Tafel slopes extrapolated from the polarization curves are ≈35 and 280 mV/decade for HER and OER, respectively (Fig. 6(b)). The OER performance can be improved by electrodepositing NiFe hydroxides on P-LIG (P-LIG-NiFe). Photographic image of P-LIG-Co-P as cathode and P-LIG-NiFe as anode, powered by two 1.5 V batteries in series are shown in Fig. 6(c)).

Fig. 6 (a) HER and OER windows of P-LIG-Co-P and P-LIG-NiFe in 1 M KOH; (b) HER and OER Tafel slopes of P-LIG-Co-P and P-LIG-NiFe; and (c) Photograph of P-LIG-Co-P and P-LIG-NiFe are powered by two 1.5 V batteries in series.

By a suitable choice of materials that can be deposited on LIG, it can be tailored to suit diverse applications such as supercapacitors and water splitting systems. The methodology opens up new avenues to engineer woods surfaces for diverse electronic applications.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Ruquan Ye et al., Laser-Induced Graphene Formation on Wood, Adv. Mater. 2017, 1702211, DOI: 10.1002/adma.201702211

Freestanding, Hydrophobic, Flexible, Lightweight 2D Transition-Metal Carbide Foams for Electromagnetic-Interference Shielding

The deleterious effect of electromagnetic radiation on human health and sensitive electronic devices is matter of concern. The vast growth in use of portable and wearable smart electronics warrant development of thin, lightweight and flexible electromagnetic-interference (EMI) shielding materials.

Researchers at State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, China and School of Materials Science and Engineering, Henan Polytechnic University, China have demonstrated  the fabrication of freestanding, hydrophobic, lightweight, and flexible 2D transition-metal carbide (MXene) foams by assembling MXene sheets into films followed by a hydrazine-induced foaming process.

A stack of Ti₃AlC₂ sheets were used as the precursor. The Al in Ti₃AlC₂ sheets was selectively etched using LiF/HCl. Delamination of the sheets induced during etching has resulted in the formation of loosely stacked structure of Ti₃C₂T_x (MXene) with weakened interlayer interactions. The MXene film was prepared by vacuum-assisted filtration of an aqueous suspension of MXene using a polypropylene membrane. MXene films with desired thickness were obtained by suitably adjusting the concentration and volume of the MXene suspension. The freestanding MXene film exhibits excellent mechanical flexibility and withstand repeated folding and stretching. The MXene film sandwiched between two ceramic wafers was treated with hydrazine at 90 °C. Infiltration of hydrazine molecules into the interior of the MXene film through the numerous tiny channels created during vacuum filtration process has enabled the formation of a lightweight MXene foam with a cellular structure. The various stages involved in the fabrication of MXene foam is schematically illustrated in Fig. 1 along with the photographs of MXene suspension, film and foam.

Fig. 1 Schematic illustration of the various stages involved in the fabrication of MXene foam along with photographs of MXene suspension, film and foam

The morphological features acquired at the cross-section indicate that the MXene film possesses a compact structure with its layers arranged parallel to each other (Figs. 2(a) and 2(b)). This structural arrangement enables the MXene film a good flexibility and excellent mechanical properties. During hydrazine treatment, introduction of numerous small pores between the parallel layers which is accompanied by volume expansion has enabled the formation of MXene foam with a cellular structure (Figs. 2(c) and 2(d)). The reaction of hydrazine with the oxygen-containing groups of MXene accompanied by the rapid release large amounts of gaseous species overcome the van der Waals forces that hold the sheets together, resulting in a lightweight and flexible MXene foam with a cellular structure containing numerous pores.

Fig. 2 Cross-sectional SEM of: (a, b) MXene film; and (c, d) MXene foam

The MXene film and foam exhibit distinct wetting behaviors due to their difference in chemical composition. The MXene film is hydrophilic (water contact angle: 59.5°), an attribute which is originated from the MXene sheets containing oxygen and fluorine terminal groups. In contrast, the MXene foam is hydrophobic  (water contact angle: 94.0°), resulting from the reaction of hydrazine with the oxygen-containing groups in the MXene film. The hydrophobic nature and porous structure of the MXene foam will be useful for selective absorption of organic solvents and oils.

The MXene film possesses a very high electrical conductivity of 400000 S/m   and offers an excellent EMI-shielding performance at different thicknesses; ≈29 dB (1 μm), ≈47 dB (3 μm), and ≈53 dB (6 μm). During the preparation of MXene foams, the sample thickness is increased from 1 to 6 μm, 3 to 18 μm, and 6 to 60 μm and the introduction of insulating pores has lead to a decrease in their electrical conductivity to 58820, 62500, and 58000 S/m, respectively. It is difficult to retain the high electrical conductivity while increasing the thickness of MXene films by foaming. Nevertheless, the increment in thickness of the MXene foam outweighs the decrease in conductivity and improves its EMI-shielding performance. A 6 μm thick MXene foam offers an EMI-shielding effect of 70 dB as opposed to 53 dB for MXene film of similar thickness (Fig. 3).

    Fig. 3 EMI-shielding efficiency: (a) MXene films; and (b) MXene foams

The lightweight, flexible, hydrophobic MXene foam with high strength, reasonable electrical conductivity and excellent EMI-shielding performance will be suitable for applications in defense, aerospace, and wearable electronics.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Ji Liu et al., Hydrophobic, Flexible, and Lightweight MXene Foams for High-Performance Electromagnetic-Interference Shielding, Adv. Mater. 2017, 1702367, DOI: 10.1002/adma.201702367

Capturing CO2 using metal-organic framework (MOF)

The steady increase in concentration of CO₂ in the atmosphere (from 310 ppm to > 380 ppm during the past five decades) and its continuous increasing trend until this moment, is really a matter of concern. Power plants contribute to ~ 60% of the total CO₂ emission worldwide. Hence, development of effective CO₂ capture systems that could selectively remove CO₂ from the exhaust gas is warranted. Porous metal-organic frameworks (MOFs) are promising for CO₂ capture. Nevertheless, development of MOFs for CO₂ capture directly from the exhaust gas of power plants is indeed challenging.

Researchers at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, University of Science and Technology of China, Hefei National Laboratory for Physical Sciences at the Microscale and Wenzhou University, China have designed and synthesized a Cu(II)-MOF (FJI-H14) with high density of open metal sites (OMS) and Lewis basic sites (LBS) in which both OMS and LBS interact synergistically with CO₂ and help to capture it.

A mixture of 2,5-di(1H-1,2,4-triazol-1-yl)terephthalic acid (H₂BTTA) (0.05 mM) and Cu(NO₃)₂·3H₂O (0.05 mM) in H₂O (4 ml) in a sealed Teflon vial under hydrothermal conditions at 120 °C for 3 days has lead to the formation of rod-shaped blue crystals of FJI-H14 ([Cu(BTTA)H₂O]_n·6_nH₂O) with 73% yield based on the organic ligand H₂BTTA (Fig. 1).

Fig. 1 Structural illustration of FJI-H14: (a) ligand H₂BTTA; (b) co-ordination environment of Cu(II) ions with BTTA; (c) one-dimensional nano-porous channels; and (d) topology of MOF (Cu atom, cyan; C atom, gray; O atom, red; N atom, blue; H atom, white)

The FJI-H14 is stable in boiling water as well as in acidic and basic environments (pH: 2 to 12) at temperatures as high as 373 K. It is also thermally stable up to 230 °C. The Brunauer–Emmett–Teller (BET) specific surface area of FJIH14 is 904 m²/g and its Langmuir-specific surface area is 1004 m²/g. The total pore volume of FJIH14 estimated from CO₂ isotherm is 0.45 cm³/g. The high porosity and high concentration of open active sites in the framework has lead to an increase in the extent of CO₂ uptake up to 279 cm³/g (Fig. 2(a)). The strong absorption bands at 2,340 cm⁻¹ and 2,328 cm⁻¹ in the IR spectra indicate that the CO₂ molecules tend to stack around the open Cu(II) sites, which is also in line with the theoretical calculations. Besides high adsorption capacity, reusability is an important property for any adsorbent. FJI-H14 maintains 100% adsorption capacity even after five cycles of adsorption, suggesting its suitability as a reusable adsorbent for CO₂ capture (Fig. 2(b)).

Since the flue gas from power plants contains a large amount of N₂ (73–77 %) than CO₂ (15–16 %), CO₂/N₂ selectivity is a crucial parameter in CO₂ capture applications. The CO₂/N₂ selectivity FJI-H14 (for the 15/85 CO₂/N₂ mixture at 298 K and at 1 atm) is 51. The high selectivity for  adsorption of CO₂ over N₂ suggests that the densely populated open active sites in the framework have a positive effect on CO₂ adsorption. The relatively narrow pores in FJIH14 could have easily blocked the relatively large N₂ molecules thus favouring selectivity for CO₂ (Figs. 2(c) and 2(d)). FJI-H14 is also capable of catalyzing chemical transformation of CO₂ into value-added chemicals, such as dimethyl carbonate, cyclic carbonates, N,N’-disubstituted ureas or formic acid.

Fig. 2 Experimental CO₂ adsorption by FJI-H14: (a) CO₂ adsorption isotherm for FJI-H14 at 195 K; (b) Cycles of CO₂ adsorption for FJI-H14 at 298 K; (c) N₂ and CO₂ adsorption isotherms for FJI-H14 at 298 K; and (d) CO₂/N₂ selectivity for 15/85 CO₂/N₂ mixture at 298 K.

FJI-H14 possesses the characteristics of an ideal MOF in terms of high CO₂ uptake at ambient conditions, excellent chemical and thermal stabilities, selectivity for CO₂ over N₂, reusability, direct and smooth conversion of CO₂ into corresponding cyclic carbonates and ease of preparation at large scale.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Liang et al., Carbon dioxide capture and conversion by an acid-base resistant metal-organic framework, Nature Communications, 8 (2017) 1233, DOI: 10.1038/s41467-017-01166-3