Selected Applications of Terahertz Pulses in Medicine and Industry

This article contains a brief summary of areas where terahertz technology is making an impact in research and industrial applications. We cover some of its uses in the pharmaceutical setting, where both imaging and spectroscopy play important roles. Medical applications are also being pursued in many research laboratories, primarily for imaging purposes and following on from the first results just over 20 years ago. The three-dimensional imaging capability of pulsed terahertz allows for the observation of tumours below the surface of tissue, such as basal cell carcinoma of skin. The recent use of the technology in studies of cultural heritage has shown to increase our understanding of the past. The power of terahertz is exemplified by the discussion on its importance in different industries, such as semiconductor circuit manufacturing and automotive assembly.

Characteristic terahertz absorption spectra of paramylon and paramylon-ester compounds

Paramylon is a long-chain polysaccharide, composed of glucose units connected via β-(1,3) glycosidic bonds, that spontaneously forms a three-strand helical bundle. Paramylon-esters can be made by partially or fully replacing saccharide chain hydroxide groups with carboxylic functional groups, such as stearoyl (CH₃(CH₂)₁₆CO) and palmitoyl (CH₃(CH₂)₂₄CO). The paramylon-ester with carboxylic acids has superior characteristics, including high thermal resistance, stability and transparency under visible light, which are necessary for thermoplastic applications. In this study, the absorption coefficient α(ν) and absorbance spectra of paramylons and paramylon-esters were measured in the 0.3-8.0 THz range and compared with the corresponding spectra of glucose and cellulose. Paramylon and paramylon-ester molecules were found to exhibit unique, so-called fingerprint, α(ν)peaks at 4.0, 6.0 and 8.0 THz, and 2.5 and 5.0 THz, respectively. We speculate that the spectral features observed are owing to intermolecular interaction modes of the weakly coupled polysaccharide chains. The paramylons with different molecular weights show very similar absorption features in the low-frequency side, both in spectral shapes and intensities, indicating that absorption is independent of molecular size. The paramylon-esters with varying degrees of substitution (DS) are similar spectral shapes but different intensities. A linear correlation between α(ν) peak intensity and the DS of paramylon-esters was established with the R² value above 0.99. This behavior can be used for the detection and identification of novel paramylon-ester molecules.

Molecular vibration and Boson peak analysis of glucose polymers and ester via terahertz spectroscopy

Complex permittivity spectra were obtained herein by performing broadband terahertz (THz) spectroscopy on cellulose, paramylon, and paramylon ester. Absorption peaks observed for cellulose and paramylon at approximately 3 THz are attributed to hydrogen bonds. In addition, a broad absorption peak around 2 THz was observed for all the polymers, demonstrating a general feature of polymer glasses derived from weak interatomic van der Waals forces. The boson peak was observed for cellulose and paramylon ester. The boson peak frequency for cellulose nearly equaled that for glassy glucose-a unit structure of the cellulose polymer. Additionally, the insensitivity of cellulose to the polymerization degree was consistent with recent results obtained via molecular dynamics simulations. In contrast, the boson peak frequency of paramylon ester was markedly smaller than that of cellulose. These results demonstrate the importance of hydrogen bonds as determinants of the boson peak frequency.

Far-Infrared Spectroscopy as a Probe for Polymorph Discrimination

Pharmaceutical crystalline polymorph and amorphous form detection and quantification is a standard requirement in the pharmaceutical industry. Infrared (IR) spectroscopy provides an important probe for the characterization of polymorphs. Nonetheless, characterization and discrimination among polymorphs using mid-IR spectroscopy is not always possible in part because the technique mainly probes vibrational modes arising from functional groups in the sample. In the present work, far-IR spectroscopy is demonstrated for the discrimination of polymorphs. This region is influenced by delocalized lattice vibrational modes derived from intermolecular forces and packing arrangements in the crystal structure. A total of 10 polymorphic pharmaceuticals were prepared to conduct a critical evaluation of the question, does this far-IR region add value for polymorph differentiation? It is demonstrated that the far-IR region offers high discriminating power for polymorphs compared to the mid-IR spectral region. In addition, structural similarity and dissimilarity in polymorphic packing arrangements can be derived from this analysis.

Optical Properties of Meloxicam in the Far-Infrared Spectral Region

One of the most commonly used nonsteroidal anti-inflammatory active pharmaceutical ingredient called Meloxicam has been characterized spectroscopically both by Terahertz (THz) time domain spectroscopy (THz-TDS) and by Fourier Transform Infrared (FTIR) spectroscopy in far-IR regions of electromagnetic spectrum; 0.2 THz to 20 THz. While many relatively sharp features are observed in the far-IR range between 2 THz to 20 THz as expected for being an organic substance, very distinct and relatively strong absorption bands are also observed at 1.00, 1.66, 2.07 and 2.57 THz in the THz range. These well separated, defined, and fairly strong spectral features can be used for discrimination and quantification of Meloxicam in drug analysis. Frequency dependent refractive index of the drug was determined in a range of 0.2 THz and 2.7 THz, where an almost constant index was observed with an average index of 1.75. Powder XRD, and solid-state Density Functional Theory (SS-DFT) calculations were utilized to determine the crystalline form of the Meloxicam sample in its enolic crystalline form. Single molecule DFT calculations were also performed in all four possible structures of Meloxicam. In addition, the capability of THz waves transmission through common packaging materials is demonstrated for possibility of future on-site analysis. The results suggest that drug analysis will be possible to perform not only at every stage of manufacturing without destruction but also directly at the shelf of a market after development of portable THz technologies.

Physical-chemical properties of furosemide nanocrystals developed using rotation revolution mixer

Recently, several approaches have been reported to improve the dissolution rate and bioavailability of furosemide, a class IV drug. However, to the best of our knowledge, none of them proposed nanocrystals. In the last decade, nanocrystals successfully addressed solubility issues by increasing surface area and saturation solubility, both leading to an increase in the dissolution rate of poor water soluble drugs. The preparation of furosemide nanocrystals was by a rotation revolution mixer method. Size distribution and morphology were performed using laser diffraction and scanning electron microscopy, respectively. In addition, differential scanning calorimetry, thermogravimetry, X-ray powder diffraction (XRD) and low frequency shift-Raman spectroscopy allowed investigating the thermal properties and crystalline state. Solubility saturation and intrinsic dissolution rate (IDR) studies were conducted. The thermal analysis revealed lower melting range for the nanocrystals comparing to furosemide. Moreover, a slight crystalline structure change to the amorphous state was observed by XRD and confirmed by low frequency shift Raman. The particle size was reduced to 231 nm with a polydispersity index of 0.232, a 30-fold reduction from the original powder. Finally, the saturation solubility and IDR showed a significant increase. Furosemide nanocrystals showed potential for development of innovative formulations as an alternative to the commercial products.

High solubility crystalline hydrates of Na and K furosemide salts

Furosemide–Na–trihydrate displayed aqueous solubility of about 4000 fold higher than that of furosemide while furosemide–K–monohydrate has over 10 000 times improved solubility.

Novel furosemide cocrystals and selection of high solubility drug forms

Furosemide was screened in cocrystallization experiments with pharmaceutically acceptable coformer molecules to discover cocrystals of improved physicochemical properties, that is high solubility and good stability. Eight novel equimolar cocrystals of furosemide were obtained by liquid-assisted grinding with (i) caffeine, (ii) urea, (iii) p-aminobenzoic acid, (iv) acetamide, (v) nicotinamide, (vi) isonicotinamide, (vii) adenine, and (viii) cytosine. The product crystalline phases were characterized by powder x-ray diffraction, differential scanning calorimetry, infrared, Raman, near IR, and (13) C solid-state NMR spectroscopy. Furosemide-caffeine was characterized as a neutral cocrystal and furosemide-cytosine an ionic salt by single crystal x-ray diffraction. The stability of furosemide-caffeine, furosemide-adenine, and furosemide-cytosine was comparable to the reference drug in 10% ethanol-water slurry; there was no evidence of dissociation of the cocrystal to furosemide for up to 48 h. The other five cocrystals transformed to furosemide within 24 h. The solubility order for the stable forms is furosemide-cytosine > furosemide-adenine > furosemide-caffeine, and their solubilities are approximately 11-, 7-, and 6-fold higher than furosemide. The dissolution rates of furosemide cocrystals were about two times faster than the pure drug. Three novel furosemide compounds of higher solubility and good phase stability were identified in a solid form screen.