# Therapeutic Applications of Halloysite
## 4 Recent Trends in HNT Research
### Metalized HNT Applications  
Early in human history, metal and metal salts were used for various applications. For example, Greek and Roman cultures used silver vessels to preserve liquids [84]. Colloidal silver was used extensively in medical treatment, and copper-based medicines were used in treating eczema, lupus, anemia, chorea, syphilis, and zinc salts were used in wound treatment. Understanding how metallic nanostructures interact with microorganisms is a rapidly growing area of inquiry in the biomedical field will offer significant advantages in diagnostic and therapeutic applications (Table 2). Nanoparticles offer unique physical properties that have associated benefits for drug delivery. For example, some metal nanoparticles have bactericidal effects due to their high surface-to-volume ratio and small size, allowing them to interact with bacterial cell membranes rather than release metal ions into the solution. Copper, silver, and zinc nanoparticles, in particular, are known for their excellent antimicrobial properties and have additional beneficial functionalities [84; 85]. Figure 5: (**A**) Graphic abstract of the HNTs coated PLA printed scaffolds with different pattern for HNT patterns guiding cell orientation. (**B**) FE-SEM images of HNTs coated printed pattern surfaces with various magnification. Reprinted by permission of the publisher. Surface functionalization of HNTs with different components, including metals, antibiotics, and bioactive compounds, is of increased importance for their potential use in biomedical devices, antimicrobial surface coatings, drug delivery systems, radiation absorbent composites, elastomer composites, electronic components, and as industrial catalysts (Figure 6) [86]. The outer surface of HNTs can be used for adsorption of metal NPs thus preventing their aggregation, reducing toxicity, and providing more sustained antimicrobial/viral action. [PERSON] and [PERSON] (2016) used a simple method (US Patent #9,981,074 B1) for the fabrication of HNT- supported metal nanoparticles [87]. Metals nanoparticles, such as silver, copper, gold, and other metal nanoparticles can be directly deposited onto the surfaces of HNTs. Existing fabrication methods include lengthy multi-step processes that include metal-salts or organic compounds, reducing agents, high temperatures, and expensive equipment to achieve metallization of the halloysite surface [88]. A strategy for controlled weight deposition of positively charged metal ions on negatively charged HNTs dispersed in an aqueous medium where the extent of metallization can be controlled through changes in voltage, solvent medium, time, and other electrolytic parameters. Additionally, this process does not require the use of any toxic chemicals, expensive reagents, or lengthy pre-processing steps [88]. This rapid and low-cost method was used in various applications, from 3D printing to surface coats. PLA 3D printed scaffolds incorporating metal coated HNTs for bone tissue regeneration have recently been published. [PERSON] et al. [89] used a surface modification method to reduce the hydrophobicity of PLA. This study used fetal bovine serum and \begin{table}
\begin{tabular}{c c c c c} \hline
**Antimicrobial** & **Sustained Release** & \
\begin{tabular}{c} **Targeted Drug** \\ **Delivery** \\ \end{tabular} & **Tissue Engineering** & **Wound Healing** \\ \hline antibiotics & antimicrobials & brain & bone & anti-infection \\ antifungals & anti-cancer agents & breast cancer &
\begin{tabular}{c} guided nerve \\ regeneration \\ \end{tabular} & hemostasis \\ anti-biofouling & dyes & colon cancer & cartilage & healing \\ anti-viral & growth factors & osteosarcoma & skin & revascularization \\ \hline \end{tabular}
\end{table} 
Table 2: Potential applications of metalized HNTs. Figure 6: Biomedical and industrial applications of metallized HNTs. NaOH to coat PLA scaffolds containing Zn-coated HNTs (Figure 7). The scaffolds possessed high mechanical strength and showed an osteoinductive potential.