The dispersion phase and the continuous phase are two immiscible phases that make up an emulsion. In the beginning, Schulman coined the phrase “microemulsion”. After more investigation into microemulsion, researchers discovered a kinetically stable emulsion known as nanoemulsion [1]. The definition of a nanoemulsion by the International Union of Pure and Applied Chemistry (IUPAC) is a dispersion made up of water, oil, and surfactant with a dispersed domain diameter and is isotropic and thermodynamically stable. In practice, the nanoscale emulsion is created using mechanical force. Nanoemulsions are emulsions that are smaller than tens to hundreds of nanometers in size. As the system strives to reach a state of minimal Gibbs free energy, surfactants work at the interface between the two immiscible phases to reduce surface tension and stabilize the emulsion [2]. Because they can quickly solubilize hydrophobic pharmaceuticals and lessen severe side effects, nanoemulsions have much potential [3].

2. Types, Composition, and Characterization of the Nanoemulsion System

Emulsions are a mixture of two or more phases, one of which is hydrophilic and the other hydrophobic, scattered throughout each other. When tiny oil droplets are scattered throughout water, the emulsion is called an oil-in-water (O/W) emulsion ^[4][9]. The homogenization method employed during the creation of nanoemulsions and traditional O/W emulsions determines the droplet sizes of the final products ^[4][5][9,10]. When water droplets are mixed with oil, water-in-oil (W/O) emulsions are created. In microreactors, W/O nanoemulsions regulate nanoparticle formation, including Cd [cadmium] nanoparticles and titania–silica nanoparticles ^[6][7][11,12]. As reaction media, different W/O emulsions are employed to produce ceramic nanoparticles. W/O emulsions are used in the pharmaceutical business as adjuvants for vaccines containing uncommon antigens, such as synthetic peptides, DNA, or recombinant proteins. However, by enclosing an emulsion within an emulsion, resulting in oil-in-water-in-oil (O/W/O) or water-in-oil-in-water (W/O/W) emulsions, these straightforward systems can be made more complex. Double emulsions are often created in two steps: An initial internal emulsion is created, and then a second emulsion is created to cover the initial emulsion. Double emulsions present difficulties in their formation and stabilization, including the need to preserve the integrity of the first emulsion when creating the second one, the need for both a hydrophilic surfactant and a lipophilic surfactant to stabilize each oil–water interface, and a higher propensity for coalescence and degradation because of diffusion between each phase ^[8][13]. Microfluidic devices have sparked interest in studying uniform double emulsions for use in food science and as templates for particle synthesis or microreactors ^[9][10][14,15]. In contrast to most emulsions, in which the internal phase only accounts for a negligible fraction (10%) of the overall emulsion volume, high internal phase emulsions contain an internal phase higher than 74% of the total emulsion volume ^[11][16]. Pickering emulsions are oil-in-water or water-in-oil emulsions in which minute particles operate as a steric barrier to coalescence at the oil–water interface as opposed to small molecules that alter the interfacial tension ^[12][17]. Stabilizing particles by moistening pickering emulsions with both emulsion phases is possible. This property makes them more effective than conventional surfactants at increasing emulsion stability by allowing them to sit at the oil–water interface ^[13][18]. Pickering emulsions have been investigated as an alternative to small molecule surfactants for drug delivery applications because of their increased stability and biocompatibility.

3. Nanoemulsion for Colorectal Cancer Delivery

A large fraction of cancer-related deaths worldwide is caused by colon cancer. Treatment options for colon cancer typically include surgery, herbal remedies, immunotherapy, radiation, and chemotherapy. Since nanotechnology has been proven effective for treating cancer, researchers have concentrated their efforts on this area. The most prevalent types of nanoemulsions for different types of cancer are summarized in Table 1, a summary of some recent nanoemulsions developed and applications for colorectal and other cancer. Worldwide, colon cancer is responsible for a disproportionate number of cancer deaths. This section classifies the various subtypes of colon cancer, including familial adenomatous polyposis, hereditary nonpolyposis colon cancer, cancer that develops independently, and cancer linked to colitis ^[14][94]. Surgery is the primary treatment option for colon cancer; other possibilities include chemotherapy, radiation therapy, immunotherapy, and alternative medicine. However, survival rates due to metastasis and reoccurrence continue to decline for around five years after surgery, suggesting that the tumor is not the primary cause of death ^[15][95]. Tomatoes contain lycopene, which possesses several potential health benefits if it were more stable and bioavailable, including protection from chronic diseases, antiproliferation activity against leukemia and colon cancer cells, and the induction of cell cycle arrest in some tumor cells ^[16][17][18][96,97,98]. In the described research, a nanoemulsion was created using lycopene to address the issue. Additionally, gold nanoparticles could be encased in a nanoemulsion formulation (AN). Rather than merely serving as a medication carrier, AN could also comprise cell receptor ligands to facilitate precise cell targeting ^[19][79]. Combining lycopene-derived molecules with liposomes, polymeric substances, or other lipid-based assemblies can mitigate this effect ^[20][99]. In this formulation, Tween 80^® serves as the emulsifier, oil containing lycopene is the oil phase, and an aqueous AN solution is the water phase. The HT-29 human colon cancer cell line is suitable for its application. The formulation’s effectiveness was demonstrated by testing the effects of AN alone, lycopene alone, and combining the two on a cell line ^[21][100]. Using a nanoemulsion preparation method, the author of this study investigated the anticancer and apoptotic effects of S. aromaticum L. bud essential oil (SABE) on human HT-29 colon cancer cells. Apoptosis (Cas3 overexpression and higher SubG1 peaks) and cell death (reduced HT-29 viability) were seen in response to a wavelength of 131.2 nm in this study ^[17][97]. In a phase inversion process, Sabna Kotta created a nanoemulsion of resveratrol (low energy technique). Grapes, berries, and other fruits contain resveratrol, a polyphenolic phytoalexin. Human colorectal cancer cell lines (HCT-166) were employed to test the antiproliferative effects of a resveratrol nanoemulsion. The research also showed that nanoemulsion had a concentration-dependent killing effect on HCT-166 colon cancer cells ^[22][101]. The author investigated the Lycium barbarum L. plant’s carotenoid extracts. Nine carotenoids, including neoxanthin, all-trans-zeaxanthin and its cis-isomers, all-trans—cryptoxanthin), all-trans—carotene and its cis-isomers, were analyzed by high-performance liquid chromatography-mass spectrometry with the highest extraction yield achieved using a solvent system of hexane–ethanol–acetone (1:1:1); the IC50 values for inhibiting the development of HT-29 colon cancer cells were reported to be 4.5 and 4.9 µg mL⁻¹ for carotenoid extracts and their nanoemulsion, respectively. G2/M phase arrest was caused by the extract and nanoemulsion, which also raised p53 and p21 expression and lowered CDK2, CDK1, cyclin A, and cyclin B expression ^[23][102]. The Spirulina platensis microalgae were used to prepare cerium oxide nanoparticles (CeO2-NPs) by Seyedeh and Asoodeh. Additionally, the ultrasonic emulsification process created nanoemulsions (CeO2-NE) by encapsulating cerium oxide nanoparticles (CeO2-NPs) inside nanoporous polymer shells. Quantitative PCR analysis of apoptotic and antioxidant-related gene expression in HT-29 cells treated with CeO2-NE revealed that CeO2-NE had a more potent effect on anticancer activities. Apoptosis was validated by finding overexpressed caspase-3, caspase-8, and caspase-9 in CeO2-NE. A correlation was found between the overexpression of superoxide dismutase and the redox potential of CeO2-NE in HT-29 colon cells ^[24][103]. To test the cytotoxic activity of Carum carvi oil nanoemulsions (CCONE), the researchers used the MTT assay on human HT29 colon cells. They found that increasing the nanoemulsion (CCONE) concentration significantly elevated caspase3 gene expression. These data demonstrate that CCONE promotes apoptosis in human colon cancer cells with minimal toxicity. However, further research is required both in vivo and in vitro ^[25][104]. Due to its serious adverse effects, oxaliplatin (Oxa) has had a limited therapeutic role in the management of colon cancer. Many distinct mutational patterns of p53 are seen in colon cancer, affecting the effectiveness of chemotherapeutic drugs such as Oxa. For this reason, it is crucial to discover an alternate therapeutic strategy that mitigates the side effects of Oxa but also enhances its efficacy in the fight against colon malignancies. In this study, Al-Otaibi and Al-Motwaa created a nanoemulsion of Teucrium polium L. essential oil (TPO-NANO). They tested its efficacy in treating colon cancer cells that differed in their sensitivity to the chemotherapeutic agent oxaliplatin based on their p53 status (HCT116 wild-type and HT-29 mutant-type). According to results from an anticancer screen, Oxa+TPO-NANO inhibited the development of both p53 wild-type HCT116 and p53 mutant-type HT-29 colon cancer cells and generated more morphological changes and apoptotic cell death than did Oxa-NS alone. Put another way, the essential oil nanoemulsion of Teucrium polium L. has a synergistic anticancer impact ^[26][105]. The antitumor activity of the combination formulas of DOX and SIM, either loaded in water (DOX-SIM-Solution) or nanoemulsions (N.E.s) (DOX-SIM-NE), was evaluated in a Swiss albino mouse model of Ehrlich ascites carcinoma. The study showed that incorporating SIM into the DOX-loaded-NE formulation remarkably improved its efficiency and reduced adverse effects ^[27][106]. Moreover, resiquimod-fortified nanoemulsion demonstrated significant antitumor immunity by TLR7/8 and TLR9 activations with induced PD-L1 upregulation in tumors. In addition, a pronounced immunostimulatory effect against polarized macrophage cells treated with test and control nanoemulsions was displayed ^[28][107]. Another safety evaluation of quercetin nanoemulsions on Wistar rat’s organ histopathology, including the liver, spleen, kidney, heart, and brain, indicated insignificant signs of atrophy, hyperplasia, necrosis, or inflammations ^[29][108]. The recent patents on nanoemulsions for cancer treatment are tabulated in Table 2.