Author: Melinda Zhao
Editors: Angela Pan, Rachel Chen, and Junyu Zheng
Artist: Kyra Wang
Many of us, or maybe our younger siblings, dread getting shots at the doctor’s office. Whether it’s the flu shot, vaccines, or other medicine, it usually means one thing: a needle. Despite being essential for maintaining a healthy body and protecting us from diseases, these shots typically require a needle to transfer the vaccine or medication into our bodies. Luckily, researchers have been searching for alternative forms of drug delivery for those who have sensitive skin or are immunocompromised—meaning their immune systems are weakened, making them more sensitive to infections and injuries, such as punctured wounds. Recently, researchers have developed a form of needle-free drug delivery using something called hydrogels.
Hydrogels are sequences of bonds that link long repeating groups of atoms into polymer chains, creating 3D structures. Due to their chemical structure, hydrogels are super-absorbent materials because of their chemical structure; hydrogels contain hydrophilic (water-attracting) groups, such as hydroxyl (-OH), carboxyl (-COOH), and amino (-NH2) groups, that draw in water molecules, allowing them to hold many times their weight in moisture, with cross-linked polymer chains that form a flexible yet stable structure, allowing them to hold their shape. In other words, hydrogels are like sponges that can stay solid yet soft like a gel. Thanks to these characteristics, they are increasingly used for medical purposes such as wound dressings, tissue engineering, and drug delivery systems, where they can interact with biological tissues in a compatible way.
In drug delivery, hydrogels function like tiny medicine containers akin to pills; the only difference is that they are absorbed through the skin. When hydrogels are applied to the skin, they begin to release the encapsulated drug in a controlled manner. This release depends on various factors such as the composition or type of the hydrogel, the size of the drug molecule, and how the drug interacts within the hydrogel environment, a moist, soft medium similar to human tissues. Hydrogels can release these drugs because of their unique ability and capacity to absorb and swell up to 99% of their volume. For instance, when the hydrogel is exposed to moisture from the skin, it absorbs water and expands. As it swells up, the polymer chains stretch and create pathways for the drug molecules to diffuse out of the hydrogel through a process known as diffusion-mediated release. This is similar to osmosis, where substances move from areas of high concentration to low concentration. In this case, the drug molecules travel from areas of high drug concentration within the hydrogel to low concentration outside. This process diffuses the drug slowly instead of all at once, allowing the body to adjust to the drug at a more stable rate and gradually produce a therapeutic effect compared to a single, rapid injection.
Although hydrogels are a huge development humans have made in the past few years, there are still imperfections that must be fixed before they can completely replace forms of drug delivery. One limitation is that certain drugs, such as large molecule vaccines or proteins (often over 1,000 Daltons), may not pass through the skin effectively and that not all drugs would work with the hydrogel’s aqueous absorbing environment as consistently as desired. Hydrogels typically work well with small to medium-sized molecules, typically under 500 Daltons, such as some antibiotics, analgesics, and hormones. Furthermore, certain hydrogels can dissolve too quickly when in contact with body fluids, which can happen due to weak cross-linking with the polymer structure, causing it to disintegrate or degrade quicker than desired. Hydrogels can also face challenges in their properties that can hinder their effectiveness, such as swelling control, which can lead to the drug releasing before the desired time or structural breakdown.
Despite these limitations, hydrogels offer several advantages over traditional needles. For instance, patients with a needle phobia can use hydrogels as a pain-free and less biologically invasive option. Additionally, patients with sensitive skin can use hydrogels as an alternative to reduce the risk of infection or tissue damage related to injections.
While hydrogels might not replace all forms of medical drug delivery, they are a step forward in further developments and inventions to improve our medical care. Other developments in similar methods have resulted in the new invention of using a nasal spray to spray mist in your nose as an alternative to flu shots. With every innovation, we grow closer to a medical industry where healthcare can be less physically invasive and more adaptable to an individual's needs. As scientists continue to refine these technologies, humanity can hopefully improve patient care globally.
Citations:
"Advantages and Disadvantages of Needle-Free Injection Devices (NFIDs) over Needle
Syringe Devices." ResearchGate, www.researchgate.net/figure/Advantages-and-
syringe_tbl1_242729583. Accessed 24 Sept. 2024.
Centers for Disease Control and Prevention. "Nasal Spray Flu Vaccine." CDC, 28 Aug. 2023,
Das, Sudipta, et al. "Hydrogels for Biomedical Applications: Structure, Characteristics, and
Synthesis." PubMed Central, 9 Apr. 2022, www.ncbi.nlm.nih.gov/pmc/articles/PMC9104731/.
de Avila, José Luis, et al. "Needle-Free Transdermal Drug Delivery via Hydrogels: A
Review." Frontiers in Chemistry, 6 Sept. 2024,
Dumitriu, Simona, et al. "Polymeric Hydrogels as Drug Delivery Systems." ScienceDirect, 25
"Hydrogel: Definition, Types, and Applications." Strouse, 23 Nov. 2020,
"Vaccines and Immunization." World Health Organization, www.who.int/health-
topics/vaccines-and-immunization#tab=tab_1. Accessed 24 Sep. 2024.
Willingham, Emily. "Explainer: What Is a Hydrogel?" Science News Explores, 23 June 2022,
Comments