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Abstract

This article systematically reviews the classification and application characteristics of subcutaneous injection needles across various medical settings, revealing a trend toward specialized segmentation in needle specifications, lengths, and designs based on different routes of administration, patient populations, and drug properties, highlighting their role as a key tool in precision medicine.

Main text

Subcutaneous injection needles are one of the most common medical devices, yet their application is far from uniform. In clinical practice, "injection" is a highly specialized procedure, with distinct needle choices corresponding to different therapeutic goals, forming a specialized and refined field.

Depending on the injection depth and target tissue, they are mainly classified into the following categories, each with distinct requirements for needles:

Intradermal injection: Commonly used for penicillin skin testing and tuberculin testing, this method involves injecting a very small amount of medication between the epidermis and dermis. Therefore, extremely short and fine needles (typically 26–27G, about 10–16 mm in length) are used, with a shallow insertion angle (approximately 5–15 degrees), to create a distinct "wheal" on the skin surface.

Subcutaneous injection: Primarily used for administering insulin, heparin, certain vaccines (such as measles vaccine), and some biological agents (like growth hormone). The drug is injected into the subcutaneous fat tissue, where absorption is slow and uniform. Needle lengths typically range from 4 mm to 12.7 mm (in recent years, shorter 4 mm and 5 mm ultra-short needles have become popular due to their enhanced safety), and they are relatively fine (31–34G). Needle selection should consider patient body type (thickness of fat layer); obese patients may require slightly longer needles, while children and underweight individuals need shorter ones to avoid accidental intramuscular injection.

Intramuscular injection: Suitable for drugs requiring rapid absorption, those that are irritating or oil-based (such as antibiotics, vaccines, and sedatives). Common injection sites include the gluteus maximus, vastus lateralis muscle, or deltoid muscle. Intramuscular needles are longer (25–38 mm) and thicker (20–23G) to ensure the medication penetrates the fatty layer and reaches the muscle accurately. The needle shaft must be stronger to withstand the dense resistance of muscle tissue.

Venipuncture and intravenous injection: Includes blood collection and IV infusion. This category has the most diverse requirements for needle types. Blood collection needles are generally thicker (21–22G) to allow smooth and rapid blood flow into vacuum tubes. Catheter materials for peripheral IVs are softer (often made of Teflon or polyurethane) to enable long-term dwell. In recent years, safety butterfly needles-featuring flexible extension tubing and protective wings for secure placement-have become widely used, especially for pediatric, elderly, and patients with poor vascular conditions.

Additionally, needles are specially designed for specific medications. For example, ultra-smooth needles (with specially treated inner diameters) are required for injecting highly viscous biological agents such as certain monoclonal antibodies; when administering suspensions, slightly thicker needles may be used to prevent clogging of the needle tip.

Specialization is also reflected in audience segmentation. Pediatric needles are shorter and finer, often featuring brightly colored sheaths to reduce children's fear. Insulin pen needles for diabetic patients aim for an ultimate "pain-free" experience, incorporating multiple technologies such as "ultra-thin walls," "five-facet needle tips," and "lubricated coatings" to achieve an extremely fine diameter of 34G (0.18mm) and a shortened length of just 4mm, significantly improving the quality of life for those requiring multiple daily injections.

Conclusion

From the tiny skin wheal of a skin test to the rapid onset of intramuscular injection, and from the life-sustaining pathway of intravenous infusion to subcutaneous injections, the specifications, lengths, and designs of subcutaneous needles are closely tied to their clinical roles. This specialized segmentation reflects the pursuit of precision and human-centered care in modern clinical medicine as it advances from "generic treatment" toward "personalized, precise therapy."