A good medical wig, also known as a cranial prosthesis, is characterized by features that prioritize the wearer's comfort, security, and natural appearance, particularly for individuals with sensitive scalps due to medical treatments like chemotherapy or conditions such as alopecia. 

Here are the key elements that define a wig as “medical grade”:

1. **Superior Comfort and Material**

   - **Hypoallergenic and Soft Materials**: The cap lining should be made of gentle, non-irritating materials suitable for sensitive scalps, such as soft cotton, bamboo, or medical-grade polymer.

   - **Lightweight and Breathable**: The cap construction must allow for excellent ventilation to prevent heat and moisture buildup, which can lead to itching or discomfort.

   - **Avoid Irritants**: Unlike fashion wigs, a good medical wig should avoid rough seams, bulky attachments, or scratchy materials, such as rubber or low-quality latex, which can irritate tender skin.

2. **Cap Construction for a Natural Look**

   - **Monofilament Top**: This is a key feature. It consists of a fine, sheer, breathable mesh where individual hairs are hand-tied. This allows the wearer to part the hair in different directions and gives the illusion that the hair is growing directly from the scalp.

   - **Hand-Tied Hair**: In many high-quality medical wigs, especially those that are 100% hand-tied, each hair is tied by hand. This allows the hair to move naturally, prevents bulkiness, and ensures the softest possible lining inside the cap.

   - **Lace Front**: A sheer lace material at the hairline creates a natural and virtually undetectable transition, allowing the hair to be styled away from the face.

3. **Secure and Proper Fit**

   - **Custom Fit/Adjustability**: While off-the-shelf options are available, a superior medical wig is often custom-sized to fit the individual's head measurements (or perimeter mold) to ensure a perfect, secure, and comfortable fit without bunching or slipping.

   - **Non-Slip Security**: The wig is designed to fit securely without the need for adhesives, glue, or clips that might be uncomfortable on a bare scalp. This is often achieved through adjustable straps, secure soft ear tabs, or medical-grade silicone strips on the cap that gently grip the skin.

4. **Hair Quality**

   - **Human Hair (Ideal for Longevity and Styling)**: High-quality human hair provides the most natural look, texture, and movement. It also allows for versatility in styling, cutting, and coloring.

   - **High-Quality Synthetic Hair**: Modern synthetic fibers can appear very natural and are often pre-styled, making them a good, lightweight option that requires less maintenance.