The Complete Filling Process of PFS Injection Pens: How Pre-Filled Syringes Are Prepared for Safe and Precise Drug Delivery

Pre-filled syringe (PFS) injection pens are becoming the global standard for delivering GLP-1 medications, peptides, insulin, biologics, and specialty injectable drugs. Their convenience, safety, and precision make them widely used in hospitals, weight-management clinics, and pharmaceutical companies.

But how does a PFS injection pen get filled?
What steps are required to ensure sterility, dosage accuracy, and stable drug performance?

This article provides a complete, SEO-optimized explanation of the PFS injection pen filling process, from raw material preparation to final sterilization and packaging.

1. What Is a PFS Injection Pen?

A PFS injection pen combines two components:

✔ A pre-filled syringe (PFS)

containing a ready-to-inject liquid medication (1 mL or 2.25 mL)

✔ A mechanical pen device

which allows the user to deliver accurate doses with stable injection force

This design eliminates manual drawing, reduces contamination, and improves patient experience.

2. Why the Filling Process Is Critical

The filling stage is one of the most sensitive steps in manufacturing injectable medications.
It must ensure:

• 100% sterility

• Precise dosage accuracy

• Particle-free, contamination-free medication

• Tight sealing and stability of the drug

• Compatibility with the pen mechanism

Because PFS pens are often used for GLP-1 treatments (Semaglutide, Tirzepatide, Liraglutide), the drug's stability must be strictly controlled.

3. Step-by-Step: The Complete PFS Pen Filling Process

Below is the full workflow used in pharmaceutical-grade filling facilities.

Step 1 — Raw Materials Preparation

Before filling begins, all materials are prepared and inspected:

• Medical-grade glass or polymer syringes

• Plungers / rubber stoppers

• Needle shields

• Drug formulation (filtered and stabilized)

• Sterile caps and sealing components

Each component must pass incoming quality inspection (IQC) before entering the cleanroom.

Step 2 — Drug Solution Preparation & Filtration

The medication is prepared in a controlled laboratory environment.

The solution must undergo:

✔ Dissolution and mixing
✔ pH adjustment
✔ Temperature-controlled blending
✔ Sterile filtration (0.22 μm filter) to remove particles and bacteria

This ensures the drug meets pharmaceutical purity and stability requirements.

Step 3 — Sterile Filling (Aseptic Processing)

This is the core stage of the entire PFS process.

Filling occurs in:

• ISO Class 5 cleanrooms

• With laminar flow hoods

• Fully automated filling lines

The filling machine dispenses the exact medication volume into each syringe (e.g., 1.0 mL or 2.25 mL).

Precision requirements:

• ±1–2% dosing accuracy

• Zero contamination

• Consistent fill levels

The entire filling area is monitored for temperature, humidity, airborne microbes, and pressure.

Step 4 — Plunger Insertion & Vacuum Stoppering

After filling, the syringe is sealed automatically.

✔ Plunger insertion

The plunger is inserted with a controlled vacuum to prevent air bubbles.

✔ Vacuum stoppering

Ensures:

• Strong sealing

• Stable drug pressure

• No leakage

This step also controls the glide force so that the injection pen can deliver smooth, stable doses.

Step 5 — Needle Shielding or Tip Capping

Depending on the PFS design:

• Some syringes receive a rigid needle shield

• Others receive a luer-lock cap

This protects the needle area and prevents contamination.

Step 6 — Visual Inspection (Automatic + Manual)

Each syringe must pass defect inspection:

Automated inspection checks:

• Fill level accuracy

• Particles / foreign bodies

• Cracks in syringe

• Air bubbles

• Plunger position

Manual inspection for:

• Cosmetic defects

• Label alignment

• Surface scratches

Any syringe with abnormalities is rejected immediately.

Step 7 — Finger Flange & Backstop Assembly

To allow smooth pen operation, syringes may be assembled with:

✔ Finger flanges
✔ Backstops
✔ Customized pen interface components

This ensures the syringe fits securely inside the PFS injection pen.

Step 8 — Integration with Injection Pen (Optional)

Some pharmaceutical partners choose ready-to-use PFS pens, where the filled syringe is pre-assembled into the pen body.

Assembly includes:

• Loading syringe into pen

• Engaging clutch mechanism

• Testing dose adjustment

• Testing injection force

• Outer housing assembly

This step must maintain aseptic control and mechanical precision.

Step 9 — Labeling & Serialization

Each unit receives:

• Product label

• Batch number

• Lot tracking code

• Serialization (required in many markets)

• Expiration date

This ensures full traceability for global regulatory compliance.

Step 10 — Final Sterilization or Pre-Sterilized Packaging

Depending on product requirements:

✔ Terminal sterilization (moist heat / EO)

for heat-stable components

✔ Aseptic packaging for pre-sterilized parts

if the drug cannot tolerate heat

PFS products are often sealed in:

• Blister packs

• Medical-grade Tyvek pouches

• Ready-to-inject cartons

4. Quality Control in the PFS Filling Process

Throughout the workflow, strict quality standards are enforced:

Quality tests include:

• Dose accuracy testing

• Plunger movement glide force test

• Leakage test

• Integrity test

• Particulate test

• Sterility test

• pH and concentration check

• Stability chamber testing

These ensure that every PFS pen is safe, consistent, and compliant with ISO 11608 and ISO 13485 standards.

5. Why PFS Injection Pens Are Preferred by Modern Pharmaceutical Brands

✔ Increased safety

No manual preparation reduces contamination risk.

✔ Higher accuracy

Pre-measured doses ensure consistent delivery.

✔ Better patient experience

Ease of use improves treatment adherence.

✔ Ideal for GLP-1 and peptide medications

Such drugs require high purity and stable environments.

✔ Ready-to-market speed for pharmaceutical companies

Fast filling + modular pen design = shorter launch cycle.

6. Conclusion: A Highly Controlled Process Designed for Precision & Safety

The filling process of PFS injection pens is one of the most advanced workflows in modern pharmaceutical manufacturing.
From sterile drug preparation to automated filling, sealing, inspection, and pen assembly, every step is engineered to guarantee:

• Sterility

• Accuracy

• Stability

• Regulatory compliance

For GLP-1 brands, peptide companies, and medical product distributors, understanding this process is essential for selecting a reliable manufacturing partner.

Need a Customized PFS Injection Pen Solution?

We provide:

✔ 1 mL / 2.25 mL PFS pens
✔ GLP-1 pen OEM / ODM customization
✔ Pen housing design, logo printing, color customization
✔ Compatibility with Semaglutide, Tirzepatide, Liraglutide
✔ Small-batch pilot orders + mass production
✔ Global shipping & technical documentation support