# How Auto-Injectors Adjust Their Dose Using a Simple Spacer > A 1977 invention for auto-injectors that uses a physical spacer to adjust the amount of medicine inside the device before it is fired. - **Patent:** US 4031893 - **Original title:** Hypodermic injection device having means for varying the medicament capacity thereof - **Owner:** Survival Technology Inc - **Granted:** 1977 - **Status:** Public domain (expired) - **Times cited:** 108 - **Field:** biotech, mechanical, consumer_electronics ## What it does This patent describes an auto-injector mechanism that allows the same device to hold different amounts of liquid medication. It achieves this by placing a spacer between the spring-loaded plunger and the piston inside the medicine cartridge. By changing the thickness or presence of this spacer, the piston is pushed further into the cartridge, effectively shrinking the volume of the medicament chamber. This ensures that the device can be calibrated for specific doses without needing to redesign the entire spring-power assembly. ## What it does NOT cover - Does not cover electronic or digital dose-setting mechanisms - Does not cover devices that use multiple chambers to mix medications - Does not cover needle-free jet injection systems - Does not cover the internal spring-power release mechanism itself ## The clever bit By using a spacer to pre-position the piston, the inventors decoupled the dose volume from the stroke length of the spring-powered plunger, allowing a single injector platform to be highly versatile. ## Real-world examples 1. EpiPen style emergency auto-injectors 2. Military nerve agent antidote injectors 3. Single-use pre-filled medication syringes ## Why it matters This design was critical for the mass production of emergency auto-injectors, such as those used for nerve agent antidotes or epinephrine. It allowed manufacturers to use a standardized spring-loaded gun body while customizing the dose for different patient needs or drug concentrations using simple, interchangeable mechanical spacers. ## Frequently asked questions ### What does How Auto-Injectors Adjust Their Dose Using a Simple Spacer cover? A 1977 invention for auto-injectors that uses a physical spacer to adjust the amount of medicine inside the device before it is fired. ### Who owns patent US 4031893? Survival Technology Inc owns this patent, granted in 1977. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 4031893 cited by? This patent has been cited by 108 later patents that build on its ideas. ### What problem does this patent solve? This design was critical for the mass production of emergency auto-injectors, such as those used for nerve agent antidotes or epinephrine. It allowed manufacturers to use a standardized spring-loaded gun body while customizing the dose for different patient needs or drug concentrations using simple, interchangeable mechanical spacers. ### What does this patent NOT cover? Does not cover electronic or digital dose-setting mechanisms **Full plain-English explainer:** https://patentbrief.org/patent/us/4031893/epipen-autoinjector-kaplan **Original patent:** https://patents.google.com/patent/US4031893 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How a Spring-Loaded Pocket Dispenser Works](https://patentbrief.org/patent/us/2620061/pez-dispenser) — A 1949 mechanical design for a pocket-sized container that uses a spring to push items like pills or candies to the top for easy access. - [Catheter System for Opening and Closing Body Passages](https://patentbrief.org/patent/us/4195637/balloon-angioplasty-catheter-gruentzig) — This 1980 patent describes a medical catheter system with a guide catheter and a special dilatation catheter that can expand to open or close body passages, like blood vessels. - [How the First Aerosol Spray Can Works](https://patentbrief.org/patent/us/2331117/aerosol-spray-can-goodhue) — A 1941 invention by Lyle Goodhue and William Sullivan that created the modern aerosol spray can by using a liquefied gas to propel liquid contents. - [How Piezoelectric Inkjet Printing Works](https://patentbrief.org/patent/us/3946398/drop-on-demand-inkjet) — A 1970 patent describing how to print images by using electrical pulses to bend a tiny crystal plate, squeezing individual ink drops out of a nozzle on demand. - [How Keurig's Original Single-Serve Coffee Pod System Works](https://patentbrief.org/patent/us/5325765/keurig-single-serve-coffee-pod) — This 1994 patent describes the original Keurig system for brewing single cups of coffee using a special filter pod that holds coffee grounds and separates them from the brewed liquid.