The human-machine interface (HMI; German: Mensch-Maschine-Schnittstelle) is the set of all elements through which information is exchanged between user and device: displays, controls, signals, and feedback. It covers both directions of the exchange and is directly safety-relevant in medical technology.
Components and directions of action
An HMI works in two directions, and both must be considered together. Toward the machine, the user transmits intentions through controls: buttons, rotary knobs, switches, touch surfaces, and increasingly also voice or gestures. Toward the human, the device returns states and results through displays, indicator lights, acoustic signals, and haptic feedback.
The core of a functioning HMI is the closed feedback loop between the two directions: Every input is followed by perceivable feedback that confirms that the input has arrived and what effect it has. The HMI thus forms the technical interface to the perception-cognition-action chain: It delivers the perceivable signals, supports their interpretation, and enables the subsequent execution of the action. If this loop is missing or ambiguous, the most common error mechanism of all arises: The user assumes a system state that does not match the actual one and continues to act on this false basis.
HMI and user interface
The terms are often used synonymously but differ in scope. "Human-machine interface" is a technically oriented term and emphasizes the direct interaction channel between user and device. The user interface in the sense of IEC 62366-1 is broader: It includes labeling, packaging, instructions for use, and training materials.
This difference is not academic. Those who speak of the HMI in a regulatory context and mean only the device risk leaving accompanying materials out of the analysis and evaluation, even though they are normatively part of the user interface and bring their own potential for error. In documentation according to IEC 62366-1, the term user interface is therefore the more robust one.
Design principles
Established principles exist for the design of interactive systems. Some are described as interaction principles in DIN EN ISO 9241-110, others come from design practice. Particularly important in medical technology are:
- Visibility of system status: The current state must be recognizable at any time without interaction, especially after interruptions and changes of responsibility.
- Unambiguous feedback: Every input is confirmed; a missing effect is indicated as such and not silently ignored.
- Conformity with expectations: Operating logic follows the expectations of the target group, for example from comparable devices. Deviations from this create negative transfer and thus systematic errors.
- Consistency: The same elements mean the same thing everywhere, within the device as well as across a product family.
- Redundant coding: Safety-relevant information is never conveyed by color alone but additionally through shape, position, text, or symbol.
- Error tolerance and forcing functions: Critical inputs are safeguarded, and safety-relevant mix-ups are excluded by design; see error tolerance.
Particularities in medical technology
Medical devices are operated under conditions that make many interaction patterns borrowed from the consumer world unsuitable. Gloves considerably change the usability of capacitive touch surfaces; requirements for wipe disinfection influence the choice between membrane keypads, touch, and mechanical elements; and in sterile environments, only part of the team may be allowed to touch the device at all.
In addition, there is competition for attention. Every piece of information for which a user has to look away from the patient or the surgical site costs attention elsewhere. And because several devices are often active at the same time at a workstation, acoustic design can never be considered in isolation: Alarms that are clearly distinguishable on their own can, in combination, create a situation of strain that pushes cognitive load beyond a tolerable level. In usability engineering, these conditions are systematically captured as use environment factors. For critical tasks, reliable HMI design is particularly important, because safety-relevant information there must still be perceived, understood, and acted on correctly under these difficult conditions.
Typical sources of error
- Mode errors. The device is in a different operating mode than the user assumes; the same operating action then has a different effect. A classic and particularly consequential mechanism, because it systematically escapes the user's attention.
- Opaque states. After an interruption, alarm acknowledgment, or handover, it is not recognizable whether a process is running, paused, or aborted.
- Color as the sole carrier. Safety-relevant distinctions made by color alone fail with color vision deficiency, unfavorable lighting, or viewing angle.
- Confusable connectors and containers. Physically compatible but functionally different connections are a design invitation to error.
- Inconsistency within a product family. Devices from the same manufacturer with different operating logic produce errors, especially among experienced users.
Regulatory reference
In medical device standardization, the relevant term is the user interface according to IEC 62366-1; "HMI" is more common in general technical standards and in development jargon. Design principles for interactive systems are described in the DIN EN ISO 9241 series, whose Part 110 formulates the interaction principles.
Product-specific requirements are added, for example IEC 60601-1-8 for alarm systems of medical electrical equipment. Annex I of the EU Medical Device Regulation (MDR) requires that risks from use errors be reduced taking into account the ergonomic features of the device, a requirement that directly targets the design of the interface.
The human-machine interface covers both directions of information exchange: input and feedback. Its safety-critical core is the closed feedback loop: Where the displayed and the actual system state diverge, use errors arise that the user cannot notice.
In documentation according to IEC 62366-1, the broader term user interface is the relevant one. It includes labeling, instructions for use, and training material.
Frequently asked questions (FAQ)
Should one speak of HMI or of user interface?
In a regulatory context, "user interface" is the more robust term, because according to IEC 62366-1 it also includes labeling, packaging, instructions for use, and training materials. "HMI" is technically oriented and refers more narrowly to the direct interaction channel between user and device.
What is a mode error?
An error that arises because the device is in a different operating mode than the user assumes. The same action then has a different effect. Mode errors are particularly insidious because the user has no reason to question their assumption. An effective countermeasure is a permanently visible, unambiguous status display.
Why is color coding not enough?
Because it can fail under real conditions: with color vision deficiency, unfavorable lighting, a shallow viewing angle, or poor display contrast. Safety-relevant distinctions should therefore be coded redundantly, additionally through shape, position, text, or symbol.
Are touchscreens unsuitable for medical devices?
Not in principle, but they must be matched to the context of use. Gloves, moisture, disinfectants, and the lack of haptic feedback considerably change usability. For safety-critical inputs under time pressure or with gloves, physical controls are often the more robust solution.
Are you designing the operation of a medical device and want to rule out safety-critical sources of error early? We support you from analysis to summative evaluation.
More about our usability engineeringSources
- IEC 62366-1:2015+AMD1:2020, Medical devices, Part 1: Application of usability engineering to medical devices
- IEC 60601-1-8:2006+AMD1:2012+AMD2:2020, Medical electrical equipment, Part 1-8: General requirements for basic safety and essential performance, Collateral standard: General requirements, tests and guidance for alarm systems in medical electrical equipment and medical electrical systems
- ISO 9241-110:2020, Ergonomics of human-system interaction, Part 110: Interaction principles
- Regulation (EU) 2017/745 on medical devices (MDR)