Fruit tissue is a cellular, water-filled, viscoelastic material. Under slow compression it shows a roughly linear region followed by a bioyield point at which cells begin to rupture, and the classical treatment of these properties, including the use of Hertz contact theory for convex bodies pressed against a plate or indenter, is that of Mohsenin (1986). A bruise is the visible consequence of that rupture: cell contents oxidise and the tissue browns beneath skin that may itself be unbroken. Opara and Pathare (2014) review how bruises are measured and note that susceptibility changes with ripeness, temperature, turgor and cultivar, so a limit established for one batch does not carry over automatically to the next. Li and Thomas (2014) distinguish damage from impact, compression, abrasion and vibration, a distinction that matters here because a gripper can cause the first three in a single cycle.

Shintake et al. (2018) sort soft grippers into those that grasp by actuation, those that grasp by controlled stiffness and those that grasp by controlled adhesion. For packing work five practical families follow from that scheme, and Table 1 compares them. Passive structures such as the Fin Ray finger bend towards an applied load and wrap the object without any internal actuator (Crooks et al., 2016). Pneumatic network actuators are elastomer fingers with internal chambers that curl when pressurised (Ilievski et al., 2011; Mosadegh et al., 2014), and fibre-reinforced variants constrain the swelling so that more of the input pressure becomes bending (Polygerinos et al., 2015). Granular jamming grippers press a loose membrane onto the object and then harden it by evacuating the air (Brown et al., 2010). Tendon-driven underactuated fingers adapt through their kinematics (Manti et al., 2015). Suction cups, the oldest of the group, remain the default wherever the surface allows a seal (Monkman et al., 2007).

For the fingers themselves two modelling traditions are available. Flexure-based and ribbed designs in stiffer polymers can be analysed with the pseudo-rigid-body method, which replaces a flexible segment with rigid links and torsional springs (Howell, 2001). Elastomer fingers undergo strains far beyond the linear range and need a hyperelastic material law; the reduced polynomial form of Yeoh (1993) is widely used because it can be fitted from a uniaxial test alone.

The first output is a number the gripper must not exceed. Following the practice described by Mohsenin (1986), samples spanning the ripeness range handled on the line are compressed between a flat plate and a curved indenter, and the force and deformation at bioyield are recorded. Hertz theory converts these into an allowable contact pressure for the radii of curvature involved. Because bruising may appear only after storage, the limit should be confirmed by holding loaded samples for the usual shelf period and inspecting them, using one of the bruise measures reviewed by Opara and Pathare (2014). A margin is then applied for the softest fruit expected, not the average.

The second step fixes what the line demands: the range of item sizes and masses, the cycle time, the accelerations of the arm, the approach direction, the spacing of neighbouring items and the geometry of the tray. The peak acceleration matters most, since the holding force must resist inertial load as well as weight, and a fast pick-and-place robot can impose accelerations several times that of gravity. The envelope also records whether the item must be reoriented, which separates tasks that need an enclosing grasp from those a single-sided grip can serve, in the sense of the power and precision grasps classified by Cutkosky (1989).

With a pressure limit and an envelope in hand, the families in Table 1 can be screened. Suction is tried first because it is fastest and cheapest, and rejected if the skin is rough, wet, hairy or too weak to carry the item by a patch of its surface. Passive adaptive fingers suit lines where a servo or pneumatic jaw is already present. Fluidic fingers are preferred where the pressure limit is low, since the grasp force is governed by supply pressure and not by jaw position. Jamming pads are reserved for mixed or irregular items. The choice and the reasons for rejecting the alternatives are written down, because a later change of crop will reopen the question.

Finger length follows from the largest item, which the fingers must reach around past its widest section, and finger width from the room between neighbouring items. Wall and rib thicknesses are first estimates at this stage. The material is chosen from grades cleared for food contact, which in the European Union means compliance with the framework regulation on food contact materials (European Parliament and Council, 2004) and in practice points to platinum-cured silicones and certain thermoplastic polyurethanes. A softer grade lowers contact pressure and raises the stroke for a given input, at the cost of tear strength and fatigue life.

The trial geometry is now checked against the limit from Step 1. For a ribbed passive finger a pseudo-rigid-body model gives the deflected shape and the reaction at each contact (Howell, 2001). For a fluidic finger the analytical model of Polygerinos et al. (2015) relates input pressure to bending angle and tip force, and a finite element model with a Yeoh material law refines the result where the geometry is unusual. The quantity of interest is the peak pressure on the fruit at the holding force required by the envelope, with the fruit represented by its measured stiffness and not as a rigid body. If the peak exceeds the limit, the designer returns to Step 4 and lengthens the contact, softens the material or adds a finger.

A gripper that touches food must be cleanable. Crevices, exposed threads, open cable runs and porous printed surfaces harbour residue and should be designed out; moulded one-piece fingers with a smooth skin are preferable to assemblies. Fingers are consumables, so they should release from the base without tools, and the air or vacuum connection should seal on its own when a finger is fitted. Where the line runs several crops, the base is kept common and only the fingers change.

The last step produces a test plan, not a result. It should specify a holding test under the peak acceleration of the envelope, a damage test in which gripped and ungripped fruit from the same batch are stored and compared after the shelf period, a cycle test to the service life expected of a finger, and a cleaning test. Pass criteria are fixed before any test is run.