HapWRAP: Soft Growing Wearable Haptic Device

Nathaniel Agharese1, Tyler Cloyd1, Laura H. Blumenschein1, Michael Raitor1,Elliot W. Hawkes2, Heather Culbertson3, and Allison M. Okamura1

Abstract— Soft robotics and pneumatic actuation present op-portunities for lightweight wearable haptic devices that providedistributed touch feedback to the skin. Ideally, such deviceswould be easily donned and doffed, since permanent coverageof a large area of the skin is undesirable. Here we present thedesign and evaluation of a concept device called HapWRAP: agrowing Haptic device constructed from Wearable Restricted-Aperture Pneumatics. Controlled air flow through flexiblelow density polyethylene allows HapWRAP to grow out of acompact housing unit and provide a combination of directionaland force feedback to a user. When activated, HapWRAPgrows up and around the forearm; its loops form a temporarysleeve. After growth, pneumatic actuators inflate and deflateto stimulate mechanoreceptors in the skin at distinguishablelocations. This paper describes the design and manufacturingof HapWRAP, reports its performance metrics, and tests itssuitability as a haptic feedback device. Participants were ableto interpret force and direction cues from HapWRAP with92.5% accuracy. These findings suggest that HapWRAP can besuccessfully used for applications where both force and directioncues are necessary.

I. INTRODUCTION

Wearable haptic devices have the potential to providehumans with motion guidance, navigation cues, and realisticinteractions with virtual or remote (teleoperated) environ-ments. Such wearable devices fall into two main categories:tactile (stimulating the skin) and kinesthetic (body-groundeddevices that provide force or torque over a movable joint).Tactile devices are especially advantageous, due to theirgenerally decreased size and weight requirements comparedto kinesthetic feedback devices; stimulating the skin typicallyrequires less force than generating noticeable forces/torquesover a moving joint [1]. In addition, recent work on skindeformation feedback [2], [3], [4], [5] and novel vibrationsignals [6], [7] shows the potential for tactile feedbackto provide natural and easy-to-interpret direction cues inmultiple degrees of freedom. Most existing wearable tactiledevices are placed on the hand, especially the fingertips, dueto the density and small receptive field of mechanoreceptorsin glabrous (non-hairy) skin, which makes tactile feedback

*This work was supported in part by the National Science Foundationgrant 1441358, a National Science Foundation Graduate Fellowship, andStanford University.

N. Agharese, T. Cloyd, L. H. Blumenschein, M. Raitor, and A. M.Okamura are with the Department of Mechanical Engineering, StanfordUniversity, Stanford, CA 94305 USA.(e-mail: {agharese, tcloyd, lblumens, mraitor, aokamura}@stanford.edu).

E. W. Hawkes is with the Department of Mechanical Engineering,University of California Santa Barbara, Santa Barbara, CA 93106 USA.(email: ehawkes@engineering.ucsb.edu).

H. Culbertson is with the Department of Computer Science, Universityof Southern California, Los Angeles, CA 90089 USA.(email: hculbert@usc.edu)

Fig. 1. HapWrap grows around the user’s forearm to create a total of 4 coils(left). The Pneumatic pouches placed on the underside of HapWRAP canthen be inflated to provide directional cues as well as a squeezing sensation.The HapWRAP is a fully compliant haptic device when deployed (right).

easy to sense and localize. Moving tactile feedback toother locations of the body is desirable in order to leavethe hands available for manipulation tasks, but the lowerdensity of mechanoreceptors in locations like the forearmmeans that the device must provide more spatially distributedfeedback [8].

In an effort to expand the utility and applicability of wear-able technology, many novel concepts for wearable hapticdevices and robots have been developed in recent years.For example, Dementyev et al. created miniature robotsthat move freely over a user’s clothing, displaying multi-modal haptic feedback including vibration and skin drag [9].In addition, some wearable devices create extranumeraryfingers [10] and limbs [11]. Other researchers have createdwearable haptic devices that make use of haptic illusions,such as the hanger reflex [12], and novel interfaces withthe skin, such as water [13] and air [14]. Soft and flexibleactuators, such as the tactile display presented in [15], arekey to maximizing wearability of haptic devices.

Wearable pneumatic haptic devices are especially promis-ing because they are comfortable, relatively low profile, anddisplay salient cues. The style of haptic actuation used in thiswork has been termed WRAP (wearable, restricted-aperturepneumatics), because it uses inelastic material that restricts

pressurized body

P

P

eversion

P

P

pump pump

Fig. 2. The deployment of the HapWRAP is based on the concept oftip-extending soft robots adapted from [17].

the aperture to which a pneumatic actuator can inflate [16].WRAP actuators display salient cues by changing shape asthey inflate, stimulating multiple types of mechanoreceptors.Raitor, et al. [16] demonstrated the ability of WRAP ac-tuators worn in a wristband to display pulsing translationand rotation cues with an overall identification accuracy of99.4%. These actuators are versatile as they successfullydisplay direction cues when wrapped around a user or aroundan object manipulated by a user. However, the WRAPwristband had a relatively small, fixed contact area withthe skin. Ideally, contact area would be increased when thedevice is in use and decreased when it is not in use.

To construct a variable coverage device, we developeda mechanism that extends the WRAP in a spiral fashionaround the forearm. This mechanism is based on a methodfor extending soft robots that is conceptually similar tothe manner in which vines grow from their tip [17]. Theextension, or “growth”, is driven by pneumatic pressureinside the body of the device, which unfurls or everts newmaterial at the tip, resulting in length change (Fig. 2). Growthrate is controlled by the tubing’s internal pressure. The shapeof the extending body can be either controlled actively orbe preset at design time; here we use the latter method forsimplicity, since the desired path of growth is predictable andset by the shape of the user’s arm.

II. DESIGN AND MANUFACTURING

There are three main components of the HapWRAP: thesoft robot, the housing, and the pneumatic system. Poweredby the pneumatic system, the soft robot grows out of itswrist-mounted housing, wrapping around the user’s forearm.Previously-developed wearable haptic devices have beenplaced at both the wrist and the forearm, areas known forhaving large myelinated afferents [16][18][19]. This place-ment results in a dependable perceived sensation from thehaptic actuators [18]. In addition, the HapWRAP is designedin two sizes, small and large, so that it can be used withdifferent sized arms.

A. Soft Robot

The soft robot grows around the arm and is used toprovide haptic feedback. The largest component of the softrobot body is a 1.8 cm diameter tube of 25.4 micron thicklow-density polyethylene (LDPE) that grows out of itself,bringing the other components of the soft robot with it.

Thin-walled LDPE tubes have proven to be effective formanufacturing soft growing robots [17]. This material evertsat low pressures and can be easily shaped or sealed byapplying heat [20]. We refer to this tubing as the robot’spneumatic support. Once pressurized, the support grows fromits housing on a predetermined path that forms several tightcoils around the user’s arm (Fig. 1). It can be pre-shapedinto different sized coils in order to grow around arms ofvarious sizes. Two 1 mm diameter vent holes in its tip allowcontinuous pressurization without the risk of popping. Theflexibility of the soft robot approach allows the HapWRAPto be contained in a housing unit a fraction of its inflatedsize.

After growth, HapWRAP provides haptic feedback viapneumatic actuators attached to the support. These actuatorsare sandwiched between the support and the user’s arm,providing discrete direction and force cues. Directions arecued on the left (ulnar), top (dorsal), right (radial), andbottom (palmar) sides of the wrist by co-located actuators.Force cues are given by one actuator that is one coil lengthlong. Direction and force cues combine to provide directionalcues of varying strength.

The directional actuators are 4 cm x 2.5 cm LDPE poucheson the small model, and 4.5 cm x 2.5 cm pouches on thelarge model. The force actuator is 22 cm x 2.5 cm on thesmall model, and 25 cm x 2.5 cm on the large model. Allactuators have channels that run along the support down tothe outlet of the housing, where they are connected to thepneumatic system. These channels are 0.8 cm wide and aremounted to the support in a manner that prevents overlapbetween channels (Fig. 3). This allows for continuous airflowto the actuators and minimizes the thickness of the soft robot,improving the growth performance of the support [21]. Thechannels are connected to the actuators at either the centerof the actuator, as seen in the left actuator, or at one of theactuator corners (Fig. 3). Fig. 4 shows the final placementsof the actuators on the arm after the device is grown.

B. Housing

The housing provides the soft robot a place to residebefore deployment, mounting it at a specific location andorientation on the user. Previous housing designs for softgrowing robots incorporated a large airtight chamber for therobot to grow out of [17]. The robot was wrapped around aspool that rotates within the chamber at a speed controlled bya DC motor. With the chamber pressurized, the spool rotated,allowing the robot to grow. Such a chamber allows for thecompact storage of a large amount of material, and the rigidconstruction can withstand the pressure needed for growth.We initially took this same approach with our housing,planning to have a sized-down version of this chamber rest onthe back of the user’s hand. However, the coiled shape of theHapWRAP made it unable to successfully grow after beingwrapped around a spool. This meant that we had to manuallyreset the HapWRAP each time, a process that requires easyaccess to housing. We were unsuccessful in creating a rigid

Fig. 3. The design of the actuators and their channels. The view is looking down at a HapWRAP that has been split open along the top and uncoiled.

Fig. 4. The layout of the actuators when on a user. The actuators areintegrated into the inside of the helix of the HapWRAP, against the skin ofthe user.

chamber that simultaneously allowed the device to be reseteasily and limited air leaks that negatively impact growth.

We found that we could instead create a sufficiently air-tight housing unit using the same material as the pneumaticsupport. The HapWRAP can be stored in this tube in acrumpled state due to its fully compliant structure. Thisdesign shift allowed the housing to be a flexible unit thatformed a more compact wearable device. The final housingdesign is a set of UV curable acrylic rings attached to anelastic band with extra length of support tube secured inthese rings (Fig. 5A). The elastic band is strapped aroundthe user’s wrist and secured with Velcro pieces. On the smalldevice, the elastic band is 18 cm x 5.25 cm, while the largedevice has a band that is 23 cm x 5.25 cm. The sectionof support tube incorporated into the housing is shapedinto a coil, allowing it to naturally wrap around the wrist.The rings secure this tube so that it stays in place whenbeing inflated. The channels for the pneumatic actuators aresecured in the ring closest to the outlet of the soft robot,keeping the pneumatic support fixed on the elastic band.There are multiple rings so that the elastic band can stretchto fit various wrist sizes while still being securely attached.

C. Pneumatic System

The pneumatic system controls the growth of the deviceand the actuation of the haptic cues. To determine therequirements for growing the device, we ran two types oftests: burst tests and growth tests. Burst tests determinedthe maximum pressure that our system could withstand. Byslowly inflating a section of the pneumatic support tubinguntil it burst, we determined that the body of the HapWRAPcould maintain up to 50 kPa. This was used to limit the entiresystem, as we expected the burst pressure for the actuatorsto be higher because of the smaller diameter of the actuatorpaths. Growth tests determined the minimum requirementsfor the air pump that would grow the soft robot. By growingthe soft robot at pressures between 10 and 30 kPa froma source that provided air at about 1.5 liters per minute(LPM), we determined that a minimum of 20 kPa at 1.5 LPMwould allow for consistent growth. The pneumatic support isdirectly connected to a 12V micro diaphragm air pump thatprovides up to 1.75 LPM at 25 kPa.

We inflated the actuators at pressures between 5 and20 kPa to test for cue perception. We found that the pneu-matic actuators were able to fully inflate almost immediatelywith 7 kPa from a source that provided air at about 1.5 LPM.The haptic feedback system is connected to a 6V miniaturediaphragm air pump that provides up to 1.5 LPM at 10 kPa.Air is directed from the 6V pump to the pneumatic actuatorswith five miniature solenoid valves that are 1 ⇥ 1 ⇥ 2 cm(Fig. 6).

The pneumatic system requires between 6.5 and 9 W tooperate, depending on the cue. This power is supplied fromtwo sources, a 12V source for the pumps that can provide atleast 920 mA, and a 3V source for the logic that can provideat least 170 mA.

D. Manufacturing Techniques

It currently takes about 6 hours to manually create oneHapWRAP, not including time for 3D-printing parts. Despitethis lengthy process, manufacturing the HapWRAP usesaccessible, low-cost materials. The soft robot is constructedso that it always inflates into a coil. To achieve this, weadapted fabrication methods that involve creating molds forheat-shrinking the LDPE tubing to the desired shape [20].The molds are three-quarters of one loop of the desired coil

B

A

1cm

Fig. 5. A: The housing for the HapWRAP, with an expanded view of thering that holds the air tubes in place. B: The small (left) and large (right)sized molds used for making the pneumatic support part of the HapWRAP.

Fig. 6. The pneumatic system. Left, Top, Right, Bottom, and Forcerepresent the pneumatic actuators, and Tube represents the pneumaticsupport.

(small or large) (Fig. 5 B). The small mold has a centerdiameter of 7.2 cm and a thickness of 1.7 cm, while thelarge mold has a center diameter of 8.2 cm and a thicknessof 1.8 cm. We chose these sizes to fit snuggly around theportion of the forearm closest to the wrist. We estimated therequired circumference with data on average forearm andwrist circumferences, favoring wrist measurements since theHapWRAP grows from the wrist [22]. We confirmed theseestimations by testing the fit from the resulting pneumaticsupports. For manufacturing, we take a 150 cm long piece ofthe 25 micron thick tubing, and heat-shrink the entire lengtharound the mold. We heat seal one end of the pneumaticsupport, and cut the tubing such that it is 4.5 coils long wheninflated. We attach the open end of the pneumatic support toan air tube with tape, and we reinforce it with hot glue. Theair tubes connect the pneumatic system to the soft robot.

The pneumatic actuators are the most labor intensive part

of constructing the HapWRAP; any errors in the constructionof the actuators can adversely affect the performance of theHapWRAP in growth and haptic feedback. The pouches aremade from 12.7 micron thick LDPE tubing. We opt for thisthinner tubing to reduce the negative impact that the addedmaterial has on growth [21]. We heat seal and cut out thedesign of the actuators and their channels. All actuators havetheir corners sealed and cut to 45 degree angles 1 cm from theedge to allow for smoother growth of the pneumatic support.These cuts are taken 1 cm in from the edge of the actuator.We place about 3 cm of double-sided tape every 2 cm alongthe channel starting from the part of the channel closest tothe actuator. There are two pieces of tape on the channelfor the left actuator, three pieces for the top actuator, andso on in a clockwise manner through the force actuator. Theactuators must be attached to the pneumatic support whileit is inflated to ensure proper placement. We attach the leftactuator three quarters of a loop from where we want theoutlet of the HapWRAP to be on the support tube. Thisguarantees that the outlet is always towards the top (dorsal)part of the user’s wrist.

The flexible housing for HapWRAP consists of a Velcrosecured elastic band and several acrylic rings. Using hot glueand double-sided tape, we fasten the 3D printed rings to theelastic band in a straight line from one corner of the bandto the opposite corner. We feed the actuator channels andair tubes through the openings in the outlet ring (Fig.5A),attaching the tubes to the actuator channels with hot glue.This creates a detachable interface between the soft robotand the housing unit, allowing the soft robot to be replacedwithout creating an entirely new housing unit. Since, thesoft robot degrades over time, the versatility of the multi-ring configuration reduces the setup time for HapWRAPreplacements.

III. HUMAN-USER STUDY: DEVICE GROWTH ANDHAPTIC CUES

We conducted a human-user study to test the ability ofHapWRAP to grow onto a participant’s arm, correctly posi-tion the pneumatic pouches, and deliver force and directioncues. 10 right-handed users (5 female and 5 male, ages 20 to23) participated in the study. Five participants had previousexperience with haptic devices, and five participants had nosuch experience. The protocol was approved by the StanfordUniversity Institutional Review Board, and all participantsgave informed consent.

A. Experimental setup

Two sizes of HapWRAP, small and large, were producedfor the study following the design described in Section II.To determine the appropriate size for each participant, wemeasured the circumference of the wrist at two locations:first, just below the pisiform bone (the knobbly bone inthe proximal row of the wrist), and second, 3.25 cm be-neath the pisiform bone. If the first measurement was lessthan 16.75 cm and the second measurement was less than17.75 cm, the participant received the small HapWRAP,

otherwise, the user received the large HapWRAP. We placedthe HapWRAP around the participant’s left wrist so thatthe elastic band was directly below the base of the user’shand and the outlet of the HapWRAP was at the dorsalend of the user’s wrist near the scaphoid as shown inFig. 1. The left palm was placed on the desk, and an elbowsupport was provided to prevent fatigue. A divider blockedthe HapWRAP from the user’s view to prevent the aid ofvisual cues. Participants wore noise-canceling headphonesthat played white noise at a frequency similar to that of theair pumps to prevent the aid of sound cues.

B. Experimental procedure

The experiment consisted of four stages: (1) growing theHapWRAP around the participant’s forearm, (2) if needed,making manual adjustments to improve positioning of thepneumatic pouches, (3) calibration, and (4) testing the par-ticipant’s ability to identify direction and force (magnitude)cues provided by the pouches.

The HapWRAP was grown onto the forearm once foreach participant using air pressurized to 25 kPa. Duringeach HapWRAP growth, we recorded video to providemeasurements of the growth time, smoothness of growth,and final positioning of the loops of the tube.

After growth, we examined the resulting placement ofthe pneumatic actuators (pouches). The directional actuatorsmust be distributed in the manner specified in Fig. 3 in orderto provide the desired direction cues: left (ulnar), top (dorsal),right (radial), and bottom (palmar). The pneumatic actuatorthat displays the force cue wraps once around the entirearm, so its location did not have to be verified. We notedthe locations of the pouches and made slight adjustments ifnecessary before proceeding to the haptic cue portion of thestudy. These adjustments also sometimes included shiftingthe base location around the wrist and shifting the placementof the air tubes within the housing unit.

In the haptic cue identification stage of the study, partici-pants were asked to identify direction and magnitude (force)cues by feel alone. For the direction cues, one of the fourdirectional pneumatic actuators was inflated for 0.15 secondsand deflated for 1.4 seconds. We began by calibrating thedevice to confirm that the actuators were working properlyand that the participants could distinguish the cues presentedby each actuator. We individually displayed each of thefour directional cues and each of the two force cues to theparticipant, while telling the participant what cue was beingdisplayed so they could confirm their interpretation of it.Starting with the directional cues, each actuator pulsed fivetimes in the following order: top, left, bottom, right. Theparticipants were given the option of feeling the cues again.If the participants experienced issues with interpreting thecues, we adjusted the device as needed.

We then displayed the two force cues by pulsing the forceactuator to the low force level three times, and then to thehigh force level three times; this process was repeated once.The low force was described as ”near” and the high forcewas described as ”far” in an analogy to navigation guidance.

The participants again had the option of repeating the cues. Ifthe participants experienced any issues, we would adjust thedevice as needed. During the calibration phase, each inflationwas 0.7 seconds and each deflation was 0.85 seconds.

After calibration, each participant completed 40 trainingtrials and 40 experiment trials. During the training andexperimental trials, the cues followed the same inflationand deflation timing as during calibration. The order of thecues was randomized for each participant. The HapWRAPcreated the direction and force cue sequence as follows:inflate the force pneumatic actuator to the correct level (nearforce cue: 7 kPa at 0.75 LPM; far force cue: 10 kPa at1.5 LPM), deflating the force actuator while inflating theappropriate directional actuator for 0.15 seconds at 1.5 LPMto 10 kPa, and then deflating all actuators for 0.7 seconds.The directional and force cues were presented separately sothat the participant would receive directional cues with thesame pressure. The participant interface presented cues thathad a ”near” force simply as the direction, and cues that hada ”far” force as the direction preceded by the word ”far”(e.g. ”top”, ”far top”).

Before the start of each set of trials, participants felt a cuefor ”top” followed by a cue for ”far top” to remind themof the relative force levels of the magnitude cues. Duringtraining, participants received a cue from the HapWRAP, se-lected the cue they interpreted, and received visual feedbackabout their performance. If the participants chose the correctcue, they were shown the cue in green. If the participantschose an incorrect cue, they were shown the correct cuein red. During the experimental trials, participants did notreceive visual feedback; they only received confirmation thattheir response was recorded. The cues was displayed untilparticipants selected an answer. Participants took a shortbreak in between the training and experiment trials.

Each participant completed a post-experiment survey toassess the difficulty of the discrimination task. On a 5-pointLikert scale from 1=“Very Easy” to 5=“Very Hard”, theyrated the ease/difficulty of the different directions and typeof cue (direction vs. force). After each participant completedthe trials, the HapWRAP was manually removed from thearm and reset for the next participant.

C. Results

1) Growth and Placement of Pneumatic Actuators: TheHapWRAP grew correctly onto the arm for 7 of the 10participants on the first attempt. In the other three cases thefirst attempt failed, and we switched to a backup devicethat achieved growth on its first attempt. Examination ofthe failed HapWRAP devices revealed that holes torn in thepneumatic support were the cause of these failures. Theseholes were likely caused when the HapWRAP was reset,which is currently done by manually pushing the HapWrapback into its center tube from the tip.

The successful growths took between 10 and 38 secondsfor 8 of the 10 participants. Two participants experiencedexceptionally slow growth of 90 and 160 seconds. Theobserved slow growth was caused by irregularities in friction

TABLE ICONFUSION MATRIX SHOWING ACCURACY FOR ALL CUES.

Numbers shown are percentage of responses in a category for a given displayedcue. Green cells represent correct interpretation of the cue. Yellow cells representcorrect interpretation of the direction, but not the force. Red cells represent correctinterpretation of the force, but not the direction.

during feeding of new material through the tubes due tothe high curvature of the arm loops. These irregularitiesare not surprising given our manual resetting process – theHapWRAP is never reset in exactly the same configuration.This changes the distribution of the HapWRAP within thehousing, causing it to grow at varying speeds. The Hap-WRAP always grew slower in the beginning stages, whenthe length and thickness of the material feeding through thetubes was largest.

Due to variability in participant arm size, even within asize group, our approach of placing the pneumatic actuatorsat predetermined locations on the growing tube means thatactuator placement is not identical for each participant. How-ever, each actuator was always at least partially in the correctlocation, such that it overlapped with the ideal footprint forthe actuator’s associated direction cue. The wrist sizes of theparticipants ranged from 14.5 cm and 19.75 cm in diameteras measured at the pisiform bone. All participants except onewore the small HapWRAP; the remaining participant worethe large HapWRAP.

2) Accuracy and Timing of Response to Haptic Cues:With 10 participants performing 40 experimental trials each,we recorded a total of 400 cues and responses. Table I isa confusion matrix that shows the percentage each responsewas given for a displayed cue. Over all trials, only 30 cueswere incorrectly identified, resulting in 92.5% accuracy rate.The correct force cue was identified 93.25% of the time,while the correct direction cue was identified 99.25% of thetime. The most mistakes made by an individual participantwas 7 (82.5% accuracy), and the fewest mistakes made byan individual participant was 0 (100% accuracy). While theoverall accuracy was 92.5%, the accuracy of directional cueswas almost perfect (99.25%). This is comparable to resultswith the directional pneumatic feedback device that inspiredour design, which had an accuracy of 99.4% [16].

Figure 7 shows the individual participant and overall re-sponse time for each displayed cue. On average, participantsresponded to cues in about 4.3 seconds, after less than 3cycles of the 0.6 Hz cues. The near right cue had the fastestaverage response time of 3.5 seconds, while the far top cuehad the slowest average response time of 5.1 seconds. Thelongest time spent responding to a cue was 29.5 seconds,and the shortest time it took a participant to respond was1.5 seconds. The average response time for all of the cues

Top Right Bottom Left

Cue Displayed

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4

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pons

e Ti

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(s)

Overall Mean and ErrorIndividual Means

FarTop

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Fig. 7. Mean response time for each cue, for individual participants andthe group. Error bars on the group mean show standard deviation. Therewas no significant difference in response time for different directions orforces.

were within 1.6 seconds of each other; which is about theamount of time of a single cycle.

3) Survey: In a post-experiment survey, participants con-sistently reported the force cues as more difficult to interpretthan the directional cues. On the 5-point Likert scale, partic-ipants gave the force cue an average rating of 3.2, and thedirectional cues an average rating of 1.6. Participants oftennoted that some directions were easier to distinguish thanothers, but there was no group trend as to which directionswere easier and which were harder.

D. Discussion

We demonstrated the ability of a pneumatic haptic deviceto be delivered from a bracelet form factor to a distributedset of tactile elements on the arm. Most of the HapWRAPstested grew successfully, but repeated use caused damage.This motivated the need for more robust materials while stillmaintaining flexibility, as well as consistent manufacturingmethods.

Since HapWRAP is a soft robot, its shape can adjust tofit a variety of people while keeping all actuators in solidcontact with the skin. Using pneumatic actuators allowsHapWRAP to successfully present haptic cues of varyingintensity, regardless of whether its shape was adjusted for theuser’s arm. However, different arm sizes affect the placementof the individual actuators and thus the locations of thedirection cues. As a result, the system could benefit from theability to adjust the location of the actuators automaticallybased on the participant’s arm size and/or desired task.

The results of the haptic cue portion of the study demon-strates the ability of HapWRAP to give both force anddirection cues to a participant. Every response had the correctdirection or force, or both were correct. Force cues weremore difficult to recognize than the directional cues, likelybecause they relied on a difference in pressure rather thana difference in contact location. Furthermore, the magnitudedifference between the two force values was limited by thechoice of hardware. In future iterations, more distinct force

levels could be created using hardware capable of higherpressures. Following the previous WRAP concept [16], theexcitation of skin deformation-sensing mechanoreceptors atdifferent locations in the skin are highly distinguishable.While accuracy was high, the current response time may betoo slow for expected applications such as navigation.

IV. CONCLUSIONS AND FUTURE WORK

This paper describes the design, manufacturing, and per-formance of a soft growing wearable haptic device: Hap-WRAP. The HapWRAP demonstrates that soft growingrobots can be used to provide accurate haptic feedback. Thisis preferable to previous wearable haptic feedback devicesbecause the HapWRAP can emerge from a compact unit tocover a large surface area on the user’s arm. The success withusers of varying wrist sizes demonstrates that the HapWRAPis adaptable to various populations.

There are several directions for future work. The currentdesign of the HapWRAP lacks a retraction method, so thearea in contact with the user only varies during growth. Weplan to create a mechanism for retraction so that we canactively control the length that the HapWRAP coils aroundthe arm. One possible method for achieving this is withstring attached to a motor that runs through the inside ofa HapWRAP and is secured to the tip. The HapWRAP alsorequires the user to keep their arm in a specific orientation.To be used in mobile applications, HapWRAP should be ableto determine the orientation of the user’s arm so that it alwaysinflates the correct actuator for a given directional cue in theuser’s body frame of reference. This can be achieved by hav-ing an inertial measurement unit attached the housing [23].Increasing the cue frequency would allow users to experiencemore cues in less time, making HapWRAP more appealingto applications that require fast reaction times. Changing thepneumatic system to a vacuum-pump system rather than asimple vent would decrease deflation time, increasing cuefrequency. Also, decreasing the manufacturing time wouldmake HapWRAP more appealing. We can explore this byautomating the manufacturing of the actuators using methodsthat involve heat drawing or stamping a design, as was donein [24]. We will also explore other options for removing thesound cues from the pumps, one example being placing thepumps within a sound-proof box.

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