[Sample SDC Paper]
- Bouncer Chair
Kun Liang, Vincent Ling, Yejay Ying, Kevin Caves, Richard Goldberg
Cerebral palsy is a disorder in which damage to the brain during pregnancy, childbirth, or shortly after birth inhibits proper motor control development. This is characterized by abnormal muscle tone, reflexes, and coordination. Because infants with cerebral palsy cannot fully control certain muscles, one of the challenges faced by their parents is in trying to develop those muscles as much as possible through physical therapy. Our client is a two year old boy who, as a result of cerebral palsy, has limited trunk support and limited control of his limbs. Our project’s goal was to provide a device that would exercise and strengthen our client’s trunk and limbs. Our final design accomplishes this by using a bouncing chair concept using springs, one that can bounce both up-and-down as well as side-to-side. By kicking his legs, our client would be able to cause a gentle up-and-down bouncing motion. By twisting his body, he would be rewarded with a side-to-side bouncing motion. Through continued use of the bouncer, the client would develop his leg and trunk muscles while performing an activity he enjoys.
Our client, Xavier, is a two year old boy with cerebral palsy. He has limited control of his limbs and lacks trunk support, which leads him to over-recruit muscles when attempting to perform tasks such as pulling or kicking. One of Xavier’s favorite activities is to lie in his bouncing chair and kick his legs, causing a bouncing movement that he enjoys. However, his current bouncer is quickly becoming too small for him, and the fact that it can only bounce in one direction, up-and-down, limits its use as a physical therapy tool.
Our goal was to build for Xavier a new bouncer that would not only bounce up-and-down, but also side-to-side. The device would have the ability to lock out either bouncing motion or have both occurring at the same time. Furthermore, the chair would have the ability to adjust from an upright position to a more relaxed reclined position.
Xavier needs a larger bouncing chair that can provide side-to-side bouncing in addition to up-and down motion. Thus, the goal is to design a bouncing chair for Xavier that can bounce side-to-side in addition to up-and-down in order to develop leg and trunk muscle control as well as provide entertainment.
Design and Development
The base of the device is made of furniture grade PVC pipe and serves to elevate the chair from the ground so that it can safely bounce without making contact with the floor. PVC piping is lightweight but very strong, and can be easily cut down to whatever specifications are needed. Furniture grade PVC can be ordered is various colors to enhance aesthetic appeal, which in the final device was chosen to be Duke blue.
- Figure 1. Furniture-grade PVC pipe base
- The chair used in the final design was extracted from a stroller chair. Made of lightweight metal and plastic, the chair features a clever mechanism that allows for simple reclining of the chair’s back. A safety harness made of neoprene and secured with buckles was added in order to ensure Xavier’s safety while using the device.
The chair is anchored to the base by four springs, two in the back and one on each side. The springs can be swapped out for stronger ones as Xavier grows and gets stronger, and included with the device are springs of varying load strengths and spring rates.
- The front of the chair has two plastic strips attached to it that are bolted to the base. This mechanism still allows full freedom of motion in the up-and-down and side-to-side directions. In order to lock out side-to-side motion, a pin is inserted through the base and through the plastic strips, preventing the chair from moving sideways. To lock out up-and-down motion, a strap and buckle mechanism attached to the back of the chair and looped around the base can be tightened, negating the effects of the springs and preventing the chair from bouncing up-and-down.
The final device was evaluated to assess durability, client ease of use, device effectiveness and safety. By client’s request, different sets of springs with various tensions were provided to the clients in order to adjust for client’s growth and increase in body weight. Durability was assessed during the design phase and the chair and all the springs were able to withstand the weight and kicking of the client. Ease of use was assessed by observation and actual testing by the client. Bouncer effectiveness was also assessed by client testing and evaluation. Detailed hazard analysis and risk assessment were conducted and risks identified were addressed through redesign.
Prototypes made from PVC pipes and wood were presented to a panel made of professors, biomedical engineering students, and physical therapists. Feedback and suggestions were provided during these presentations. Modifications were made based on information gathered from client testing of our prototypes and final device.
Discussion and Conclusions
In conclusion, the bouncer chair met all of the project goals outlined at the beginning of the project. Specifically, it has all of originally the proposed features:
- Larger size to fit client’s growth
- Bounce side-to-side
- Bounce up-and-down
- Switch between side-to-side and up-and-down bouncing, but not simultaneously
- Recline at different angles
- Provide neck support*
*Neck support will be provided by a custom neck pillow that the client’s family will place in the chair.
As a result, the chair will be able to meet all of the overall project goals:
- Accommodate growth
- Improve muscle development
- Increase durability and stability
Currently, the 2-year-old client is still getting accustomed to the chair, which is understandable. He was uncomfortable at first due to unfamiliarity, but the parents expect him to quickly adjust because this device replicates the angle, comfort level, and up-and-down bouncing motion of his current commercial bouncer chair that he likes. Furthermore, he also had a transition period getting used to the current bouncing. The physical therapist also has full confidence that the client will soon grow accustomed to using this bouncer chair.
During the design of the bouncer chair, the most difficult challenges came from finding the appropriate spring strengths and developing mechanisms to lock specific bouncing motions.
To replicate the bouncing motion that the client liked about his old chair, springs that were strong yet responsive were needed. After ordering many sets of springs off of the internet and having varying degrees of success with each set, the decision was made to make the springs replaceable. Thus, the springs can be interchanged based on the client or physical therapist’s preference and also with the client’s growing strength. Right now, the weakest springs (back springs: 10 lbs loading force; side springs: 30 lbs loading force) are installed in the device. However, stronger springs will also be delivered with the device (see user’s manual) so that they can be switched out at will.
To lock side-to-side and up-and-down bouncing separately, several ideas were proposed and rejected. For example, one idea was to use a system of suspended straps to grab the front legs of the chair; once tightened, they would lift the chair off of the base and prevent side-to-side bouncing. This idea turned out to be too cumbersome and would detract from the aesthetics of the device. Another idea to stop up-and-down bouncing was to use hinges to lock that motion. However, the hinges turned out to have so much friction that they would have locked out all bouncing motions, not just up-and-down bouncing. The search for an elegant, simple, yet effective solution proved to be more difficult than originally imagined.
In the end, a simple strap and buckle system was used in the back of the chair to lock up-and-down bouncing. This mechanism was low profile and easy to use. Meanwhile, one of the instructors, Kevin Caves, suggested the mechanism for locking side-to-side bouncing. Instead of trying to use so many straps, he recommended simply fixing a plastic piece to the front of the chair to lift it off of the base then drilling a hole through it for a locking pin to prevent side-to-side motion. After further testing, this mechanism was found to be very effective, low profile, and simple to use, so it was implemented in the final design.
Regarding safety, the final device was modified to minimize safety hazards. Rubber caps were placed on all sharp edges of the chair’s legs, and any protruding screws were sawed off to keep the device free of tripping, scraping, and cutting hazards. The side springs were adjusted to prevent the chair from tipping too far to the side and thus minimize the risk of the child falling out. In addition, a comfortable safety harness was added to keep the child from slipping out of the front of the chair. Moreover, the chair was kept as low off of the ground as possible.
Thus, the bouncer met all design specifications, project goals, client needs, and safety targets.
We’d like to thank our client and his parents, Amanda and Nate, for accommodating us throughout the project and allowing us to work so closely with Xavier. It was truly a pleasure and an eye-opening experience working with them. We’d also like to thank Ilana Levin, our client’s physical therapist, for being at all our client meetings, contributing her expertise, and giving constructive feedback and advice for our device. Special thanks go out to Harry Phillips of Triangle Orthopedics for graciously providing us with the neoprene material for the chair’s harness. In addition, Steve Earp of the Duke student machine shop provided essential guidance in the machine shop for milling and cutting some of our parts, and Nobel Vale was a constantly helpful TA. Finally, this would not have been possible without Dr. Richard Goldberg and Kevin Caves, who gave us this unique opportunity and proved to be dedicated, positive, and wise instructors during the semester. Funding for this project was made possible by NSF grant # 0118558.